It has been proposed that bodily self-consciousness is based on the brain's multisensory integration of visual, vestibular, somatosensory, proprioceptive and motor signals (Haggard et al., 2003; Blanke et al., 2004; Blanke and Mohr, 2005). This theory distinguishes between personal (including also peripersonal) space, which is a volume of space occupied by the physical body and the space immediately surrounding the body, and extrapersonal space, that is the space outside of personal space. The theory proposes that the vestibular system is critically involved in integrating sensory signals from personal space (e.g., somatosensory, proprioceptive, visual, and auditory signals) with sensory signals from extrapersonal space (e.g., visual and auditory signals). It was proposed that particularly otolithic vestibular signals about constant gravitational acceleration provide a world-centered reference for the bodily self. By means of multisensory integration between personal and extrapersonal space the brain generates a spatial representation of the body as a whole, with a given location and orientation with respect to the external world, i.e., bodily selfconsciousness. In line with this theory, Lopez et al. (2008) argued that vestibular otolithic signals are highly relevant for spatial aspects of bodily self-consciousness, i.e., self-location and firstperson perspective, which depend on signals from both personal and extrapersonal space, and that vestibular signals may be less relevant for non-spatial aspects of bodily self-consciousness, e.g., self-identification, which depend mainly on signals from personal space and relate mostly to the body itself, rather than to the body relative to the external world (see also Blanke, 2012).

## **Clinical data**

The strongest support for the proposal that vestibular processing contributes to bodily self-consciousness comes from observations in neurological patients suffering from out-of-body experience who show a three-way disembodiment of their bodily selfconsciousness (Devinsky et al., 1989; Blanke et al., 2002, 2004; Brandt et al., 2005; De Ridder et al., 2007; Ionta et al., 2011; Pfeiffer et al., 2013). During an out-of-body experience patients typically identify with an illusory body in external space (disembodied self-identification), feel to be elevated above their physical body (disembodied self-location), and to have an elevated visuospatial perspective directed back downward to the physical body (disembodied first-person perspective).

Out-of-body experience in some neurological patients were caused by damage (Ionta et al., 2011), dysfunction (Blanke et al., 2004), or electrical stimulation (Blanke et al., 2002) at the TPJ, i.e., a brain region that receives strong vestibular inputs (Lopez et al., 2008, 2012b; zu Eulenburg et al., 2012). In addition to out-of-body experiences, electrical stimulation at TPJ also induced vestibular, visual, and kinesthetic hallucinations (Blanke et al., 2002). Vestibular processing and out-of-body experience were linked at the phenomenal level in a different study on healthy individuals. Cheyne and Girard (2009) found that humans suffering from sleep paralysis (i.e., a sleep disorder that is associated with immobility after awakening from sleep) often experienced vestibular-motor hallucinations as well as out-of-body experiences. According to self-report these experiences occurred mostly in supine posture and began mostly with vestibular-motor hallucinations that were followed by out-ofbody experiences.

Out-of-body experiences most frequently occur in supine posture when otolithic vestibular signals are altered with respect to the vertical body axis (Green, 1968), suggesting that otolithic vestibular processing is critical for these changes in bodily selfconsciousness (Lopez et al., 2008). Together, these reviewed data suggest that altered vestibular processing at temporoparietal cortex is associated with disturbances in bodily self-consciousness during out-of-body experiences.

## **Experimental data**

Similar changes in bodily self-consciousness can be studied in healthy humans using different body illusions, such as the bodyswap illusion (Petkova and Ehrsson, 2008), the out-of-body illusion (Ehrsson, 2007) or the full-body illusion (Lenggenhager et al., 2007). During a classic version of the full-body illusion (Lenggenhager et al., 2007) a participant views (from a thirdperson viewpoint) a virtual body being stroked at the back, i.e., visual stroking, and simultaneously feels stroking at his or her physical body, i.e., tactile stroking. Importantly, the visual stroking of the virtual body and the tactile stroking at participant's physical body are spatially separated. Synchronous visuotactile stroking typically increases self-identification with the virtual body and increases self-location in the direction of the virtual body, when compared with an asynchronous stroking control condition (comprehensive reviews and comparison to similar illusions in Blanke, 2012; Serino et al., 2013).

Using such a full-body illusion setup we recently showed that the subjectively experienced direction of first-person perspective, together with self-location, was altered by directional conflict between otolithic vestibular and visual gravitational signals (Ionta et al., 2011; Pfeiffer et al., 2013). **Figure 2** shows the experimental setup and results. Participants viewed a virtual body from an elevated visuospatial viewpoint, seeing visual gravity in downward direction, and simultaneously lay in supine body posture, receiving otolithic vestibular signals about their body being upward directed relative to gravity. Under these conditions individuals differed in terms of their experienced first-person perspective: upgroup participants experienced an upward-directed first-person perspective and an upward-directed change in self-location during the full-body illusion. In contrast, down-group participants experienced a downward-directed first-person perspective and downward-directed change in self-location. Interestingly, individual differences in first-person perspective and self-location were reflected in changes in neural processing, as revealed by fMRI, in the bilateral TPJ, or more precisely in the posterior superior temporal gyrus (pSTG), a region close to the lesion overlap found in a group of patients with out-of-body experiences, i.e., angular gyrus (Ionta et al., 2011).

Pfeiffer et al. (2013) found at the behavioral level that individual differences in the subjective first-person perspective depended on individual differences in the way individuals weight visual and vestibular information, as assessed by subjective visual vertical judgments (Oltman, 1968). Participants oriented a visual line with respect their subjective vertical. A tilted frame around the line induced a small bias in subjective visual vertical judgments in some of the participants (visual independent group), while inducing larger subjective visual vertical biases in other subjects (visual dependent group). We found that assignment of participants to visual field dependent-independent groups, depending on their performance in the visual vertical judgment task, was predictive of their subjective first-person perspective during the full-body illusion. Specifically, participants from the visual independent group more likely experienced an up-looking first-person perspective during the full-body illusion, meaning that their subjective first-person perspective was congruent with vestibular signals. On the other hand, participants from the visual dependent group were more likely to experience a down-looking first-person perspective during the full-body illusion, meaning that their subjective first-person perspective was in line with visual signals.

Together, these studies support the hypothesis that the vestibular system contributes to whole-body spatial representation underlying bodily self-consciousness (Blanke et al., 2004; Blanke, 2012). One may wonder whether also body-part spatial representations depend on vestibular signals. Indeed, body-part representations are related to whole-body representations (Petkova et al., 2011; Ehrsson, 2012) and several studies observed vestibular effects on touch localization and shape perception of the hand

(Lopez et al., 2010, 2012a,c; Ferre et al., 2011, 2013). However, these studies did not test whether vestibular stimulation also affected spatially integrated whole-body representations that underlie spatial aspects of bodily self-consciousness.

## **PART ONE: CONCLUSION AND OPEN QUESTIONS**

Multisensory conflicts, i.e., between vestibular otolithic and visual gravitational signals in healthy subjects, as well as vestibular hallucinations, i.e., due to functional interference at TPJ in neurological patients, have been associated with changes in bodily self-consciousness, most consistently regarding it's spatial aspects: first-person perspective and self-location. Phenomenal experiences during these illusions included vestibular hallucinations, i.e., illusory reversals of the visuospatial first-person perspective with respect to gravity. Furthermore, ambiguous visual gravitational and vestibular otolithic signals induced changes of both first-person perspective and self-location. These observations suggest a critical role of vestibular cortical processing underlying spatial aspects of bodily self-consciousness.

Yet, very little is known about the functional and neural mechanisms underlying these effects. For instance, the vestibular peripheral system was never been directly stimulated during an out-of-body experience and a full-body illusion. It is thus not well studied how otolithic, semicircular, or both signals together affect spatial aspects of bodily self-consciousness. Furthermore, little is known about how vestibular processing contributes to a volumetric representation of the body, and how this spatial volume is related to representations of the external world. Finally, the vestibular system signals movement of the head and of the body. However, most studies on spatial aspects of bodily selfconsciousness have used static body conditions. We think that these are important research questions for the future.

## **PART TWO: VESTIBULAR CONTRIBUTIONS TO BODILY SELF-RELATED COGNITIVE AND PERCEPTUAL FUNCTION**

The second part of the review summarizes empirical research showing vestibular effects on mental spatial transformation, selfmotion perception, and body representation. These cognitive processes involve spatial representations of the body, the external world, and the relationship between body and external world. We argue therefore that bodily self-related processes closely resemble spatial aspects of bodily self-consciousness, which require volumetric representation of the body with respect to the external world and spatial reference frames.

## **MENTAL SPATIAL TRANSFORMATION**

Mental spatial representations are an important aspect of selfconscious experience. For example, the capacity to take the visual perspective of other humans is important for spatial cognition (Maguire et al., 1998), theory-of-mind (Baron-Cohen et al., 1985; Saxe and Kanwisher, 2003; Frith and Frith, 2006) and bodily selfconsciousness (Newen and Vogeley, 2003).

Mental spatial representations have been extensively studied by mental imagery tasks involving objects, body parts, or entire bodies at different locations and orientations in external space (Shepard and Metzler, 1971). Mental imagery of these objects involves mental spatial transformation without participants actually moving their body or the perceived object. Performance in these tasks, i.e., reaction times and error rates, generally depend on the object rotation angle and the shortest path of rotation (Shepard and Metzler, 1971; Parsons, 1987a; Wexler et al., 1998). Mental imagery of body parts or entire bodies additionally depends on anatomical constraints of the physical body (Parsons, 1987b, 1994) and on participant's body posture while performing the mental imagery task (Ionta and Blanke, 2009; Ionta et al., 2013).

A long tradition in cognitive neuroscience has studied egocentric imagery, which is self-centered mental spatial transformation of the own whole body or visuospatial perspective. In egocentric imagery tasks participants judge spatial attributes of objects in their environment from a location or perspective that differs from their actual location or perspective. For example, participants may judge whether a marker is at the left or right side of their imagined body location. Some researchers referred to egocentric imagery in the context of body-part imagery (Zacks and Michelon, 2005) which we argue does not necessarily draw on global representations of the whole body, but rather depends on body-part centered reference frames (Klatzky, 1997; Blanke, 2012). Therefore, we choose to refer to egocentric imagery for imagined own whole-body or perspective transformations. Egocentric imagery is typically compared to allocentric imagery, which is imagining transformations of objects in external space in order to judge their spatial attributes. Several studies have shown that egocentric vs. allocentric imagery depend on distinct functional neural activations (Mast et al., 1999; Wraga et al., 2005). For instance, egocentric, but not allocentric, imagery exhibits brain activity at the TPJ (Arzy et al., 2006) the same brain region involved in spatial aspects of bodily selfconsciousness, in out-of-body experience (Blanke et al., 2002, 2004; Blanke and Mohr, 2005) and in full-body illusions (Ionta et al., 2011).

While most previous research comparing egocentric with allocentric imagery focused on visual, motor, and proprioceptive contributions, more recent studies have shown very specific contributions of vestibular signals to egocentric mental spatial transformation. For instance, Grabherr et al. (2011) compared mental imagery in patients with vestibular loss (i.e., peripheral vestibular damage) with performance of healthy individuals (i.e., intact peripheral vestibular system). These authors found that bilateral vestibular impairment affected egocentric imagery when compared with unilateral loss or intact vestibular processing. Vestibular damage vs. intact vestibular processing did not affect allocentric imagery, thus highlighting the relevance of peripheral vestibular signals (intact or semi-intact) in egocentric imagery. Notably, egocentric imagery is known to rely on cortical activation of the TPJ (see above).

Likewise, highly specific effects of vestibular processing on egocentric imagery were found by Lenggenhager et al. (2008). Healthy participants received vestibular stimulation by left/right anodal GVS while viewing left/right rotated bodily or nonbody object. Egocentric imagery was facilitated by side-congruent vestibular-visual stimulation, but only if participants viewed bodily objects. GVS had no effect on allocentric imagery and did not influence mental imagery of non-body objects. These results not only show vestibular modulation of egocentric imagery, but also vestibular processing specifically affecting body-related mental transformations for multisensory congruent directions. These results are congruent with clinical observations linking vestibular, visual, and kinesthetic processing at the TPJ and with changes of spatial aspects of bodily self-consciousness during out-of-body experience (Blanke et al., 2002).

While Grabherr et al. (2011) and Lenggenhager et al. (2008) studied the effects of vestibular damage and artificial stimulation on mental imagery, van Elk and Blanke (2014) used natural vestibular stimulation and found comparable results. Passive whole-body yaw rotations (activating the horizontal semicircular canals) facilitated egocentric body-related mental imagery if actual rotations and shortest paths of mental rotation were sidecongruent. While general bilateral vestibular loss in the study by Grabherr et al. (2011), and GVS in the study by Lenggenhager et al. (2008), involved altered vestibular signals from both otoliths and semicircular canals, the study by van Elk and Blanke (2014) showed that selective stimulation of the semicircular canal signals affected egocentric mental imagery.

These data indicate that mental spatial transformation depends on vestibular signals. Vestibular processing enhances egocentric imagery when related to a visually seen bodily object. Vestibular signals from semicircular canals and otolith organs facilitate mental imagery in a spatial direction specific fashion.

Given that egocentric mental imagery draws on similar spatial representations and neural processing as spatial aspects of bodily self-consciousness, then it is likely that vestibular signals from semicircular canals contribute to spatial aspects of bodily self-consciousness and that they are processed at TPJ. To our knowledge, this hypothesis has not been studied directly. Instead, previous work on spatial aspects of bodily self-consciousness studied the effects of otolithic vestibular signals and representations of the static gravitational field.

Egocentric imagery recruits functional neural activation at the TPJ, suggesting that egocentric imagery engages similar representations, as do spatial aspects of bodily self-consciousness. Indeed, the strategy during egocentric imagery involves mental spatial displacement of one's own body or perspective to a location in external space, whose analogous physical movements would activate otolithic and semicircular canals respectively. The effects found for rotational-direction specific contributions of vestibular signals to egocentric imagery suggest that cortical processing of semicircular canal signals may contribute to spatial aspects of bodily self-consciousness. Finally, vestibular signals facilitates egocentric imagery when viewing a human body shape, suggesting that egocentric imagery and spatial aspects of bodily selfconsciousness are highly tuned to visual representations of the human body.

## **SELF-MOTION PERCEPTION**

Most everyday activities imply bodily movement in the environment. Planning and controlling these actions require accurate self-motion perception with respect to the environment and for this the brain must be able to monitor body movements based on multisensory signals. Furthermore, self-motion perception is important for balance, walking, and tracking the motion of objects under the influence of gravity. Research has shown that self-motion perception depends on integrating redundant sensory signals about body movement from vestibular, visual, proprioceptive, auditory and kinesthetic signals. Although vestibular signals alone indicate head posture and movement with respect to the environment, they are poor at sensing very slow movements (Kolev et al., 1996) and prolonged constant-velocity movements (Brandt et al., 1998). Similarly, the otoliths cannot distinguish between linear acceleration from head motion and constant gravitational acceleration (Einstein, 1907). Research on self-motion perception studied therefore multisensory integration mechanisms, i.e., most extensively visual-vestibular integration, in non-human primates (Andersen et al., 2000; Bremmer et al., 2002; Gu et al., 2007; Bremmer, 2011). These studies found that in the non-human primate brain the medial temporal region and dorsal MST region integrate optokinetic stimuli and vestibular signals about head rotation and heading direction. Another area integrating vestibular, visual, and somatosensory signals relevant for self-motion perception is VIP (Bremmer et al., 1999; Chen et al., 2013a). Neuroimaging in humans found comparable activation for visual-vestibular integration for self-motion in posterior parietal, parietooccipital, and medial temporal regions (Brandt et al., 1998; Kleinschmidt et al., 2002; Kovács et al., 2008; Becker-Bense et al., 2012).

While these studies showed that self-motion perception depends on an optimal comparison of dynamic multisensory stimuli, including vestibular signals about bodily movement, more recent studies have shown that also constant gravitational acceleration signals are important for self-motion perception. For instance, De Saedeleer et al. (2013) found that under normal terrestrial conditions (with constant gravitational acceleration acting in the downward direction), the velocity of perceived self-motion depends on the spatial direction of visual implied motion, and that self-motion velocity perception shows an asymmetric pattern for upward vs. downward, but not for leftward vs. rightward motion. Specifically, visual self-motion is experienced as slower when directed upwards (opposite to the downward direction of gravitational acceleration) than when directed downward (in the same direction as gravitational acceleration). In microgravity, when no otolithic vestibular signals are present, this upwarddownward asymmetry is abolished. Interestingly, the transition between asymmetric to symmetric perceptual bias is delayed by several days when astronauts in microgravity are presented with tactile cues that mimic foot sole pressure, as if they were standing upright in a gravitational field. These results suggest that constant gravitational acceleration, but also multisensory cues, affect selfmotion perception.

Neural correlates of self-motion perception as related to the gravitational field have been studied by Indovina et al. (2013). During fMRI, these authors presented visual self-motion cues in a virtual rollercoaster. For motion in the vertical, but not in the horizontal, direction the PIVC region was activated—a key region in the cortex receiving vestibular inputs. The activation depended on motion acceleration constant and showed strongest activation for direction-acceleration congruent motion at earthgravity constant 9.81 m/s<sup>2</sup> .

Several studies from the same research group have previously shown that an internal model of gravity is recruited for visual motion perception. An internal model of gravity during these tasks recruited activation at of PIVC region, which was similarly activated by peripheral vestibular stimulation (McIntyre et al., 2001; Indovina et al., 2005). More recently, Maffei et al. (2010) found that visual object motion with a gravitational acceleration profile activated insula cortex and inferior parietal cortex. Both visually seen motion and unseen apparent motion cues similarly activated these regions. Activations were stronger when these signals were behaviorally relevant during an object interception task as compared to passive observation.

These recent studies in human subjects showed that selfmotion perception is not only based on dynamic signals about body movement, but also on vestibular signal about the static gravitational field. Behavioral responses and functional neuroimaging suggest that the brain accounts for the effects of gravity on self- and environmental object motion by using an internal model of gravity that was found to overlap with cortical processing of vestibular signals in the PIVC region (Indovina et al., 2005, 2013)—a key region for vestibular input to the cortex (see Section Part Three: Vestibular Cortex and Spatial Aspects of Bodily Self-Consciousness of this review). Together, these findings suggest that vestibular signals about movement and position of the head are critical for self-motion perception, which draws on spatially representing one's own-body movements with respect to the external environment.

Experiments on self-motion perception have extensively inquired about participants' subjective experience of whether or not, and in which direction, they experienced to be moving. These are self-related perceptual judgments that are likely based on multisensory spatial representations of the bodily self ("I") and the external world. Thus, self-motion perception likely draws on similar neural representations underlying the spatial aspects of bodily self-consciousness, i.e., self-location and first-person perspective. It is important for the brain to spatially update selflocation and first-person perspective while the body is in motion, and to withhold from spatial update when there is motion in the environment. However, research on bodily self-consciousness has mostly studied static body conditions and thus to date the exact relationship between functional and neural representations of self-motion perception and spatial aspects of bodily selfconsciousness is not well understood.

## **BODY REPRESENTATION**

Spatial aspects of bodily self-consciousness include a volumetric spatial representation of the body. Yet, no single sensory modality in isolation encodes such volumetric body representation. Instead, the brain integrates multisensory, body-related signals from the somatosensory, proprioceptive, visual, and, as it has been shown more recently, the vestibular system.

Longo and Haggard (2010) developed a task to assess perception of hand shape. They found that hand shape judgments were deformed in a manner partially resembling the cortical representation of the hand in primary somatosensory cortex. Using a similar task, Lopez et al. (2012c) studied the effect of vestibular stimulation by CVS on body representation and found that hand size judgments were generally enlarged by vestibular stimulation. A different study by Ferre et al. (2013) applied vestibular stimulation by GVS during a homologous task and found that finger representations were enlarged while hand dorsum was shrunk by vestibular stimulation. The specific differences between the results in these studies, i.e., enlargement or shrinkage of hand shape judgments, may reflect differences in the spatial directionality of the vestibular signals applied. Specifically, vestibular stimulation by CVS mostly activates the horizontal canals that encode yaw rotation, whereas GVS activates mostly the vertical canals (i.e., anterior and posterior canals) that encode roll and pitch rotation (Lopez et al., 2012b). Alternatively, these results may be based on additional factors to the stimulation technique utilized; for instance, sensory coactivation of thermal and nociceptive sensory signals. Despite differences between studies, both findings show that vestibular stimulation deforms hand shape representation. Thus, in addition to visual, somatosensory and proprioceptive signals (Serino and Haggard, 2010), the brain also integrates vestibular signals in order to determine the volumetric representation of the body.

Vestibular stimulation temporarily altered participant's perception of the internal spatial configuration of the hand in the studies by Lopez et al.(2012c) and Ferre et al.(2013). These results differ from experienced changes of hand location during the rubber hand illusion (Botvinick and Cohen, 1998). Specifically, participants experience their own hand at a location different from their physical hand, but do not experience changes of hand shape. It seems that vestibular signals differently contribute to human position sense of implicit hand representations and overall hand location in external space. Two studies provide indirect support for this idea by showing that vestibular stimulation during the rubber hand illusion did not affect proprioceptive drift (Lopez et al., 2010, 2012a).

Generally, adult physical bodies undergo little change of shape over time, but vestibular stimulation immediately affected the internal representation of the hand shape. This suggests that highly plastic mechanisms underlie volumetric representations of the body. Such representations may be critical for spatial aspects of bodily self-consciousness, which can be manipulated rapidly during full-body illusions.

## **PART TWO: CONCLUSION**

We reviewed data showing that vestibular signals from otolith organs and semicircular canals, as well as internal models of gravity, contribute to cognitive, sensorimotor, and perceptual functions. These self-related functions depend on vestibular processing at the TPJ, the intraparietal sulcus, the parietal-occipital and the medial temporal cortices. Because the TPJ also encodes spatial aspects of bodily self-consciousness, it is likely that vestibular processing at the TPJ is involved in both self-related processes and spatial aspects of bodily self-consciousness.

Vestibular signals are special sensory signals because the peripheral vestibular organs are fixed with respect to the head and therefore signal head movement relative to the external environment. Vestibular signals are thus likely to contribute in locating and updating location during movement of the body in the external world. However, vestibular signals alone are not sufficient, as they are signaling head position, but not the position of other body parts with respect to the external world. A multisensory integrated global representation of the whole body is necessary for bodily self-consciousness and thus vestibular signals need to be integrated with other spatially informative multisensory signals from the body. A full body representation can be achieved only by integrating multisensory body-related signals within a unique body-centered reference frame. Together vestibular world-related signals, when integrated with multisensory bodily signals, can provide a representation of the volumetric spatial body and its momentary position and orientation in space. Such representation of the whole body in space must be dynamically updated as the body and its parts continuously move. In this function, the vestibular signals are important to signal self-motion and thus to update spatial aspects of bodily self-consciousness with respect to the environment.

We think that for these functions, i.e., the spatial relationship between external world and a global full-body representation, and the update of the body-environment relationship in motion, vestibular processing in posterior brain regions is critical. In the final part of the present review we will present evidence supporting this view.

## **PART THREE: VESTIBULAR CORTEX AND SPATIAL ASPECTS OF BODILY SELF-CONSCIOUSNESS**

What are the neural correlates of vestibular processing contributing to bodily self-consciousness? Empirical data shows that in the right hemisphere posterior cortical regions process both vestibular signals and spatial aspects of bodily self-consciousness (Dieterich et al., 2003; Blanke and Mohr, 2005; Ionta et al., 2011). In the third part of this review we summarize the functional characteristics of three important posterior vestibular cortex regions, i.e., PIVC, MST, and VIP, and a region causally involved in bodily self-consciousness, i.e., TPJ, which together may encode selflocation and first-person perspective.

## **PIVC**

It is commonly accepted that PIVC is a key region of vestibular input into the animal cortex (Grüsser et al., 1990a,b). This area also receives somatosensory and proprioceptive inputs (Lopez and Blanke, 2011). There is no consensus about the exact location and function of the PIVC in the human cortex. Different authors localized PIVC in the posterior insular and retroinsular cortex (Fasold et al., 2002; Indovina et al., 2005; Lopez et al., 2012b), in the parietal operculum (zu Eulenburg et al., 2012) and in different regions in the TPJ (Bense et al., 2001; Deutschländer et al., 2002; Lopez et al., 2012b). The available functional neuroimaging data in humans show that PIVC encodes vestibular signals from artificial stimulations by GVS and CVS (Fasold et al., 2002; Lopez et al., 2012b), proprioceptive signals from the neck (Fasold et al., 2008), and also visual signals (Brandt et al., 1998; Bense et al., 2001; Brandt et al., 2002; Deutschländer et al., 2002; Indovina et al., 2005, 2013). Although from non-human primate electrophysiology there is evidence for visual processing in PIVC (Grüsser et al., 1990a) there are also reports of no visual encoding in this region (Chen et al., 2010). Brandt et al. (1998) proposed that human PIVC and parietal occipital region encode visual and vestibular signals related to self-motion by a reciprocal visual vestibular inhibition mechanism. Specifically, these authors proposed that vestibular input activates PIVC and simultaneously deactivates parietooccipital region. Optokinetic stimulation, on the other hand, would activate parietooccipital region and simultaneously deactivate PIVC. Accordingly, the dynamic interaction between activation and inhibition from PIVC to parietooccipital region and *vice versa* would allow for determining self-motion. The PIVC projects to all other vestibular cortex regions, which is why some authors have discussed PIVC as the main vestibular input region to the human cortex (zu Eulenburg et al., 2012).

What could be the role of PIVC in encoding the spatial aspects of bodily self-consciousness? Because PIVC can be considered a subregion of the TPJ (see **Figure 3**), on top of the evidence for PIVC as a major input area of vestibular signals into the cortex, in addition to PIVC's strong connection to pSTG region, the PIVC seems to be critical in encoding vestibular signals contributing to self-location and first-person perspective. During experimentally induced changes in self-location and first-person perspective, vestibular otolithic signals play a critical role (Ionta et al., 2011) and these otolithic inputs as well as internal models of gravity have been reported to be encoded by PIVC and immediately neighboring regions (Indovina et al., 2005). It is thus likely that PIVC encodes body orientation and motion in the gravitational field and that these signals interact with neural processing regions at the TPJ coding for spatial aspects of bodily self-consciousness. Determining a clear functional and anatomical localization of PIVC in humans and its distinction from other neighboring regions involved in bodily self-consciousness will be an important goal for future research.

## **MST**

In non-human primates, the dorsal MST region is located in the extrastriate cortex. It processes visual optic flow stimuli, in addition to vestibular signals from body translation and rotation (Bremmer et al., 2002; Gu et al., 2007). Recent models proposed that MST neurons process the perceptual decision about self-motion by integrating visual and vestibular cues according to a Bayesian optimal integration model (Tanaka et al., 1986; Duffy and Wurtz, 1991; Gu et al., 2008; Fetsch et al., 2013; Chen et al., 2013a). While in primates next to MST also VIP neurons process optic flow, both regions are different in terms of their reference frame encoding such vestibular signals. While VIP encodes vestibular signals in bodyand world-centered coordinates, MST encodes vestibular signals in eye-centered coordinates (Chen et al., 2013a,b). These data suggest that in primates, MST is a critical region of visuovestibular integration and self-motion perception. Due to morphological changes of the cortical structures between nonhuman primates and humans, the exact human homologue

multisensory signals and computes reference frames transformation to common body and world-centered spatial reference frames; MST integrate vestibular and visual signals necessary for self-motion perception. Area in

creative commons.)

in out-of-body experience and full-body illusion, and also the vestibular cortex region PIVC is part of the TPJ. (Image is a derivative of work licensed under

of MST (in terms of functional properties) is not precisely located in the human, however, functional neuroimaging studies have shown optic flow induced activity in the parietooccipital region (Brandt et al., 1998, 2002; Deutschländer et al., 2002). It is likely that the human homologue of MST is contributing to spatial aspects of bodily self-consciousness during self-motion by integrating visuovestibular signals. Therefore, vestibular processing in MST may play an important role in updating self-location and first-person while the body is in motion.

## **VIP**

VIP is a critical region for multisensory spatial coding. First of all, several findings both in humans and animals show that VIP processes visual, tactile, proprioceptive, and auditory stimuli (Duhamel et al., 1997, 1998; Bremmer et al., 2001; Avillac et al., 2005; Schlack et al., 2005; Sereno and Huang, 2006; Huang et al., 2012). A main function of VIP neurons is to integrate spatial information from different sensory modalities, which initially encode space in peripheral sensory system centered coordinates (e.g., visual stimuli in retinotopic coordinates; auditory stimuli in head coordinates; somatosensory stimuli in somatotopic coordinates) into common body-centered reference frames. Most neurons in area VIP respond selectively to visual stimuli presented close the animal's body. Indeed, about half of VIP neurons respond best to visual stimuli within 30 cm of the body, and many neurons respond only within a few centimeters range (Colby et al., 1993). However, more distant space is also represented in VIP, since some neurons have visual receptive fields that are not confined in depth. In most neurons in VIP visual stimuli are encoded in body-part centered reference frames (typically centered at the head), some neurons are encoded in visual (retinal) reference frames, and some neurons have mixed reference frames (Avillac et al., 2005). Therefore, most VIP neurons preferentially represent the space near the body, in body-centered reference frames (Colby et al., 1993; Bremmer et al., 2002; Schlack et al., 2005). Although some neurons in VIP also encode visual-based representations of extrapersonal space, these extrapersonal space representations and the body-centered spatial representations are implemented in rather distinct neural populations within VIP (Colby et al., 1993), which supports the idea of distinct representations for near and far space, rather than a continuous representation from near to far space.

Interestingly, VIP also receives vestibular input. For instance, linear translations of the body, that are signaled by the otoliths, are encoded in VIP in body- or world-centered reference frames (Chen et al., 2013a). VIP may thus integrate vestibular with multisensory signals to compute spatial representations of the whole body—which are an important aspect of self-location (Blanke and Metzinger, 2009; Blanke, 2012; Metzinger, 2013). For all these reasons, computational models have proposed that VIP plays a critical role in coordinates transformation (Pouget et al., 2002; Avillac et al., 2005) and suggest that this region, together with other portions of the posterior parietal cortex plays a key role in remapping multisensory body-related signals into a common, whole-body centered, reference frames. Such computation is necessary to build a multisensory representation

of the body in space, which is critical for spatial aspects of bodily self-consciousness.

## **TPJ**

The TPJ can be defined as a larger region including the pSTG, angular gyrus, supramarginal gyrus, and the parietal operculum (**Figure 3**, gray region). The TPJ receives somatosensory, visual, and vestibular inputs (zu Eulenburg et al., 2012; Bzdok et al., 2013). Note that PIVC is a subregion of the TPJ (Lopez and Blanke, 2011; Lopez et al., 2012b). The TPJ is important for multisensory signal coding (Downar et al., 2000), theory of mind (Saxe and Kanwisher, 2003), and bodily self-consciousness (Blanke, 2012). Several findings presented in the first and second part of this review show that damage or stimulation at the TPJ can induce changes in self-location and first-person perspective (Blanke et al., 2002, 2004; Ionta et al., 2011). In the same vein, changes in self-location and first-person perspective, induced in healthy subject by the full-body illusion, are encoded at the TPJ, and in particular in the pSTG region. Thus, the TPJ seems to be a critical region for encoding spatial aspects of bodily selfconsciousness. We think that vestibular inputs from PIVC, MST, and VIP to the TPJ are critical for that function. In particular, TPJ might integrate inputs from VIP contributing to a global body representation, from MST to update body orientation and direction during movement, from PIVC for the orientation of the body in the gravitational field. When these vestibular inputs are absent or in conflict with other sensory signals, e.g., visual or somatosensory, the brain may generate an inaccurate spatial representation of the bodily self, inducing illusions in healthy participants or disorders of bodily self-consciousness in patients.

## **CONCLUSION**

The vestibular system processes head posture relative to constant gravitational acceleration and head motion in threedimension space, thus providing important information related to the body with respect to the earth gravitational system, which is essential for coding the spatial orientation of the body in the external world. By reviewing recent data about bodily illusions, mental spatial representations, self-motion perception, and body representation, we argue that vestibular information is integrated with other sensory modalities to underlie bodily self-consciousness. Visual-vestibular interactions and internal models of gravity are processed at the TPJ, contributing to self-location and first-person perspective. We propose that this information depends on neural processing in the posterior cortical areas, which integrates and computes multisensory signals to build body representations in global whole-body centered reference frames and therefore contributes to stable representations of the bodily self. Integration of vestibular signals in PIVC, MST, and VIP, and further processing at the TPJ might be critical for the experience of the self as placed within a body, which occupies a specific location of space and faces the world from the first-person perspective. Vestibular processing may thus serve as a spatial reference for these spatial determinants of bodily self-consciousness.

## **ACKNOWLEDGMENTS**

We would like to thank Jean-Paul Noel for his help with the manuscript. Christian Pfeiffer and Olaf Blanke are supported by grants to Olaf Blanke from the Swiss National Science Foundation (SINERGIA CRSII1-125135), the European Science Foundation (FP7 project VERE) and the Bertarelli Foundation. Andrea Serino is supported by the Volkswagen Stiftung (the UnBoundBody project, ref. 85 639), by the University of Bologna (RFO) and by the Bertarelli Foundation. The funders had no role in the decision to publish or in the preparation of the manuscript.

## **REFERENCES**

**Conflict of Interest Statement**: The authors declare that this research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Received: 29 November 2013; paper pending published: 19 February 2014; accepted: 20 March 2014; published online: 17 April 2014*.

*Citation: Pfeiffer C, Serino A and Blanke O (2014) The vestibular system: a spatial reference for bodily self-consciousness. Front. Integr. Neurosci. 8:31. doi: 10.3389/fnint.2014.00031*

*This article was submitted to the journal Frontiers in Integrative Neuroscience*.

*Copyright © 2014 Pfeiffer, Serino and Blanke. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms*.

## The relationship between vestibular function and topographical memory in older adults

#### **Fred H. Previc <sup>1</sup>\*, Wesley W. Krueger <sup>2</sup> , Ruth A. Ross <sup>1</sup> , Michael A. Roman<sup>1</sup> and Gregg Siegel <sup>1</sup>**

<sup>1</sup> Biomedical Development Corporation, San Antonio,TX, USA

<sup>2</sup> Ear Institute of Texas, San Antonio, TX, USA

#### **Edited by:**

Stephane Besnard, INSERM U1075, France

#### **Reviewed by:**

Jocelyne Ventre-Dominey, INSERM, France Marie-Laure Machado, U 1075 COMETE UCBN/INSERM, France

#### **\*Correspondence:**

Fred H. Previc, Biomedical Development Corporation, 620 East Dewey Place, San Antonio, TX 78212, USA e-mail<EMAIL_ADDRESS>
Research during the past two decades has demonstrated an important role of the vestibular system in topographical orientation and memory and the network of neural structures associated with them. Almost all of the supporting data have come from animal or human clinical studies, however. The purpose of the present study was to investigate the link between vestibular function and topographical memory in normal elderly humans. Twenty-five participants aged 70 to 85 years who scored from mildly impaired to normal on the Montreal Cognitive Assessment (MoCA) received three topographical memory tests: the Camden Topographical Recognition Memory Test (CTMRT), a computerized topographical mental rotation test (TMRT), and a virtual pond maze (VPM). They also received six vestibular or oculomotor tests: optokinetic nystagmus (OKN), visual pursuit (VP), actively generated vestibulo-ocular reflex (VOR), the sensory orientation test (SOT) for posture, and two measures of rotational memory (error in degrees, or RM◦ , and correct directional recognition, or RM→). The only significant bivariate correlations were among the three vestibular measures primarily assessing horizontal canal function (VOR, RM◦ , and RM→). A multiple regression analysis showed significant relationships between vestibular and demographic predictors and both the TMRT (R = 0.78) and VPM (R = 0.66) measures. The significant relationship between the vestibular and topographical memory measures supports the theory that vestibular loss may contribute to topographical memory impairment in the elderly.

**Keywords: vestibular, topographical memory, hippocampus, Alzheimer disease, elderly**

## **INTRODUCTION**

The topographical orientation system, also known as the spatial navigation, topokinetic, and action-extrapersonal systems, is one of the four major networks in the brain governing our interaction with our 3D environment (Previc, 1998). It is believed to be comprised mainly of three posterior regions—the hippocampus, posterior cingulate, and parietal-temporal cortex (Berthoz, 1997; Previc, 1998)—and frontal-striatal structures such as the caudate nucleus (Maguire et al., 1998). It is the system that is responsible for scene and route memory, presence in the world, and topographical orientation in the plane of the Earth's surface (Previc, 1998).

The most widely studied of the topographical regions is the hippocampus. Activation of the hippocampus occurs during recall of topographical routes (Maguire et al., 1997), and damage to it results in a profound amnesia for spatial landmarks, loss of spatial maps, and severe topographical disorientation (Aguirre and D'Esposito, 1999). In maintaining a cognitive map of the environment, the hippocampus integrates inputs from various sensory modalities, the two most important of these being distal visual inputs representing large regions of mainly the upper visual field (Previc, 1998; Arcaro et al., 2009) and those emanating from the vestibular system. In recent decades, a large literature has emerged concerning the role of vestibular inputs to the hippocampus (see Smith, 1997, for an early review). Aside from a few studies with humans (e.g., Vitte et al., 1996; Brandt et al., 2005), the evidence for a major role of the vestibular system in hippocampal function has come from animal studies involving vestibular stimulation or lesions (e.g., Ossenkopp and Hargreaves, 1993; Horii et al., 1994; Sharp et al., 1995; Russell et al., 2006; Tai et al., 2012). The primary labyrinthine inputs to the hippocampus and the topographical memory system in general are from the lateral/horizontal semicircular canals (Taube et al., 1996), which signal angular rotation of the head in the plane of the Earth's surface (i.e., the domain of the topographical as opposed to gravitational orientation system). The hippocampus also receives inputs from the utricle (Cuthbert et al., 2000), which signals linear acceleration in the plane of the Earth's surface. The importance of the vestibular inputs to the hippocampus may be due to the fact that the head is the anchor for the topographical memory system (Previc, 1998)—hence, the reference to it ("heading") when describing movements in topographical space.

Despite the above links, there is no clear evidence that vestibular function is related to topographical memory in normal humans. The purpose of this study was to assess the relationship between various measures of vestibular and related oculomotor function and topographical memory in a sample comprised of cognitively and physically "normal" elderly participants (70–85 years). This age group was selected because of the recent hypothesis that declining vestibular function in the elderly may be associated with the onset of Alzheimer's disease (Previc, 2013), which is heralded by a loss of topographical orientation and memory and atrophy and/or metabolic deactivation of key components of the topographical neural network (Huang et al., 2002; Johnson et al., 2005; Berti et al., 2010; Pengas et al., 2012; Lithfous et al., 2013). Because it has proven difficult to test individuals with dementia to determine the presence of vestibular loss, the present study tested ostensibly "normal" older individuals to determine any relationship between subtle topographical memory loss and vestibular function.

To test topographical memory, three tests were used: the Camden Topographical Recognition Memory Test (CTRMT), a computerized topographical mental rotation test (TMRT) similar to the "four mountains" test used by Bird et al. (2010) and Hartley and Harlow (2012), and a virtual version of the widely used Morris water maze used in animals (Sharma et al., 2010) and humans (e.g., Moffat and Resnick, 2002; Brandt et al., 2005). While the CTRMT has not been specifically linked to hippocampal function, scene memory in general has been linked to the hippocampus (Epstein and Kanwisher, 1998; Arcaro et al., 2009; Bonnici et al., 2012). Memory for scenes requiring topographical mental rotation is also dependent on the hippocampus (Hartley and Harlow, 2012), and the critical role of the hippocampus in the water maze has been repeatedly shown in both animals (Sharma et al., 2010) and humans (Brandt et al., 2005). The standard vestibular and oculomotor tests used in this study, some of which have been linked to cognitive impairment in Alzheimer's disease (Previc, 2013), included visual pursuit (VP), optokinetic nystagmus (OKN), the actively generated vestibuloocular reflex (VOR), the sensory orientation test for postural control, and two measures of rotational memory—perceived rotation error in degrees and correctly perceived direction of rotation.

The relationships among the various topographical and vestibular measures were explored using a combination of correlational and multiple regression analyses.

## **METHOD**

## **PARTICIPANTS**

A total of 25 individuals between the ages of 70 and 85 (*M* = 76.37, *SD* = 4.47) participated in this study. The sample consisted of 16 females and 9 males. Nine of the 25 participants were of Hispanic origin and 21 were Caucasian, with 3 Asian-Americans and 1 designated "other". In terms of education, there were 7 participants with high school degrees, 6 with undergraduate degrees, and 12 with graduate degrees. All participants were prescreened for dementia, with anyone scoring less than 19 on the Montreal Cognitive Assessment (MoCA) excluded from the study. The MoCA scores ranged from 19 to 28 (*M* = 24.88, *SD* = 2.42).

All participants were prescreened for any previous balance or vestibular problems, and only two had any previous vestibular problems, both involving single episodes of dizziness or vertigo many years previously (These two participants each scored within one standard deviation of the mean or above on five of six "vestibular" tests, with one scoring low on VP gain and the other scoring low on the postural test—see later descriptions). Although no tests of visual function were performed, no participant reported difficulty in seeing any of the stimuli during the various tests. Anyone with neck surgery or hip replacement surgery was excluded, although knee replacements were permitted if they were not within the past year and were not causing the person any pain. Other exclusionary criteria included a history of brain trauma (including stroke, seizures, or traumatic brain injury), a diagnosed neurological condition, or current use of psychoactive drugs. Caffeine use not exceeding three cups of coffee or its equivalent per day, moderate social consumption of alcohol, and use of nonnarcotic pain medication was acceptable for inclusion into the study.

## **TESTING SITES AND EQUIPMENT**

All spatial memory testing was performed in the neuropsychology office of Dr. Michael Roman in San Antonio, TX and required approximately 90 min. Two of the tests created for this study were computerized versions of previously used tests the virtual pond maze (VPM), which tested spatial navigation and memory, and the topographical mental rotation test, which required participants to remember the original view of a scene and then rotate it mentally. Both of these tests were executed using a Hewlett-Packard Compaq NC6400 computer under moderately dim illumination with the participant seated in a quiet room. The third spatial memory test was the CTRMT, which was administered from a test booklet by the neuropsychologist sitting directly across from the participant in a well-lit room.

The "vestibular" tests were conducted at the Ear Institute of Texas clinic in San Antonio, TX and required slightly less than one hour on average to complete. The OKN, VOR, and VP tasks were all administered using the Vorteq system (Micromedical, Chatham, IL). This system contains an inertial head-tracker for measuring the motion of the head in space and an infrared camera for measuring the position of each eye in the orbit. These tests were all conducted in darkness, aside from illumination of the stimuli, and were preceded by a calibration of the system using the fixation stimulus positioned at five positions on the screen (center and 21.8◦ right, left, up and down, at a viewing distance of 160 cm). The Sensory Orientation Test (SOT) tested balance under various sensory conditions and made use of the Equitest system (Neurocom, Portland, OR). Finally, the rotational memory test was administered on a barber-style chair locked in place on a rotating industrial turntable whose motion was controlled electronically (Carousel, Monrovia, CA). The turntable had an accuracy of ∼0.5◦ for the profiles and velocities and displacements used in this study (25◦ /s; up to 40◦ left or right). When seated in the chair, which was centered on the turntable, the participant was situated at the approximate radius from a large white circular Mylar half-screen (diameter = 155 cm) containing a scale that measured laser-pointing accuracy to 0.5◦ .

## **TESTS**

## **Camden Topographical Recognition Memory Test (CTRMT)**

This task required participants to view photographs of 30 urban scenes, presented according to the standard instructions (Warrington, 1996) in which participants viewed each scene for up to 3 s and had to determine if the photograph was shot by an amateur or a professional. Immediately after the completion of the presentation sequence, participants were shown each of the same 30 scenes in a different order along with two similar versions of it shot from different perspectives or poses. In this threealternative self-paced forced-choice recognition task, participants were required to point to the scene that was actually presented and to avoid guessing. Each score was based on the total correct out of 30.

## **Topographical Mental Rotation Test (TMRT)**

Participants viewed a scene containing a set of three objects (e.g., red cylinder, blue sphere, green cube) and were then shown a figure instructing them to rotate their viewpoint 90◦ left, 90◦ right, or 180◦ opposite (see **Figure 1**, left panel). They were then shown three scenes from each of the viewpoints and asked to use the mouse to click on the image that depicted the correct viewpoint shift. A yellow box indicated their choice while a green box (only presented during practice trials) showed the correct viewpoint. Participants had 12 s to view each scene and 14 s to make their response, with a timer appearing during the last 5 s of the forced-choice interval. After being presented with instructions, which included demonstrations with actual objects, participants viewed a set of 12 practice trials (they could view a second set if they chose to) and then were presented with 15 test trials. Each score was based on the total number of correct responses out of 15.

## **Virtual Pond Maze (VPM)**

In the pond maze, participants started from one of six locations around a virtual pond. Each starting location faced a different set of virtual buildings and vegetation to provide distinct spatial landmarks (see **Figure 1**, right panel). Participants were required to navigate using only the left and right cursor arrows to a fixed platform slightly offset from the middle of the pond, with the forward speed set by the software at a simulated 7 m/s. The location and size of the platform was set such that participants had to make at least one correction to reach the platform from each of the starting points. After approximately 20 min of instructions and practice, participants were tested on 18 trials, consisting of one platform-visible trial followed by two platform-nonvisible trials, in which spatial memory was required to reach the platform. Performance on the 6 visible and 12 nonvisible trials was measured by the time taken to reach the platform, the virtual distance traveled, and the number of corrections made. The starting position on each trial varied in a random-without-replacement order, such that all starting positions were sampled twice in the nonvisible conditions and once in the visible conditions. Because the number of corrections involved more strategy than memory and the virtual distance was almost perfectly correlated with time to reach the platform, only the latter was used in the final analysis.

Also, because time taken to reach the platform on the visible trials showed little variation among participants, presumably because forward speed was set by the software, only time to reach the platform on the nonvisible trials was used to assess navigational memory.

## **Visual pursuit**

In the VP task, participants were required to follow a small red square as it moved from left to right. They were instructed to not move their head, and the experimenter additionally held the head in place from behind to prevent it from moving. The small red square had a diameter of 0.5◦ and moved from 21.8◦ left to 21.8◦ right at increasing sinusoidal velocities (0.1–0.4 Hz). Pursuit gain relative to the movement of the square was recorded at the highest frequency (0.4 Hz) and averaged across the left and right eyes.

## **Optokinetic nystagmus**

In the OKN task, participants were required to gaze at an imaginary fixation spot in the center of the screen as columns of dots swept past at a velocity of 30◦ /s, first leftward then rightward. Each dot was 3.96◦ in diameter and each column was comprised of five dots spanning 37◦ vertically, with an inter-column distance of 13.34◦ . Participants were instructed to count the number of times a dot column crossed his or her imaginary fixation spot during a 30-s interval. They were also instructed to not move their head, with the experimenter again holding the head in place from behind to help prevent it from moving. The gain of the slow phase of the OKN relative to the movement of the dots was averaged across both eyes and both directions of dot-field motion.

## **Vestibulo-ocular reflex**

The VOR was measured while participants fixated on the same red square as in the VP task, which remained stationary in the center of the visual field. They actively generated left-right head movements to the sound of a metronome at frequencies beginning at 1 Hz and increasing to 3 Hz (i.e., within the natural range of head movements). Only data when the head was moving at the proper frequency were used for analysis. Because it took some delay for the participant to begin synchronizing the head autorotations to the starting frequency of 1 Hz and because many participants could not easily generate the proper head motions at 3 Hz, only the VOR gains at 2 Hz were used in the final analysis. VOR gain was averaged across two trials and both eyes. Phase of the VOR was also recorded, but it was not analyzed because the reliability was very low (see Results section).

## **Postural control**

Postural tests were conducted using the Equitest's Sensory Orientation Test (see http://resourcesonbalance.com/neurocom/ products/EquiTest.aspx). The SOT provides an "equilibrium score" based on variations in the center-of-pressure under six different postural conditions in which the visual surround is either fixed or moves with the person's postural axis (swayreferenced) or the platform is fixed or moves with the person (sway-referenced). The six conditions involve: (1) visual scene and platform both fixed; (2) eyes-closed with platform fixed; (3) sway-referenced visual scene with platform fixed; (4) visual scene fixed with sway-referenced platform; (5) eyes-closed with sway-referenced platform; (6) sway-referenced visual scene with sway-referenced platform. Equilibrium scores were recorded for two trials in each condition, usually the first and second trials. Occasionally a third trial was run if the participant was outside of normal levels or suffered a fall on the first trial, which was almost entirely restricted to conditions 5 and 6. During each condition, participants were situated in a safety harness while their feet were evenly spaced from the center point and facing forward. Condition 1 is considered the baseline condition (normal standing), whereas Condition 5 relies on vestibular inputs given that visual and somatosensory inputs (the other two major sensory inputs to the postural control system) are eliminated due to the fact that the eyes are closed and the platform moves with the participant's postural sway (Black et al., 1995). Vestibular function was assessed in the standard manner by calculating the ratio of postural sway in Condition 5 to Condition 1.

## **Rotational memory**

In this task, participants were seated in a barber chair that was locked and secured to the rotating turntable underneath. Participants were blindfolded and deprived of auditory inputs by means of ear plugs and ear muffs, which required a reliance on vestibular and, to a lesser extent, somatosensory cues. Participants received a sequence of nine trials in which they were rotated from the same starting point. Four of these trials involved single left or right rotations (to +/−30◦ or +/−40◦ ), four involved combinations of eight left-right rotations (again ending at +/−30◦ or +/−40◦ ), while the middle trial consisted of eight left-right rotations but ended up at the original starting position and served to disguise the pattern of the other eight end-positions. The five combination trials alternated with the single-rotation trials in a fixed order, with the control trial always being the fifth of nine. The nine trials consisted of the following sequences of rotations, in degrees: 1) 40R,20L,30R,20L,10R,40L,40R,30L,40R,20L (net = 30R); 2) 40L; 3) 20L,50R,20L,40R,30L,30R,40L,20R,30L,40R (net = 40R); 4) 30R; 5) 30L,50R,20L,10R,40L,10R,20L,30R,20L,30R (net = 0); 6) 30L; 7) 20R,50L,20R,40L, 30R,30L,40R,20L,30R,40L (net = 40L); 8) 40R; 9) 40L,20R,30L,20R,10L,40R,40L,30R, 40L,20R (net = 30L). On each trial, participants were instructed to point with arms outstretched to their perceived "straight-ahead" on the screen in front of them prior to the rotation, with the experimenter adjusting their arms to align the mark on the screen with the true straight-ahead. Immediately after the cessation of rotation, participants were instructed to aim the laser pointer to the spot on the screen where their original perceived straightahead intersected it, and the offset of the second mark from the first one reflected their rotational memory error, measured in degrees (RM◦ ). After their laser pointer position was measured and recorded and their arms were relaxed, participants were then required to state whether they had ended up at or to the left or right of their original starting position (rotational direction memory (RM→). Participants were provided with some demonstrations and two practice trials to ensure they understood the task.

## **ANALYSIS**

There were a total of 12 variables analyzed in this study. Six of them were independent variables related to vestibular or oculomotor function: VP, OKN, VOR, SOT, RM◦ , and RM→. Three additional demographic variables were included as covariates: age, education level (high school vs. college and above), and gender (male vs. female). The three topographical memory tasks (CTRMT, TMRT, and VPM) served as the dependent variables. In addition to descriptive statistics and bivariate correlations, backward stepwise multiple regressions were performed using SPSS software (IBM, Chicago, IL) for each topographical memory test, using the vestibular and demographic variables as predictors.

## **RESULTS**

The means and standard deviations for the three topographical memory and six vestibular measures are shown in **Table 1**. The descriptive statistics showed a slightly less than perfect gain for the VP and optokinetic responses (0.83 and 0.88, respectively) and a slightly greater than unity gain (1.29) for the VOR, which involved actively generated head movements. On average, 1.6 of the direction judgments in the rotational memory task were incorrect, which is consistent with the fact that the mean rotational pointing error was 19.71◦ (*SD* > 7 ◦ ) relative to the +/−30◦ and +/−40◦ endpoints. The most variable measures across participants were the TMRT and VPM and the SOT and RM◦ , where the standard deviation was a minimum of 30% of the mean. Part of the variability in the TMRT task was due to a very large difference between males (*M* = 12.44, *SD* = 2.3) and females (*M* = 7.8, *SD* = 3.51).

## **BIVARIATE CORRELATIONS**

The bivariate correlation matrix involving the three topographical memory measures and six vestibular measures revealed only 2 of the 36 correlations to be statistically significant. The correlational analysis used the nonparametrc Spearman rank-order statistic, since a Shapiro-Wilk test showed significant violations of normality present for seven of the nine measures.

The highest correlations among the topographical memory measures themselves were between the TMRT and VPM scores (ρ = −0.34) and the CTRMT and VPM scores (ρ = −0.37), but both of these correlations were nonsignificant (The negative

values indicate that as performance increased on the memory tasks, time taken to reach the platform in the maze task decreased).

The correlations among the "vestibular" measures were almost entirely nonsignificant, with only the measures most directly assessing horizontal semicircular canal performance (VOR, RM◦ , and RM→) significantly correlated with each other (ρ = −0.56, *p* < 0.01 for RM→/RM◦ and ρ = 0.53, *p* < 0.01 for RM◦ /VOR). The significant negative correlation between VOR gain and degrees of rotational memory indicates that higher VOR gains were associated with poorer rotational memory performance. The correlations among the vestibular measures and between the vestibular and topographical measures suffered partly because the reliabilities of the vestibular measures themselves were not very high. For those measures where multiple trials were gathered, the reliabilities from one trial to the next (for the VOR and SOT measures) or from the first four trials to the second set of four (for the RM◦ and RM→ measures) ranged from 0.26 and 0.43 for the RM→ and RM◦ measures, respectively, to 0.49 and 0.62 for the VOR gain and SOT measures, respectively. It is impressive, therefore, that some of the *inter-vestibular* measures attained correlation coefficients in the range of the reliability of the individual measures themselves (>0.5).

Finally, there were only two correlations out of 18 between the vestibular measures and the topographical memory measures that exceeded 0.3 (ρ = 36 for RM→/TMRT and ρ = −0.34 for RM→/VPM); both of these were nonsignificant, however.

## **MULTIPLE REGRESSIONS**

The regression models for the TMRT and VPM measures are listed in **Tables 2A** and **2B**. The regression model for the CTMRT measure was excluded because, despite an *R* value of 0.575, it did not prove statistically significant. For the TMRT measure, the significant regression model with the largest number of predictor variables included RM→, gender, VP, SOT, OKN, RM◦ , education level, VOR, and age. The combined *R* was 0.78 (*R* <sup>2</sup> = 0.608), while the largest adjusted *R* 2 (which takes into account the number of variables and sample size) was for the combination of RM→ and gender (*R* <sup>2</sup> = 0.504). The significant regression was based mainly on the fact that poorer rotational memory performance was associated with poorer TMRT scores; gender contributed as well because the TMRT means were so different between men and women. For the VPM measure, the model with the largest number of predictor variables included RM→, SOT, RM◦ , education level, and VOR. The combined *R* was 0.661 (*R* <sup>2</sup> = 0.437), while the largest adjusted *R* <sup>2</sup> was 0.289, for those same five predictors. The regression was based on the negative relationships of RM→, RM◦ , SOT, and educational level with the time to reach the platform and the positive relationship of VOR gain with time to reach the platform (the latter indicating that higher VOR gains were associated with poorer VPM performance).

## **DISCUSSION**

The results of this study reveal that there is a linkage between vestibular function and topographical memory in a normal elderly human population. Because this was a correlational study,

#### **Table 2B | Regression analysis for the Virtual Pond Maze**.

no causal inference can be made about the vestibular role in topographical memory on the basis of these results alone. However, previous evidence from animal and human clinical studies does suggest a causal contribution of vestibular function to topographical memory.

The bivariate correlation matrix revealed modest but nonsignificant relationships among the three topographical measures and much stronger relationships among the three measures most directly linked to horizontal semicircular canal function (VOR, RM◦ , RM→). It is somewhat surprising that the correlations among the three topographical memory measures were not stronger, given that all have been linked to hippocampal/parahippocampal processing. However, it is possible that they tap into different aspects of the topographical memory process. For instance, only the TMRT measure showed a large difference between males and females, as found in other mental rotation tasks (Nazareth et al., 2013) involving the inferior parietal lobe. Also, the Camden topographical memory test involves less active mental transformations than do the pond maze and topographical mental rotation tasks.

It was not surprising that the correlation between the two rotational memory measures was significant, in that the measure of rotational direction occurred after the participant had made his or her mark with the laser pointer to indicate the original starting direction. The high correlation of rotational direction accuracy with VOR gain was more impressive in that it was about the same as the individual reliabilities for the two measures. It is noteworthy that the correlation between VOR gain and rotational accuracy was negative, with higher gains associated with lower accuracy. Unlike the passively generated VOR, actively generated VORs by means of head autorotation tend to have above-unity gains (Hirvonen et al., 1997), and it may be speculated that higher active VOR gains provided by corollary discharge boost performance in those individuals with diminished canal function. It is unlikely that the higher gains in individuals with poorer overall vestibular function were caused by a difference in the amplitude of the head autorotations, since preliminary measurements indicated that the size of the head movement did not affect actively generated VOR gain.

The relationship between the various measures of vestibular function and the topographical memory measures was more complex. On the one hand, no vestibular measures correlated significantly with any of the topographical memory measures. On the other hand, the significant regression models incorporating the greatest number of vestibular and oculomotor predictors and adjusted for gender, age, and education accounted for over 60% of the variance for the TMRT test and over 40% of the variance for the VPM measure. As previously noted, it is likely that the regression predictions would have been even better had they not been limited by the reliability of the vestibular measures themselves. On the other hand, there is a danger in incorporating a large number of variables with a relatively small sample size of 25, although even the adjusted *R* 2 , which takes into account number of variables and sample size, proved impressive for the TMRT measure. It should be noted that the three measures of horizontal semicircular canal function tended to have slightly better predictive power in the regression analyses than did the SOT measure of vestibularly mediated postural control. This is consistent with the fact that the major vestibular inputs to the hippocampus emanate from the semicircular canals, signaling head rotation in the place of the Earth's surface (Taube et al., 1996; Previc, 2013).

 The results of this study demonstrate that vestibular function and topographic memory are clearly related, but it is not clear why. In contrast to evidence that vestibular damage induces behavioral, neuroanatomical, and neurophysiological alterations in the topographical memory system (Ossenkopp and Hargreaves, 1993; Brandt et al., 2005), it has never been shown that hippocampal or neocortical damage seriously disturbs vestibular function; hence, it is more likely that the causal direction flows from the vestibualar system. Because, however, both the vestibular system and the topographical memory system can be affected by general physiological deterioration (e.g., cardiovascular decline, diabetes, traumatic brain injury) (see Previc, 2013), it is also possible that the vestibular-topographical relationship can be explained by an outside factor affecting each separately.

The clinical implications for estimating any vestibular contribution to dementia of the Alzheimer's type (see Previc, 2013) are also unclear. As noted in the Introduction, topographical memory impairment, whether or not caused by vestibular dysfunction, is one of the earliest signs of Alzheimer's disease (Huang et al., 2002; Johnson et al., 2005; Berti et al., 2010; Pengas et al., 2012; Lithfous et al., 2013). By design, none of the participants in this study tested positive for dementia, but several of them showed clear impairments in topographical memory and may be at risk for developing more severe cognitive decline in the future. It cannot presently be ascertained whether a topographical memory and/or vestibular/oculomotor model could be used with diagnostic accuracy to predict who might later develop Alzheimer's. If, however, assessment of vestibular function can at least help identify those individuals at elevated risk for developing Alzheimer's, vestibular therapy, general exercise, and perhaps other vestibular interventions may serve as preventive measures to stave off the later cognitive decline (see Previc, 2013).

Unfortunately, it has been difficult to test Alzheimer's patients for vestibular performance. It can be a challenge to test even many normal elderly participants because of difficulties in moving their hips, knees and neck, keeping their eyelids from obscuring the view of the eye, and a host of other problems. That makes the rotational direction measure even more potentially valuable in this regard. It was the vestibular measure best correlated with the topographic memory measures and also the easiest to administer in that it did not involve any motor response on the part of the participant. This measure should be considered as a standard part of any otolaryngological assessment in the elderly population.

In summary, this study involving normal elderly participants has confirmed previous findings from the animal and human clinical literatures that a relationship between vestibular function and hippocampally mediated topographical memory exists. Future studies are required to better understand this relationship in both normals and those at greatest risk for Alzheimer's disease.

#### **ACKNOWLEDGMENTS**

We wish to acknowledge the assistance of Sam Washburn (computer programming), Victor Espinoza (vestibular testing), Dr. Dora Angelaki (experimental protocol), Karl McCloskey (administrative) and Ben Perez (fabrication). Research reported in this publication was supported by the National Center for Advancing Translational Research (NCATS), the National Institute on Aging (NIA), and the National Heart, Lung and Blood Institute (NHLBI) under Award #TR000645-01. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

## **REFERENCES**

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Received: 11 November 2013; accepted: 15 May 2014; published online: 02 June 2014*.

*Citation: Previc FH, Krueger WW, Ross RA, Roman MA and Siegel G (2014) The relationship between vestibular function and topographical memory in older adults. Front. Integr. Neurosci. 8:46. doi: 10.3389/fnint.2014.00046*

*This article was submitted to the journal Frontiers in Integrative Neuroscience*.

*Copyright © 2014 Previc, Krueger, Ross, Roman and Siegel. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms*.

## From ear to uncertainty: vestibular contributions to cognitive function

## *Paul F. Smith\* and Yiwen Zheng*

*Department Pharmacology and Toxicology, School of Medical Sciences, and the Brain Health Research Centre, University of Otago, Dunedin, New Zealand*

#### *Edited by:*

*Stephane Besnard, INSERM U1075, France*

#### *Reviewed by:*

*Alberto E. Pereda, Albert Einstein College of Medicine of Yeshiva University, USA Stephane Besnard, INSERM U1075, France*

#### *\*Correspondence:*

*Paul F. Smith, Department Pharmacology and Toxicology, School of Medical Sciences, and the Brain Health Research Centre, University of Otago, Frederick St., 9045, Dunedin, New Zealand e-mail: paul.smith@ stonebow.otago.ac.nz*

In addition to the deficits in the vestibulo-ocular and vestibulo-spinal reflexes that occur following vestibular dysfunction, there is substantial evidence that vestibular loss also causes cognitive disorders, some of which may be due to the reflexive deficits and some of which are related to the role that ascending vestibular pathways to the limbic system and neocortex play in spatial orientation. In this review we summarize the evidence that vestibular loss causes cognitive disorders, especially spatial memory deficits, in animals and humans and critically evaluate the evidence that these deficits are not due to hearing loss, problems with motor control, oscillopsia or anxiety and depression. We review the evidence that vestibular lesions affect head direction and place cells as well as the emerging evidence that artificial activation of the vestibular system, using galvanic vestibular stimulation (GVS), can modulate cognitive function.

**Keywords: vestibular, spatial memory, cognition, hippocampus, vestibular lesions**

## **INTRODUCTION**

Recent epidemiological studies have demonstrated that vestibular disorders occur in more than 35% of adults aged 40 or older; between the ages of 60 and 69, the prevalence increases to almost 50% and between 70 and 79, it is 69% (Agrawal et al., 2009). Saber Tehrani et al. (2013) has estimated that, of 3.9 million patients visiting a Hospital Emergency Department for dizziness or vertigo in the USA in 2011, 25.7% were attributable to otological or vestibular causes, costing US \$757 million. Vestibular dysfunction therefore represents a substantial and increasing burden on healthcare systems.

The most obvious effects of poor vestibular function are oscillopsia and ataxia (see Curthoys and Halmagyi, 1995 for a review); however, vestibular dysfunction involves a more complex syndrome characterized not only by reflex deficits, but also by attention and memory deficits, and anxiety disorders (see Smith et al., 2010 for reviews). It is evident that, in addition to the role of the vestibular system in the vestibulo-ocular and vestibulo-spinal reflexes (VORs and VSRs), the vestibular information provided in the ascending pathways to the limbic system and neocortex is required for an accurate internal representation of the relationship between the self and the spatial environment (Angelaki et al., 2009; chen et al., 2013). In the absence of this information, this internal representation becomes inaccurate, ambiguous, and cognitive performance is affected. The aim of this review is to summarize and critically evaluate the current literature relating to the effects of vestibular function on cognition.

## **ANIMAL STUDIES OF THE EFFECTS OF VESTIBULAR LESIONS ON MEMORY**

Many of the early animal studies of spatial navigation suggested that non-visual, idiothetic cues such as vestibular and proprioceptive information, along with external, allocentric cues, were used by animals in order to remember how to navigate their way through a familiar environment (Beritoff, 1965; Potegal et al., 1977; Etienne, 1980; Mittelstaedt and Mittelstaedt, 1980; Horn et al., 1981; Potegal, 1982; Miller et al., 1983; Etienne and Jeffery, 2004). It was speculated that vestibular information must be transmitted to the hippocampus, in order to be integrated with other sensory information relevant to spatial memory (Wiener and Berthoz, 1993; Berthoz, 1996; McNaughton et al., 1996; Etienne and Jeffery, 2004). Ultimately it was reported that place cells in the hippocampus, that respond to specific places in the environment, were modulated by vestibular stimulation (Gavrilov et al., 1995; Wiener et al., 1995), which was supported by numerous animal behavioral studies showing that the disruption of normal vestibular function resulted in spatial memory deficits (Potegal et al., 1977; Horn et al., 1981; Potegal, 1982; Miller et al., 1983; Petrosini, 1984; Mathews et al., 1988, 1989; Semenov and Bures, 1989; Chapuis et al., 1992; Ossenkopp and Hargreaves, 1993; Stackman and Herbert, 2002; Wallace et al., 2002; Russell et al., 2003a; Zheng et al., 2003, 2006, 2007, 2008, 2009a,b, 2012a,b; Baek et al., 2010; Besnard et al., 2012; Machado et al., 2012a,b; Smith et al., 2013).

The early studies were open to the interpretation that what appeared to be spatial memory impairment following vestibular damage might be a direct result of oscillopsia, due to VOR deficits, or ataxia, due to VSR deficits. Such deficits never completely compensate even following a unilateral vestibular lesion (Smith and Curthoys, 1989; Curthoys and Halmagyi, 1995); therefore, this was a reasonable possibility. The first sophisticated study of spatial memory following bilateral vestibular lesions was reported by Wallace et al. (2002), who employed a foraging task, in which rats had to remember their way back to a home base, at 2 weeks post-op. This study, which used chemical lesions of the vestibular labyrinth with intratympanic sodium arsanilate, was well-controlled and used an electronic tracking system to quantify the rats' behavior. They found that rats with bilateral vestibular deaffferentation (BVD) exhibited profound spatial memory deficits in darkness, when visual cues were not available. Over the last decade in particular, many such studies have been conducted at much longer points after the lesion. Some compensation for the vestibular reflex deficits has occurred in the intervening period and yet the spatial memory deficits still persist (Zheng et al., 2006, 2007, 2008, 2009a,b, 2012a,b; Baek et al., 2010). In the study by Baek et al. (2010), which employed the longest post-operative time interval to date, rats that were 14 months post-BVD were more severely impaired in a spatial memory foraging task in darkness than at 5 months post-op. (see **Figures 1**, **2**).

## **EFFECTS OF UVD vs. BVD IN A SPATIAL MEMORY TASK IN DARKNESS**

Especially important is that the spatial memory deficits exhibited by BVD rats were substantially more severe than for animals with unilateral vestibular deafferentation (UVD) (Zheng et al., 2006, 2009b). Rats with UVD showed spatial memory deficits in darkness at 3 months following the lesion, but performed at levels similar to sham controls at 6 months post-op. (Zheng et al., 2006). However, at 5–6 months post-op., while BVD rats had only minimally impaired performance in a foraging task in light, their performance deteriorated substantially in darkness (Zheng et al., 2009b). By 14 months post-op., rats with the same kind of BVD lesions were severely impaired in darkness (Baek et al., 2010; see **Figures 1**, **2**). In this latter study, even treatment with a cannabinoid receptor agonist, which would normally be expected to cause spatial memory impairment, could not increase the severity of the spatial memory deficits (**Figure 2**). These deficits in darkness suggest that oscillopsia is not necessary for the spatial navigation impairment to occur.

## **SPATIAL MEMORY DEFICITS IN LIGHT FOR BVD RATS**

BVD rats have also been demonstrated to exhibit spatial memory deficits in light. At 6 weeks post-op., rats with BVD performed significantly worse than sham controls in a radial arm maze task even in light (Russell et al., 2003a,b). During spatial alternation in a T maze task in light, rats with BVD exhibited some improvement in performance over time, but at 5 months post-op. their percentage of correct responses was still significantly below normal (Zheng et al., 2007). This result was recently replicated by Zheng et al. (2012a,b), using rats at 4–5 months following BVD (see **Figure 3**).

Besnard et al. (2012) and Machado et al. (2012a,b) have recently demonstrated similar spatial memory deficits in light using the radial arm maze and Y maze in rats that had sequential unilateral chemical labyrinthectomies using sodium arsanilate (**Figure 4**). In an automated 5 choice serial reaction time task (5-CSRTT), which is a task commonly used to assess attentional performance in light, rats with BVD made significantly fewer correct responses, significantly more incorrect responses, with no more omissions, and responded with a reduced latency, compared to sham controls (Zheng et al., 2009a,b; see **Figure 5**). This study

95% CIs for the BVD animals were unreliable due to the low concentration of vectors. The inner circles (dotted line) indicate the number of observations for the given vectors (blue triangles). Reproduced with permission from Baek et al. (2010).

in particular shows that the deficits of BVD rats in these cognitive tasks is not due to an inability to respond, but to incorrect responses.

These studies demonstrate that bilateral vestibular lesions have different effects in different cognitive tasks (i.e., radial arm maze, foraging task, Y and T mazes and 5-CSRTT) and that poor performance is not necessarily contingent upon the animal being in darkness, or light, where the oscillopsia would be expected to be worse, or due to an inability to respond. Therefore, the deficits are more likely to be due to an interaction between reduced cognitive ability and altered sensory input than altered sensory input alone. Studies over the last few years have aimed at elucidating the extent to which the spatial memory deficits may be due to other

**exhibited in the same foraging task described in Figure 1, for the sham and BVD animals at 14 month post-op. in darkness, showing the effects of surgery, drug treatment with a cannabinoid receptor agonist, WIN55,212-2 (which one would normally expect to make spatial memory worse), and their interaction. (A)** Effects of surgery; **(B)** Drug treatment; and **(C)** Drug dose. Data are represented as mean ± s.e.m. Reproduced with permission from Baek et al. (2010).

**FIGURE 3 | Mean % correct responses in the spatial T maze task in light over 8 days for the BVD and sham animals at 4–5 months post-op.** Reproduced with permission from Zheng et al. (2012a,b). Data are expressed as means ± a 95% confidence interval.

**task. (A)** Number of reference memory errors. **(B)** Number of working memory errors. Data are represented as mean ± s.e.m. Reproduced with permission from Besnard et al. (2012).

complications of BVD, such as changes in locomotor activity and anxiety.

## **POSSIBLE RELATIONSHIP BETWEEN SPATIAL MEMORY DEFICITS AND LOCOMOTOR HYPERACTIVITY**

Most studies of rats with BVD indicate that they are hyperactive rather than hypoactive (Russell et al., 2003a,b; Goddard et al., 2008; Zheng et al., 2008, 2009a,b, 2012a,b; Baek et al., 2010; Besnard et al., 2012; Stiles et al., 2012; Machado et al., 2012b; see **Figure 6**). While this makes it difficult to explain poor performance in cognitive tasks in terms of an inability to move, it is conceivable that the hyperactivity prevents the animals from performing accurately in these various cognitive tasks. However,

Baek et al. (2010) used regression analyses to show that the poor performance of BVD rats in a foraging task could not be predicted by their hyperactivity (**Figure 7**). We have recently revisited this issue using multiple linear and random forest regression and found that hyperactivity cannot predict the poor performance of rats in a spatial T maze alternation task, but that the best predictors were whether the animals had received a BVD and the duration of their rearing in an open field maze (Smith et al., 2013; see **Figure 8**). These results suggest that the poor spatial memory performance of BVD rats cannot be explained by their locomotor hyperactivity. Further evidence in support of this conclusion is that even rats treated with diazepam and exhibiting anxiolytic behavior, still demonstrated the same spatial memory deficits in a radial arm maze task (Machado et al., 2012b).

## **POSSIBLE RELATIONSHIP BETWEEN SPATIAL MEMORY DEFICITS AND ANXIETY**

It is possible that the association between vestibular dysfunction and anxiety and depression has some connection with the observed cognitive deficits that accompany vestibular

lesions (Balaban and Thayer, 2001; Balaban, 2002; Staab and Ruckenstein, 2003; Staab, 2006). However, at least in animal studies, there is some evidence to disentangle anxiety and cognition in relation to BVD. Machado et al. (2012b) used the black and white box test to show that rats with BVD exhibited increased

anxiety and then administered diazepam to reduce it. Although the diazepam appeared to decrease the animals' anxiety, it had no effect on their poor performance in the radial eight-arm maze (**Figure 9**). Zheng et al. (2012a,b) attempted a similar experiment using the non-benzodiazepine anxiolytic drug, buspirone. In this case, the BVD animals did not exhibit increased anxiety in the elevated plus maze and buspirone had no effect on the time spent in the open arms of the maze, but neither did it have any effect on the animals' poor performance in a spatial forced alternation T maze task. Similarly, an anxiogenic drug, FG-7142, had no effect either on their spatial memory performance. Both of these studies suggest that, at least in animals, spatial memory deficits following BVD may be independent of anxiety.

## **VESTIBULAR vs. AUDITORY DAMAGE**

One of the important limitations of the animal studies in this area is that chemical or surgical lesions of the vestibular labyrinth usually involve damage to the cochlea as well, and therefore it is possible that any cognitive effects of the lesions are partly due to hearing loss. For example, auditory stimulation, including noise trauma, has been reported to affect place cell function (Sakurai, 1990, 1994; Goble et al., 2009). For this reason, in most of our behavioral studies, sham control animals have their tympanic

membranes removed so that sound is no longer transmitted effectively to the malleus, incus and stapes. This can serve as only a partial auditory control, however, because some sound will still be transmitted to the cochlea. Nonetheless, we have consistently found that rats without vestibular lesions but with the tympanic membrane removed perform significantly better in cognitive tasks than animals with vestibular lesions (Zheng et al., 2006, 2007, 2008, 2009a,b, 2012b; Baek et al., 2010). This result suggests that hearing loss is not the major cause of the spatial memory deficits in animals subjected to BVD, and is consistent with the results from studies of patients with vestibular dysfunction (e.g., Brandt et al., 2005). In addition, animal studies, using different kinds of aminoglycosides (i.e., streptomycin and neomycin) with different toxicities for the auditory and vestibular hair cells, have shown that the effects of auditory and vestibular lesions on learning and memory are different (Schaeppi et al., 1991). In a radial arm maze task, rats treated with streptomycin, which lesioned the auditory and the vestibular systems, exhibited impaired working memory; however, rats treated with neomycin, which lesioned only the auditory system, did not (Schaeppi et al., 1991). In our tinnitus studies we have found that a unilateral acoustic trauma has no significant effect on spatial memory (Zheng et al., 2011).

It is not clear whether the cognitive deficits associated with vestibular dysfunction in animals are limited to spatial memory impairment. Some animal studies using surgical BVD have also reported deficits in attention (Zheng et al., 2009a) and in object recognition memory that has no spatial component (Zheng et al., 2004). However, others using sequential chemical UVDs have reported no deficits in object recognition memory (Besnard et al., 2012); therefore, this issue remains to be resolved due to methodological differences between the existing studies.

## **HUMAN STUDIES OF THE VESTIBULAR LESIONS ON MEMORY**

One of the first clinical studies of cognitive function following vestibular loss was reported by Grimm et al. (1989). They described patients with a perilymph fistula syndrome, who reported symptoms such as positional vertigo but also a variety of psychological symptoms, including memory and attention deficits. From a total of 102 patients, more than 85% of them reported memory loss. The patients had a normal level of intellectual function; however, their performance on digit symbol, block design, paired associate learning and picture arrangement tasks, was impaired. Following this, several studies in the 1990's examined the effects of vestibular damage on spatial navigation in humans. They showed that patients with vestibular disorders exhibited deficits in path navigation (Peruch et al., 1999; Cohen, 2000; Cohen and Kimball, 2002; Borel et al., 2004). However, the tasks involved movement, and therefore it could be argued that the deficits resulted from a complex interaction between cognition and postural control. Other studies examined cognitive performance more directly. Black et al. (2004) found that memory problems were common in patients with vestibular loss due to gentamicin ototoxicity. Jauregui-Renaud and colleagues have published a number of studies reporting that patients with vestibular disorders have high rates of depersonalization/derealization symptoms, which include difficulty focussing attention and thoughts seeming blurred (Sang et al., 2006; Jauregui-Renaud et al., 2008a,b; see Gurvich et al., 2013, for review). Other studies have shown that as postural tasks become more challenging, patients with vestibular disorders exhibit deficits in reaction time and memory (Yardley et al., 2001, 2002; Redfern et al., 2004). In a study by Redfern et al. (2004), patients with unilateral vestibular loss showed large increases in complex and inhibitory reaction time, even while seated. A similar result was reported by Talkowski et al. (2005).

## **BILATERAL vs. UNILATERAL VESTIBULAR LESIONS**

The most sophisticated and well-controlled studies of spatial memory following vestibular lesions in humans, have been reported by Schautzer et al. (2003) and Brandt et al. (2005), who examined the performance in a spatial memory test, the virtual Morris water maze, of 10 patients with bilateral vestibular loss and compared them with age-, sex- and education-matched controls. The task involved moving only a cursor on a computer screen, using a mouse; therefore, performance in the spatial memory task could not have been confounded by VSR deficits. The patients had received bilateral vestibular neurectomies 5–10 years previously for the treatment of neurofibromatosis type 2. They found that the patients with bilateral vestibular loss showed impaired performance in the task when they had to remember a navigation path to a previously visible target; however, they showed no deficit when the target was visible, therefore the effect was specific to memory (see **Figure 10**). Brandt et al. used the Weschler memory test to show that the patients had normal or above normal non-spatial memory performance. Compared to the controls, the patients also exhibited a significant decrease in the volume of the hippocampus (16.9%). This effect was bilateral, specific to the hippocampus and there was no significant reduction in the total brain volume or in the volume of gray matter or white matter, only a significant increase in cerebrospinal fluid volume. Importantly, only one of the BVD patients had total post-operative hearing loss.

In a study of patients with UVD, Hüfner et al. (2007) reported results from the virtual Morris water maze task that were more complex. During place learning, males with a right UVD and females with a left UVD exhibited a significantly smaller decrease in heading error over the repeated trials compared to the controls and other UVD groups. In addition, inspection of the swim paths showed that only 2 out of the 8 right UVD patients used direct paths to the platform, compared to 12 out of the 16 control subjects, a difference that was statistically significant. In the probe trial, the right UVD patients were found to have a greater heading error than the left UVD patients or control subjects. Finally, in the cued navigation, females with a left UVD performed worse than the other groups in the first visible platform trial. By contrast with the study of Brandt et al. (2005) of patients with BVD, Hüfner et al. (2007) found no significant differences in the hippocampal volume of UVD patients compared to controls. In a later study, Hüfner et al. (2009) reported some subtle deficits in spatial memory in patients with UVD as well as an atrophy of the ipsilateral supramarginal nucleus, the postcentral and superior temporal gyrus and the MT/V5 area, as well as the contralateral thalamus and tegmentum of the mesencephalon. However, zu Eulenburg et al. (2010) reported that patients who

had recovered from unilateral vestibular neuritis, exhibited a significant decrease in the volume of the left posterior hippocampus, irrespective of the laterality of the vestibular neuritis. Helmchen et al. (2013) have recently reported the results of a resting state fMRI study of patients with vestibular neuritis. They identified a network of brain regions that was significantly different from controls, including the parietal lobe, the medial aspect of the superior parietal lobule, the posterior cingulate cortex, the middle frontal gyrus, the middle temporal gyrus, the parahippocampal gyrus, the anterior cingulate cortex, the insular cortex, the caudate nucleus, the thalamus and the midbrain.

Consistent with these studies, Hüfner et al. (2010) reported structural changes in the hippocampi of professional dancers and slackliners, who have unusual spatial memory experience. Hufner et al. found that trained subjects exhibited a smaller anterior volume, and a larger posterior volume, in the hippocampal formation, although they showed no difference in spatial memory compared to controls, according to the virtual Morris water maze test.

It is apparent from these studies that the volume of the human hippocampus is sensitive to vestibular input. While the effects of BVD are the most dramatic (Brandt et al., 2005), the effects of unilateral vestibular loss are more circumscribed (Hüfner et al., 2007; zu Eulenburg et al., 2010; Helmchen et al., 2013).

Vestibular disorders have also been reported to impair mental imagery tasks that involve imagined rotations or translations of objects relative to the environment, which are also spatial in nature (Péruch et al., 2011). Grabherr et al. (2011)found that only patients with bilateral vestibular loss, and not unilateral vestibular loss, exhibited impaired ability to mentally transform images of bodies and body parts compared to controls.

It is not entirely clear whether the cognitive deficits associated with vestibular dysfunction in humans are limited to spatial memory impairment. While some studies have reported that other aspects of memory, attention and general intelligence are normal (Schautzer et al., 2003; Brandt et al., 2005), others have reported deficits in attention and concentration (Sang et al., 2006; Jauregui-Renaud et al., 2008a,b) and even dyscalculia (Risey and Briner, 1990-1991; Andersson et al., 2002, 2003; Yardley et al., 2002; see Smith, 2012 for a review).

## **POSSIBLE CONNECTION WITH VESTIBULAR REFLEX DYSFUNCTION AND VERTIGO**

As with the animal studies, it is possible that any memory impairment following vestibular damage might simply be a result of vestibular reflex deficits, for example, the inability to see clearly due to oscillopsia or ataxia as a consequence of impaired VORs and VSRs (respectively) (Gizzi et al., 2003). However, studies such as those by Schautzer et al. (2003) and Brandt et al. (2005) were conducted 5–10 years following bilateral vestibular neurectomy. Although the patients would never have recovered normal VOR and VSR function, they would have compensated for the severe acute symptoms (Smith and Curthoys, 1989; Curthoys and Halmagyi, 1995).

Given the severity of some vestibular disorders such as Ménière's disease, it is possible that any cognitive dysfunction is an indirect consequence of symptoms such as vertigo. However, studies of patients with chronic vestibular loss without vertigo, have reported that the patients still exhibit spatial memory impairment (Guidetti et al., 2008). Overall, it seems that patients with at least bilateral vestibular loss, suffer from cognitive problems, particularly spatial memory impairment, that are not simply a direct result of vestibular reflex dysfunction (see Smith et al., 2005a,b; Hanes and McCollum, 2006; Smith et al., 2009, 2010; Gurvich et al., 2013, for reviews).

## **POSSIBLE CONNECTION WITH ANXIETY AND DEPRESSION**

In the case of humans, it is much more difficult to separate poor performance in cognitive tests from emotional disorders. Vestibular dysfunction in humans is often associated with anxiety disorders, including panic attacks and phobias, as well as depression (Eagger et al., 1992; Asmundson et al., 1998; Balaban and Thayer, 2001; Balaban, 2002; Furman and Jacob, 2001; Monzani et al., 2001; Grunfeld et al., 2003; Persoons et al., 2003; Pollak et al., 2003; Godemann et al., 2004a,b, 2009; Best et al., 2006; Staab, 2006; see Gurvich et al., 2013, for review). Anxiety may be a direct consequence of vestibular dysfunction; however, it has also been reported that anxiety disorders can cause dizziness of vestibular origin (Asmundson et al., 1998; Venault et al., 2001; Bolmont et al., 2002; Staab et al., 2002; Tecer et al., 2004; Best et al., 2006; Furman et al., 2006) and antidepressants such as selective serotonin reuptake inhibitors (SSRIs) have been reported to relieve dizziness associated with anxiety (Staab et al., 2002; Simon et al., 2005; Horii et al., 2007). It possible that emotional disorders arise indirectly from cognitive impairment. However, Halberstadt and Balaban (2006) have reported that the same neurons in the dorsal raphe nucleus that release serotonin, send projections into the amygdala, as well as the brainstem vestibular nucleus. This finding suggests that changes in emotional tone may directly influence the vestibular system.

It is worth noting here that the hippocampus is as much an emotional brain region as one that contributes to spatial memory (Gray and McNaughton, 2003). While the rat dorsal hippocampus (approximately equivalent to the posterior hippocampus in humans) is involved in spatial information processing and spatial memory, the rat ventral hippocampus (approximately equivalent to the anterior hippocampus in humans) processes emotional stimuli (Bannerman et al., 1999, 2004). Therefore, it may be difficult to disentangle the effects of vestibular loss on cognition and emotion.

## **EFFECTS OF VESTIBULAR LESIONS ON HEAD DIRECTION CELL AND PLACE CELL FUNCTION**

Taube and colleagues have shown that bilateral inactivation of the vestibular labyrinth, using intratympanic tetrodotoxin, results in the dysfunction of thalamic head direction cells (Stackman and Taube, 1997; see Brown et al., 2002 for a review). In other studies, they have shown that head direction cell activity is degraded during inverted locomotion (Calton and Taube, 2005) and as a result of the loss of vestibular information from either the otoliths (Yoder and Taube, 2009) or the semi-circular canals (Muir et al., 2009). Also using intratympanic tetrodotoxin, Stackman et al. (2002) first reported that loss of vestibular function resulted in a disruption of the selective firing of hippocampal place cells in alert rats. One of the most important aspects of this result was that the disruption to place cell firing patterns was immediate, and it recovered over time, indicating that long-term changes in hippocampal structure were unnecessary for the changes in place cell function. This result was replicated by Russell et al. (2003b) using permanent surgical BVD in rats.

Hippocampal theta rhythm is a large amplitude, quasisinusoidal EEG rhythm of ∼5–12 Hz which is believed to serve a cohesive function for the firing of hippocampal place cells (Hasselmo, 2005; Vertes, 2005). Many studies in alert animals have reported that theta rhythm in the frequency range of 6– 9 Hz can be recorded during movement (see Zou et al., 2009 for a review). In fact, theta frequency has been shown to increase with increasing speed of locomotion (Jeewajee et al., 2008; Lever et al., 2009). Very few studies have investigated the effects of vestibular lesions on theta rhythm. Stackman et al. (2002) investigated theta in one rat in a study in which they transiently inactivated the vestibular system. In the one animal in which they analyzed theta, they found no significant difference from control animals. Russell et al. (2006) used permanent surgical BVD to investigate the effects of vestibular loss on theta rhythm. In contrast to Stackman et al. (2002), they found that hippocampal theta rhythm was severely disrupted. Not only was the power of theta reduced following BVD, but the quasi-sinusoidal character of the waveform was corrupted. Although the BVD animals were hyperactive, theta rhythm was abnormal across the entire range of movement velocities. These results have recently been replicated by Neo et al. (2012), who tried but failed to reverse the spatial memory and emotional deficits caused by BVD by electrically stimulating the septum in order to provide an artificial theta rhythm. Tai et al. (2012) also recently showed that rats that are administered sodium arsanilate intratympanically exhibit a reduction in theta power.

Taken together, these animal studies support the view that vestibular information is important for the generation of spatial memories (Wiener and Berthoz, 1993; Berthoz, 1996; McNaughton et al., 1996, 2006; Etienne and Jeffery, 2004; Smith et al., 2005a,b, 2009, 2010). It is still unclear how vestibular information reaches the hippocampus. Electrical stimulation of one vestibular labyrinth or of the vestibular nucleus has been reported to evoke field potentials, single unit activity and neurotransmitter release in the hippocampus, albeit with a long latency (Horii et al., 1994, 2004; Cuthbert et al., 2000). Caloric or electrical stimulation of the human labyrinth has been shown to cause activation of the hippocampus (Vitte et al., 1996; De Waele et al., 2001) and glucose uptake is reduced in the hippocampus in patients with acute vestibular neuritis (Bense et al., 2001). The thalamus is certain to be one important relay station for the transmission of at least some ascending vestibular information. However, the number of different vestibulo-hippocampal pathways and their precise nature, remains to be determined (Smith, 1997; see Shinder and Taube, 2010 and Hufner et al., 2011 for recent reviews). It must also be kept in mind that the hippocampus is only one part of a highly complex system of limbic-neocortical pathways that are responsible for spatial memory (Guldin and Grusser, 1998; Hanes and McCollum, 2006; Gu et al., 2007; Shinder and Taube, 2010; Lopez and Blanke, 2011). In humans, fMRI has revealed that areas of significant activation by galvanic vestibular stimulation (GVS) include the posterior insula, the retroinsular regions, the superior temporal gyrus, parts of the inferior parietal lobule, the intraparietal sulcus, the post-central and pre-central gyrus, the anterior insular, the inferior frontal gyrus, the anterior cingulate gyrus, the precuneus and the hippocampus (Lobel et al., 1998; see Karnath and Dieterich, 2006 for a review). Activation of cortical networks during GVS is not symmetrical; it appears to be stronger in the non-dominant hemisphere, in the hemisphere ipsilateral to the stimulated ear, and in the hemisphere ipsilateral to the fast phase of vestibular nystagmus (see Karnath and Dieterich, 2006 for a review).

Despite the finding by Brandt et al. (2005) that patients with BVD exhibit a bilateral atrophy of the hippocampus, studies in BVD rats have so far failed to detect such a change. Using a sequential chemical BVD procedure, Besnard et al. (2012) found no significant change in hippocampal volume using MRI. Likewise, Zheng et al. (2012a) could find no change hippocampal volume or neuronal number using stereology. Nonetheless, in unpublished studies, we have found a significant decrease in dendritic length in the hippocampi of BVD rats. One possibility is that hippocampal volume is maintained in rats following BVD due to their locomotor hyperactivity, since movement is known to stimulate hippocampal neurogenesis. Interestingly, Zheng et al. (2012a) observed a significant increase in cell proliferation in the rat hippocampus following BVD. Besnard et al. (2012) also reported an increase in NMDA receptor density and a decrease in affinity in BVD rats. Although Zheng et al. (2013) did not find significant differences in glutamate receptor subunit expression in rats with BVD, principal component analysis did reveal subtle changes in the relationship between different NMDA receptor subunits (Smith and Zheng, 2013). Because Besnard et al. (2012) used autoradioradiography with beta imaging, their results may reflect functional NMDA receptors rather than the total receptor pool.

## **EFFECTS OF ARTIFICIAL VESTIBULAR ACTIVATION ON MEMORY IN HUMANS**

A small literature exists on the use of low amplitude, noisy GVS to enhance cognition in humans. The overlaying of a noise signal on a galvanic stimulus, termed "noisy GVS," is based upon the concept of stochastic resonance, in which a sub-threshold sensory stimulus can be made to exceed a fixed threshold if a Gaussian noise signal, with a frequency much higher than the sub-threshold stimulus, is superimposed upon it (Moss et al., 2004).

Bächtold et al. (2001) was the first to report that caloric vestibular stimulation (CVS) could improve verbal and spatial memory in humans, and the effect was greater for right ear irrigation. However, CVS induces vestibular reflexes and therefore it is difficult to separate those effects from cognitive performance. Falconer and Mast (2012) have recently reported that CVS can enhance performance in an egocentric transformation task.

In a later study, Wilkinson et al. (2008) investigated whether noisy, low intensity, bipolar GVS, which was sub-threshold for the activation of the vestibular reflexes, could affect memory for faces. They found that the mean reaction time was shorter for the GVS groups compared to the sham controls, but that the largest reduction was for the anode on the left ear and the cathode on the right ear. They concluded that GVS can improve memory for faces, perhaps by increasing blood flow to the right temporal and parietal cortices. This effect of GVS on memory was unlikely to be due to non-specific arousal since the electrical stimulation was subthreshold, the memory improvement was greater for the anode on the left, and improvement was found only when a noise signal was added to the GVS. Wilkinson and colleagues have also found that noisy GVS can attenuate prosopagnosia (Wilkinson et al., 2005) and figure copying deficits (Wilkinson et al., 2010). In the most recent study involving two patients with visuo-spatial neglect, GVS was found to have a lasting beneficial effect in a target cancellation task (Zubko et al., 2013). In one of the most systematic studies to date, Dilda et al. (2012) found that suprathreshold GVS significantly increased the error rates for match-to-sample and perspective-taking tasks compared to a subthreshold GVS group; however, reaction time, dual tasking, mental rotation and manual tracking were not significantly affected. Subthreshold GVS had no significant effect on cognitive performance compared to the pre-stimulus conditions.

At the present, there is very little evidence relating to how noisy GVS affects memory. It is not clear whether the effect has any relationship to the effects of vestibular lesions on spatial memory. One possibility is that any beneficial effect of GVS is merely a result of the fact that vestibular information reaches many regions of the neocortex (Bense et al., 2001) and that it will probably cause a change in the integration of sensory information (Wilkinson et al., 2008). However, given the importance of vestibular information to spatial memory, sub-threshold GVS may somehow "prime" the brain to process and store new information. Wilkinson et al. (2012) have recently reported that GVS increased the amplitude of the N170 potential and the power of delta and theta EEG during a face processing task. It is noteworthy that many of these effects of GVS are not restricted to spatial attention or memory. A more recent study by Kim et al. (2013) also supports the idea that GVS modulates neural oscillations.

## **CONCLUSIONS**

Over the last 12 years in particular, a substantial body of evidence has accumulated to suggest that the loss of vestibular function results in cognitive disorders, especially spatial memory deficits that cannot easily be attributed to the direct effects of reflex dysfunction, motor control problems or hearing loss. The spatial memory deficits appear to be most striking in the case of BVD, where there is a complete loss of vestibular function, although there are many more animal studies than human studies to substantiate this. It will be particularly important in future studies to further investigate the effects of unilateral and bilateral vestibular loss in humans, on different types of cognitive function.

In animals at least, the spatial memory deficits appear to be independent of anxiety, although this is less conclusive in humans. One reason to be cautious about disentangling the effects of vestibular damage on cognition and emotion, is that the hippocampus is deeply involved in both (Gray and McNaughton, 2003). While lesions of the dorsal hippocampus in rats result in spatial memory deficits and locomotor hyperactivity, lesions of the ventral hippocampus result in reduced anxiety in the elevated plus maze and hyponeophagia (Bannerman et al., 2002). It is intriguing that BVD, in addition to causing spatial memory deficits and locomotor hyperactivity in rats, has been reported to cause both reduced anxiety in the elevated plus and T mazes (Zheng et al., 2008; Neo et al., 2012) and hyponeophagia (Zheng et al., 2008). This might suggest that BVD results in behavioral effects that are similar to both dorsal and ventral hippocampal lesions and that it may be very difficult to disentangle them completely.

The cognitive effects of vestibular loss appear to be due mainly to the important contribution that the vestibular system makes to neurons involved in spatial navigation and memory, such as head direction cells and place cells, although exactly how this information is used remains to be determined. Information from the otoliths as well as the semi-circular canals seems to be necessary for the normal function of head direction cells; however, so far the different contributions of the otoliths and semi-circular canals to hippocampal place cell function have not been investigated. Furthermore, despite the attention to the thalamus and hippocampus, many different areas of the limbic system and neocortex are involved in these spatial memory processes. Since grid cells in the entorhinal cortex are thought to be responsible for the place fields of hippocampal place cells (see Giocomo et al., 2011 for a review), it might be predicted that BVD would severely disrupt grid cell firing; however, this has not been investigated to date. Future studies will need to specifically address how vestibular information is transmitted to the dorsal and ventral hippocampus in rats and the extent to which otolithic vs. semi-circular canal input is represented in different areas of the hippocampus and entorhinal cortex. At present, we know that vestibular input is necessary for the normal function of the hippocampus but we do not understand how this input is being used.

At present there is some evidence that artificial activation of the vestibular system, using noisy GVS, can modulate memory. However, at present this literature is small and it is unclear whether this effect, when it occurs, is related to the same mechanisms by which vestibular lesions affect memory.

### **ACKNOWLEDGMENTS**

This research has been supported by the Marsden Fund and the New Zealand Neurological Foundation. Yiwen Zheng was a recipient of the Health Research Council Sir Charles Hercus Fellowship.

## **REFERENCES**

vestibular deafferentation in the rat. *Behav. Brain Res.* 246, 15–23. doi: 10.1016/j.bbr.2013.02.033

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Received: 01 August 2013; paper pending published: 13 September 2013; accepted: 07 November 2013; published online: 26 November 2013.*

*Citation: Smith PF and Zheng Y (2013) From ear to uncertainty: vestibular contributions to cognitive function. Front. Integr. Neurosci. 7:84. doi: 10.3389/fnint. 2013.00084*

*This article was submitted to the journal Frontiers in Integrative Neuroscience.*

*Copyright © 2013 Smith and Zheng. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

## Plasticity of the histamine H3 receptors after acute vestibular lesion in the adult cat

## *Brahim Tighilet 1\*, Christiane Mourre2 and Michel Lacour <sup>1</sup>*

<sup>1</sup> Laboratoire de Neurosciences Intégratives et Adaptatives, UMR 7260, FR - Comportement, Cerveau, Cognition (Behavior, Brain, and Cognition),

Centre Saint-Charles, Case B, Centre National de la Recherche Scientifique, Aix-Marseille Université, Marseille, France

<sup>2</sup> Laboratoire de Neurosciences Cognitives, UMR 7291, Centre Saint-Charles, Centre National de la Recherche Scientifique, Aix-Marseille Université,

Marseille, France

#### *Edited by:*

Pierre Denise, Université de Caen Basse-Normandie, France

#### *Reviewed by:*

Catherine de Waele, Centre National de la Recherche Scientifique, France Christian Chabbert, Institut National de la Santé et de la Recherche Médicale, France

#### *\*Correspondence:*

Brahim Tighilet, Laboratoire de Neurosciences Intégratives et Adaptatives, UMR 7260, FR - Comportement, Cerveau, Cognition (Behavior, Brain, and Cognition), Centre Saint-Charles, Case B, Centre National de la Recherche Scientifique, Aix-Marseille Université, 3 Place Victor Hugo, 13331 Marseille Cedex 03, France

e-mail<EMAIL_ADDRESS>
After unilateral vestibular neurectomy (UVN) many molecular and neurochemical mechanisms underlie the neurophysiological reorganizations occurring in the vestibular nuclei (VN) complex, as well as the behavioral recovery process. As a key regulator, the histaminergic system appears to be a likely candidate because drugs interfering with histamine (HA) neurotransmission facilitate behavioral recovery after vestibular lesion. This study aimed at analyzing the post-lesion changes of the histaminergic system by quantifying binding to histamine H3 receptors (H3R; mediating namely histamine autoinhibition) using a histamine H <sup>3</sup> <sup>3</sup> receptor agonist ([ H]N-α-methylhistamine). Experiments were done in brain sections of control cats (N = 6) and cats submitted to UVN and killed 1 (N = 6) or 3 (N = 6) weeks after the lesion. UVN induced a bilateral decrease in binding density of the agonist [3H]N-αmethylhistamine to H3R in the tuberomammillary nuclei (TMN) at 1 week post-lesion, with a predominant down-regulation in the ipsilateral TMN. The bilateral decrease remained at the 3 weeks survival time and became symmetric. Concerning brainstem structures, binding density in the VN, the prepositus hypoglossi, the subdivisions of the inferior olive decreased unilaterally on the ipsilateral side at 1 week and bilaterally 3 weeks after UVN. Similar changes were observed in the subdivisions of the solitary nucleus only 1 week after the lesion. These findings indicate vestibular lesion induces plasticity of the histamine H3R, which could contribute to vestibular function recovery.

**Keywords: histamine H3 receptor, unilateral vestibular neurectomy, vestibular compensation, vestibular nuclei, tuberomammillary nuclei, inferior olive, solitary nucleus, cat**

## **INTRODUCTION**

Unilateral lesion of the peripheral vestibular system induces a syndrome of static oculomotor (nystagmus) and postural (head rolland yaw-tilt, asymmetric extensor tone in the limb and axial muscles, increase of the surface delimited by the four legs) disorders that subside quite rapidly (few days or weeks) in a process of behavioral recovery known as vestibular compensation. This unilateral vestibular damage induces also dynamic symptoms such as vestibulo-ocular reflex gain deficit toward the lesioned side that are however long-lasting or remain relatively uncompensated. The neural mechanisms underlying this vestibular function recovery has been well documented and it is admitted that the static deficits result from the asymmetric resting discharge and their compensation is associated with a rebalanced resting activity on both sides. By contrast the compensation of the dynamics symptoms involves multiple plasticity mechanisms occurring in various brain areas (Smith and Curthoys, 1989; Curthoys, 2000; Dieringer, 2003; Lacour, 2006; Paterson et al., 2006; Dutia, 2010; Lacour and Tighilet, 2010).