lateral intraparietal sulcus (lip) in macaques area 7b in macaques second somatosensory cortex (sii) temporal lobe primary auditory cortex (a1) superior temporal cortex (stg/sts/pt) audio visual cross modal interactions are known to occur in the auditory association cortex which lies directly inferior to the sylvian fissure in the temporal lobe. plasticity was observed in the superior temporal gyrus (stg) by petitto et al. (2000). here, it was found that the stg was more active during stimulation in native deaf signers compared to hearing non signers. concurrently, further research has revealed differences in the activation of the planum temporale (pt) in response to non linguistic lip movements between the hearing and deaf; as well as progressively increasing activation of the auditory association cortex as previously deaf participants gain hearing experience via a cochlear implant. anterior ectosylvian sulus (aes) in cats rostral lateral suprasylvian sulcus (rls) in cats cortical-subcortical interactions the most significant interaction between these two systems (corticotectal interactions) is the connection between the anterior ectosylvian sulcus (aes), which lies at the junction of the parietal, temporal and frontal lobes, and the sc. the aes is divided into three unimodal regions with multisensory neurons at the junctions between these sections. (jiang & stein, 2003). neurons from the unimodal regions project to the deep layers of the sc and influence the multiplicative integration effect. that is, although they can receive inputs from all modalities as normal, the sc can not enhance or depress the effect of multisensory stimulation without input from the aes. concurrently, the multisensory neurons of the aes, although also integrally connected to unimodal aes neurons, are not directly connected to the sc. this pattern of division is reflected in other areas of the cortex, resulting in the observation that cortical and tectal multisensory systems are somewhat dissociated. stein, london, wilkinson and price (1996) analysed the perceived luminance of an led in the context of spatially disparate auditory distracters of various types. a significant finding was that a sound increased the perceived brightness of the light, regardless of their relative spatial locations, provided the light's image was projected onto the fovea. here, the apparent lack of the spatial rule, further differentiates cortical and tectal multisensory neurons. little empirical evidence exists to justify this dichotomy. nevertheless, cortical neurons governing perception, and a separate sub cortical system governing action (orientation behavior) is synonymous with the perception action hypothesis of the visual stream. further investigation into this field is necessary before any substantial claims can be made. dual "what" and "where" multisensory routes research suggests the existence of two multisensory routes for "what" and "where". the "what" route identifying the identity of things involving area brodmann area 9 in the right inferior frontal gyrus and right middle frontal gyrus, brodmann area 13 and brodmann area 45 in the right insula-inferior frontal gyrus area, and brodmann area 13 bilaterally in the insula. the "where" route detecting their spatial attributes involving the brodmann area 40 in the right and left inferior parietal lobule and the brodmann area 7 in the right precuneus-superior parietal lobule and brodmann area 7 in the left superior parietal lobule. development of multisensory operations theories of development all species equipped with multiple sensory systems, utilize them in an integrative manner to achieve action and perception. however, in most species, especially higher mammals and humans, the ability to integrate develops in parallel with physical and cognitive maturity. children until certain ages do not show mature integration patterns. classically, two opposing views that are principally modern manifestations of the nativist/empiricist dichotomy have been put forth. the integration (empiricist) view states that at birth, sensory modalities are not at all connected. hence, it is only through active exploration that plastic changes can occur in the nervous system to initiate holistic perceptions and actions. conversely, the differentiation (nativist) perspective asserts that the young nervous system is highly interconnected; and that during development, modalities are gradually differentiated as relevant connections are rehearsed and the irrelevant are discarded. using the sc as a model, the nature of this dichotomy can be analysed. in the newborn cat, deep layers of the sc contain only neurons responding to the somatosensory modality. within a week, auditory neurons begin to occur, but it is not until two weeks after birth that the first multisensory neurons appear. further changes continue, with the arrival of visual neurons after three weeks, until the sc has achieved its fully mature structure after three to four months. concurrently in species of monkey, newborns are endowed with a significant complement of multisensory cells; however, along with cats there is no integration effect apparent until much later. this delay is thought to be the result of the relatively slower development of cortical structures including the aes; which as stated above, is essential for the existence of the integration effect. furthermore, it was found by wallace (2004) that cats raised in a light deprived environment had severely underdeveloped visual receptive fields in deep layers of the sc. although, receptive field size has been shown to decrease with maturity, the above finding suggests that integration in the sc is a function of experience. nevertheless, the existence of visual multisensory neurons, despite a complete lack of visual experience, highlights the apparent relevance of nativist viewpoints. multisensory development in the cortex has been studied to a lesser extent, however a similar study to that presented above was performed on cats whose optic nerves had been severed. these cats displayed a marked improvement in their ability to localize stimuli through audition; and consequently also showed increased neural connectivity between v1 and the auditory cortex. such plasticity in early childhood allows for greater adaptability, and thus more normal development in other areas for those with a sensory deficit. in contrast, following the initial formative period, the sc does not appear to display any neural plasticity. despite this, habituation and sensititisation over the long term is known to exist in orientation behaviors. this apparent plasticity in function has been attributed to the adaptability of the aes. that is, although neurons in the sc have a fixed magnitude of output per unit input, and essentially operate an all or nothing response, the level of neural firing can be more finely tuned by variations in input by the aes. although there is evidence for either perspective of the integration/differentiation dichotomy, a significant body of evidence also exists for a combination of factors from either view. thus, analogous to the broader nativist/empiricist argument, it is apparent that rather than a dichotomy, there exists a continuum, such that the integration and differentiation hypotheses are extremes at either end. psychophysical development of integration not much is known about the development of the ability to integrate multiple estimates such as vision and touch. some multisensory abilities are present from early infancy, but it is not until children are eight years or older before they use multiple modalities to reduce sensory uncertainty. one study demonstrated that cross-modal visual and auditory integration is present from within 1 year of life. this study measured response time for orientating towards a source. infants who were 8–10 months old showed significantly decreased response times when the source was presented through both visual and auditory information compared to a single modality. younger infants, however, showed no such change in response times to these different conditions. indeed, the results of the study indicates that children potentially have the capacity to integrate sensory sources at any age. however, in certain cases, for example visual cues, intermodal integration is avoided. another study found that cross-modal integration of touch and vision for distinguishing size and orientation is available from at least 8 years of age. for pre-integration age groups, one sense dominates depending on the characteristic discerned (see visual dominance). a study investigating sensory integration within a single modality (vision) found that it cannot be established until age 12 and above. this particular study assessed the integration of disparity and texture cues to resolve surface slant. though younger age groups showed a somewhat better performance when combining disparity and texture cues compared to using only disparity or texture cues, this difference was not statistically significant. in adults, the sensory integration can be mandatory, meaning that they no longer have access to the individual sensory sources. acknowledging these variations, many hypotheses have been established to reflect why these observations are task-dependent. given that different senses develop at different rates, it has been proposed that cross-modal integration does not appear until both modalities have reached maturity. the human body undergoes significant physical transformation throughout childhood. not only is there growth in size and stature (affecting viewing height), but there is also change in inter-ocular distance and eyeball length. therefore, sensory signals need to be constantly re-evaluated to appreciate these various physiological changes. some support comes from animal studies that explore the neurobiology behind integration. adult monkeys have deep inter-neuronal connections within the superior colliculus providing strong, accelerated visuo-auditory integration. young animals conversely, do not have this enhancement until unimodal properties are fully developed. additionally, to rationalize sensory dominance, gori et al. (2008) advocates that the brain utilises the most direct source of information during sensory immaturity. in this case, orientation is primarily a visual characteristic. it can be derived directly from the object image that forms on the retina, irrespective of other visual factors. in fact, data shows that a functional property of neurons within primate visual cortices' are their discernment to orientation. in contrast, haptic orientation judgements are recovered through collaborated patterned stimulations, evidently an indirect source susceptible to interference. likewise, when size is concerned haptic information coming from positions of the fingers is more immediate. visual-size perceptions, alternatively, have to be computed using parameters such as slant and distance. considering this, sensory dominance is a useful instinct to assist with calibration. during sensory immaturity, the more simple and robust information source could be used to tweak the accuracy of the alternate source. follow-up work by gori et al. (2012) showed that, at all ages, vision-size perceptions are near perfect when viewing objects within the haptic workspace (i.e. at arm's reach). however, systematic errors in perception appeared when the object was positioned beyond this zone. children younger than 14 years tend to underestimate object size, whereas adults overestimated. however, if the object was returned to the haptic workspace, those visual biases disappeared. these results support the hypothesis that haptic information may educate visual perceptions. if sources are used for cross-calibration they cannot, therefore, be combined (integrated). maintaining access to individual estimates is a trade-off for extra plasticity over accuracy, which could be beneficial in retrospect to the developing body. alternatively, ernst (2008) advocates that efficient integration initially relies upon establishing correspondence – which sensory signals belong together. indeed, studies have shown that visuo-haptic integration fails in adults when there is a perceived spatial separation, suggesting sensory information is coming from different targets. furthermore, if the separation can be explained, for example viewing an object through a mirror, integration is re-established and can even be optimal. ernst (2008) suggests that adults can obtain this knowledge from previous experiences to quickly determine which sensory sources depict the same target, but young children could be deficient in this area. once there is a sufficient bank of experiences, confidence to correctly integrate sensory signals can then be introduced in their behaviour. lastly, nardini et al. (2010) recently hypothesised that young children have optimized their sensory appreciation for speed over accuracy. when information is presented in two forms, children may derive an estimate from the fastest available source, subsequently ignoring the alternate, even if it contains redundant information. nardini et al. (2010) provides evidence that children's (aged 6 years) response latencies are significantly lower when stimuli are presented in multi-cue over single-cue conditions. conversely, adults showed no change between these conditions. indeed, adults display mandatory fusion of signals, therefore they can only ever aim for maximum accuracy. however, the overall mean latencies for children were not faster than adults, which suggests that speed optimization merely enable them to keep up with the mature pace. considering the haste of real-world events, this strategy may prove necessary to counteract the general slower processing of children and maintain effective vision-action coupling. ultimately the developing sensory system may preferentially adapt for different goals – speed and detecting sensory conflicts