acknowledged that uncertainty in sensory domains results in an increased dependence of multisensory integration. hence, it follows that cues from multiple modalities that are both temporally and spatially synchronous are viewed neurally and perceptually as emanating from the same source. the degree of synchrony that is required for this 'binding' to occur is currently being investigated in a variety of approaches. the integrative function only occurs to a point beyond which the subject can differentiate them as two opposing stimuli. concurrently, a significant intermediate conclusion can be drawn from the research thus far. multisensory stimuli that are bound into a single percept, are also bound on the same receptive fields of multisensory neurons in the sc and cortex. decreasing reaction time responses to multiple simultaneous sensory stimuli can be faster than responses to the same stimuli presented in isolation. hershenson (1962) presented a light and tone simultaneously and separately, and asked human participants to respond as rapidly as possible to them. as the asynchrony between the onsets of both stimuli was varied, it was observed that for certain degrees of asynchrony, reaction times were decreased. these levels of asynchrony were quite small, perhaps reflecting the temporal window that exists in multisensory neurons of the sc. further studies have analysed the reaction times of saccadic eye movements; and more recently correlated these findings to neural phenomena. in patients studied by gonzalo, with lesions in the parieto-occipital cortex, the decrease in the reaction time to a given stimulus by means of intersensory facilitation was shown to be very remarkable. redundant target effects the redundant target effect is the observation that people typically respond faster to double targets (two targets presented simultaneously) than to either of the targets presented alone. this difference in latency is termed the redundancy gain (rg). in a study done by forster, cavina-pratesi, aglioti, and berlucchi (2001), normal observers responded faster to simultaneous visual and tactile stimuli than to single visual or tactile stimuli. rt to simultaneous visual and tactile stimuli was also faster than rt to simultaneous dual visual or tactile stimuli. the advantage for rt to combined visual-tactile stimuli over rt to the other types of stimulation could be accounted for by intersensory neural facilitation rather than by probability summation. these effects can be ascribed to the convergence of tactile and visual inputs onto neural centers which contain flexible multisensory representations of body parts. mcgurk effect it has been found that two converging bimodal stimuli can produce a perception that is not only different in magnitude than the sum of its parts, but also quite different in quality. in a classic study labeled the mcgurk effect, a person's phoneme production was dubbed with a video of that person speaking a different phoneme. the result was the perception of a third, different phoneme. mcgurk and macdonald (1976) explained that phonemes such as ba, da, ka, ta, ga and pa can be divided into four groups, those that can be visually confused, i.e. (da, ga, ka, ta) and (ba and pa), and those that can be audibly confused. hence, when ba – voice and ga lips are processed together, the visual modality sees ga or da, and the auditory modality hears ba or da, combining to form the percept da. ventriloquism ventriloquism has been used as the evidence for the modality appropriateness hypothesis. the ventriloquism effect is the situation in which auditory location perception is shifted toward a visual cue. the original study describing this phenomenon was conducted by howard and templeton, (1966) after which several studies have replicated and built upon the conclusions they reached. in conditions in which the visual cue is unambiguous, visual capture reliably occurs. thus to test the influence of sound on perceived location, the visual stimulus must be progressively degraded. furthermore, given that auditory stimuli are more attuned to temporal changes, recent studies have tested the ability of temporal characteristics to influence the spatial location of visual stimuli. some types of evp – electronic voice phenomenon, mainly the ones using sound bubbles are considered a kind of modern ventriloquism technique and is played by the use of sophisticated software, computers and sound equipment. double-flash illusion the double flash illusion was reported as the first illusion to show that visual stimuli can be qualitatively altered by audio stimuli. in the standard paradigm participants are presented combinations of one to four flashes accompanied by zero to 4 beeps. they were then asked to say how many flashes they perceived. participants perceived illusory flashes when there were more beeps than flashes. fmri studies have shown that there is crossmodal activation in early, low level visual areas, which was qualitatively similar to the perception of a real flash. this suggests that the illusion reflects subjective perception of the extra flash. further, studies suggest that timing of multisensory activation in unisensory cortexes is too fast to be mediated by a higher order integration suggesting feed forward or lateral connections. one study has revealed the same effect but from vision to audition, as well as fission rather than fusion effects, although the level of the auditory stimulus was reduced to make it less salient for those illusions affecting audition. rubber hand illusion in the rubber hand illusion (rhi), human participants view a dummy hand being stroked with a paintbrush, while they feel a series of identical brushstrokes applied to their own hand, which is hidden from view. if this visual and tactile information is applied synchronously, and if the visual appearance and position of the dummy hand is similar to one's own hand, then people may feel that the touches on their own hand are coming from the dummy hand, and even that the dummy hand is, in some way, their own hand. this is an early form of body transfer illusion. the rhi is an illusion of vision, touch, and posture (proprioception), but a similar illusion can also be induced with touch and proprioception. it has also been found that the illusion may not require tactile stimulation at all, but can be completely induced using mere vision of the rubber hand being in a congruent posture with the hidden real hand. the very first report of this kind of illusion may have been as early as 1937 (tastevin, 1937). body transfer illusion body transfer illusion typically involves the use of virtual reality devices to induce the illusion in the subject that the body of another person or being is the subject's own body. neural mechanisms superior colliculus superior colliculus the superior colliculus (sc) or optic tectum (ot) is part of the tectum, located in the midbrain, superior to the brainstem and inferior to the thalamus. it contains seven layers of alternating white and grey matter, of which the superficial contain topographic maps of the visual field; and deeper layers contain overlapping spatial maps of the visual, auditory and somatosensory modalities. the structure receives afferents directly from the retina, as well as from various regions of the cortex (primarily the occipital lobe), the spinal cord and the inferior colliculus. it sends efferents to the spinal cord, cerebellum, thalamus and occipital lobe via the lateral geniculate nucleus (lgn). the structure contains a high proportion of multisensory neurons and plays a role in the motor control of orientation behaviours of the eyes, ears and head. receptive fields from somatosensory, visual and auditory modalities converge in the deeper layers to form a two-dimensional multisensory map of the external world. here, objects straight ahead are represented caudally and objects on the periphery are represented rosterally. similarly, locations in superior sensory space are represented medially, and inferior locations are represented laterally. however, in contrast to simple convergence, the sc integrates information to create an output that differs from the sum of its inputs. following a phenomenon labelled the 'spatial rule', neurons are excited if stimuli from multiple modalities fall on the same or adjacent receptive fields, but are inhibited if the stimuli fall on disparate fields. excited neurons may then proceed to innervate various muscles and neural structures to orient an individual's behaviour and attention toward the stimulus. neurons in the sc also adhere to the 'temporal rule', in which stimulation must occur within close temporal proximity to excite neurons. however, due to the varying processing time between modalities and the relatively slower speed of sound to light, it has been found the neurons may be optimally excited when stimulated some time apart. putamen single neurons in the macaque putamen have been shown to have visual and somatosensory responses closely related to those in the polysensory zone of the premotor cortex and area 7b in the parietal lobe. cortical areas multisensory neurons exist in a large number of locations, often integrated with unimodal neurons. they have recently been discovered in areas previously thought to be modality specific, such as the somatosensory cortex; as well as in clusters at the borders between the major cerebral lobes, such as the occipito-parietal space and the occipito-temporal space. however, in order to undergo such physiological changes, there must exist continuous connectivity between these multisensory structures. it is generally agreed that information flow within the cortex follows a hierarchical configuration. hubel and wiesel showed that receptive fields and thus the function of cortical structures, as one proceeds out from v1 along the visual pathways, become increasingly complex and specialized. from this it was postulated that information flowed outwards in a feed-forward fashion; the complex end products eventually binding to form a percept. however, via fmri and intracranial recording technologies, it has been observed that the activation time of successive levels of the hierarchy does not correlate with a feed-forward structure. that is, late activation has been observed in the striate cortex, markedly after activation of the prefrontal cortex in response to the same stimulus. complementing this, afferent nerve fibres have been found that project to early visual areas such as the lingual gyrus from late in the dorsal (action) and ventral (perception) visual streams, as well as from the auditory association cortex. feedback projections have also been observed in the opossum directly from the auditory association cortex to v1. this last observation currently highlights a point of controversy within the neuroscientific community. sadato et al. (2004) concluded, in line with bernstein et al. (2002), that the primary auditory cortex (a1) was functionally distinct from the auditory association cortex, in that it was void of any interaction with the visual modality. they hence concluded that a1 would not at all be effected by cross modal plasticity. this concurs with jones and powell's (1970) contention that primary sensory areas are connected only to other areas of the same modality. in contrast, the dorsal auditory pathway, projecting from the temporal lobe is largely concerned with processing spatial information, and contains receptive fields that are topographically organized. fibers from this region project directly to neurons governing corresponding receptive fields in v1. the perceptual consequences of this have not yet been empirically acknowledged. however, it can be hypothesized that these projections may be the precursors of increased acuity and emphasis of visual stimuli in relevant areas of perceptual space. consequently, this finding rejects jones and powell's (1970) hypothesis and thus is in conflict with sadato et al.'s (2004) findings. a resolution to this discrepancy includes the possibility that primary sensory areas can not be classified as a single group, and thus may be far more different from what was previously thought. the multisensory syndrome with symmetric bilaterality, characterized by gonzalo and called by this author 'central syndrome of the cortex', was originated from a unilateral parieto-occipital cortical lesion equidistant from the visual, tactile, and auditory projection areas (the middle of area 19, the anterior part of area 18 and the most posterior of area 39, in brodmann terminology) that was called 'central zone'. the gradation observed between syndromes led this author to propose a functional gradient scheme in which the specificity of the cortex is distributed with a continuous variation, the overlap of the specific gradients would be high or maximum in that 'central zone'. frontal lobe area f4 in macaques area f5 in macaques polysensory zone of premotor cortex (pz) in macaques occipital lobe primary visual cortex (v1) lingual gyrus in humans lateral occipital complex (loc), including lateral occipital tactile visual area (lotv) parietal lobe ventral intraparietal sulcus (vip) in macaques