Multisensory Neurons Research Articles

Explicit Encoding of Multimodal Percepts by Single Neurons in the Human Brain

Different pictures of Marilyn Monroe can evoke the same percept, even if greatly modified as in Andy Warhol's famous portraits. But how does the brain recognize highly variable pictures as the same percept? Various studies have provided insights into how visual information is processed along the "ventral pathway," via both single-cell recordings in monkeys and functional imaging in humans. Interestingly, in humans, the same "concept" of Marilyn Monroe can be evoked with other stimulus modalities, for instance by hearing or reading her name. Brain imaging studies have identified cortical areas selective to voices and visual word forms. However, how visual, text, and sound information can elicit a unique percept is still largely unknown. By using presentations of pictures and of spoken and written names, we show that (1) single neurons in the human medial temporal lobe (MTL) respond selectively to representations of the same individual across different sensory modalities; (2) the degree of multimodal invariance increases along the hierarchical structure within the MTL; and (3) such neuronal representations can be generated within less than a day or two. These results demonstrate that single neurons can encode percepts in an explicit, selective, and invariant manner, even if evoked by different sensory modalities.

Visual influences on ferret auditory cortex

Multisensory neurons are now known to be widespread in low-level regions of the cortex usually thought of as being responsible for modality-specific processing. The auditory cortex provides a particularly striking example of this, exhibiting responses to both visual and somatosensory stimulation. Single-neuron recording studies in ferrets have shown that each of auditory fields that have been characterized using physiological and anatomical criteria also receives visual inputs, with the incidence of visually-sensitive neurons ranging from 15% to 20% in the primary areas to around 50% or more in higher-level areas. Although some neurons exhibit spiking responses to visual stimulation, these inputs often have subthreshold influences that modulate the responses of the cortical neurons to sound. Insights into the possible role played by the visual inputs can be obtained by examining their sources of origin and the way in which they alter the processing capabilities of neurons in the auditory cortex. These studies suggest that one of the functions of the visual input to auditory cortex is to sharpen the relatively imprecise spatial coding typically found there. Because the extent to which this happens varies between cortical fields, the investigation of multisensory interactions can also help in understanding their relative contributions to auditory perception.

Somatosensory and multisensory properties of the medial bank of the ferret rostral suprasylvian sulcus

In ferret cortex, the rostral portion of the suprasylvian sulcus separates primary somatosensory cortex (SI) from the anterior auditory fields. The boundary of the SI extends to this sulcus, but the adjoining medial sulcal bank has been described as "unresponsive." Given its location between the representations of two different sensory modalities, it seems possible that the medial bank of the rostral suprasylvian sulcus (MRSS) might be multisensory in nature and contains neurons responsive to stimuli not examined by previous studies. The aim of this investigation was to determine if the MRSS contained tactile, auditory and/or multisensory neurons and to evaluate if its anatomical connections were consistent with these properties. The MRSS was found to be primarily responsive to low-threshold cutaneous stimulation, with regions of the head, neck and upper trunk represented somatotopically that were primarily connected with the SI face representation. Unlike the adjoining SI, the MRSS exhibited a different cytoarchitecture, its cutaneous representation was largely bilateral, and it contained a mixture of somatosensory, auditory and multisensory neurons. Despite the presence of multisensory neurons, however, auditory inputs exerted only modest effects on tactile processing in MRSS neurons and showed no influence on the averaged population response. These results identify the MRSS as a distinct, higher order somatosensory region as well as demonstrate that an area containing multisensory neurons may not necessarily exhibit activity indicative of multisensory processing at the population level.

Axon Morphologies and Convergence Patterns of Projections from Different Sensory-Specific Cortices of the Anterior Ectosylvian Sulcus onto Multisensory Neurons in the Cat Superior Colliculus

Corticofugal projections to the thalamus reveal 2 axonal morphologies, each associated with specific physiological attributes. These determine the functional characteristics of thalamic neurons. It is not clear, however, whether such features characterize the corticofugal projections that mediate multisensory integration in superior colliculus (SC) neurons. The cortico-collicular projections from cat anterior ectosylvian sulcus (AES) are derived from its visual, auditory, and somatosensory representations and are critical for multisensory integration. Following tracer injections into each subdivision, 2 types of cortico-collicular axons were observed. Most were categorized as type I and consisted of small-caliber axons traversing long distances without branching, bearing mainly small boutons. The less frequent type II had thicker axons, more complex branching patterns, larger boutons, and more complex terminal boutons. Following combinatorial injections of 2 different fluorescent tracers into defined AES subdivisions, fibers from each were seen converging onto individual SC neurons and indicate that such convergence, like that in the corticothalamic system, is mediated by 2 distinct morphological types of axon terminals. Nevertheless, and despite the conservation of axonal morphologies across different subcortical systems, it is not yet clear if the concomitant physiological attributes described in the thalamus are directly applicable to multisensory integration.

Cortex rules: The neural mechanisms differentiating multisensory integration and unisensory integration in the midbrain

Multisensory neurons in the superior colliculus (SC) integrate cues from different senses and the responses to multisensory stimuli often equal or exceed the summed influences of the modality‐specific stimulus components. This capacity is unique to multisensory information as we have shown that such rarely occurs for pairs of within‐modal stimuli (e.g., two visual stimuli)(Alvarado et al., 2007). That the same neuron integrates excitatory inputs differently depending on their source is fortuitous from a functional perspective but the architecture underlying this dual capacity is unclear. A likely candidate is a projection to the SC from the anterior ectosylvian cortex (AES)(Wallace and Stein, 1994; Jiang et al., 2001; Alvarado et al., 2007). We examined the degree to which the AES‐derived cortico‐collicular projection is multisensory‐specific by evaluating the impact of cortical inactivation on both unisensory and multisensory integration in the same multisensory neurons. Cortical inactivation nearly abolished multisensory enhancement but had no impact on unisensory integration. These findings suggest that this cortico‐collicular circuit has evolved for the purpose of combining information across multiple senses and highlights an essential distinction between brain mechanisms for integrating stimuli within and across senses. This research was supported by NIH grants EY016716 and NS36916.

Cochlear Damage Changes the Distribution of Vesicular Glutamate Transporters Associated with Auditory and Nonauditory Inputs to the Cochlear Nucleus

Integration of multimodal information is essential for understanding complex environments. In the auditory system, multisensory integration first occurs in the cochlear nucleus (CN), where auditory nerve and somatosensory pathways converge (Shore, 2005). A unique feature of multisensory neurons is their propensity to receive cross-modal compensation after deafening. Based on our findings that the vesicular glutamate transporters, VGLUT1 and VGLUT2, are differentially associated with auditory nerve and somatosensory inputs to the CN, respectively (Zhou et al., 2007), we examined their relative distributions after unilateral deafening. After unilateral intracochlear injections of kanamycin (1 and 2 weeks), VGLUT1 immunoreactivity (ir) in the magnocellular CN ipsilateral to the cochlear damage was significantly decreased, whereas VGLUT2-ir in regions that receive nonauditory input was significantly increased 2 weeks after deafening. The pathway-specific amplification of VGLUT2 expression in the CN suggests that, in compensatory response to deafening, the nonauditory influence on CN is significantly enhanced. One undesirable consequence of enhanced glutamatergic inputs could be the increased spontaneous rates in CN neurons that occur after hearing loss and that have been proposed as correlates of the phantom auditory sensations commonly called tinnitus.

Not Just for Bimodal Neurons Anymore: The Contribution of Unimodal Neurons to Cortical Multisensory Processing

Traditionally, neuronal studies of multisensory processing proceeded by first identifying neurons that were overtly multisensory (e.g., bimodal, trimodal) and then testing them. In contrast, the present study examined, without precondition, neurons in an extrastriate visual area of the cat for their responses to separate (visual, auditory) and combined-modality (visual and auditory) stimulation. As expected, traditional bimodal forms of multisensory neurons were identified. In addition, however, many neurons that were activated only by visual stimulation (i.e., unimodal) had that response modulated by the presence of an auditory stimulus. Some unimodal neurons showed multisensory responses that were statistically different from their visual response. Other unimodal neurons had subtle multisensory effects that were detectable only at the population level. Most surprisingly, these non-bimodal neurons generated more than twice the multisensory signal in the PLLS than did the bimodal neurons. These results expand the range of multisensory convergence patterns beyond that of the bimodal neuron. However, rather than characterize a separate class of multisensory neurons, unimodal multisensory neurons may actually represent an intermediary form of multisensory convergence that exists along the functional continuum between unisensory neurons, at one end, and fully bimodal neurons at the other.

Multisensory Enhancement in the Optic Tectum of the Barn Owl: Spike Count and Spike Timing

Temporal and spatial correlations between auditory and visual stimuli facilitate the perception of unitary events and improve behavioral responses. However, it is not clear how combined visual and auditory information is processed in single neurons. Here we studied responses of multisensory neurons in the barn owl's optic tectum (the avian homologue of the superior colliculus) to visual, auditory, and bimodal stimuli. We specifically focused on responses to sequences of repeated stimuli. We first report that bimodal stimulation tends to elicit more spikes than in the responses to its unimodal components (a phenomenon known as multisensory enhancement). However, this tendency was found to be history-dependent; multisensory enhancement was mostly apparent in the first stimulus of the sequence and to a much lesser extent in the subsequent stimuli. Next, a vector-strength analysis was applied to quantify the phase locking of the responses to the stimuli. We report that in a substantial number of multisensory neurons responses to sequences of bimodal stimuli elicited spike trains that were better phase locked to the stimulus than spike trains elicited by stimulating with the unimodal counterparts (visual or auditory). We conclude that multisensory enhancement can be manifested in better phase locking to the stimulus as well as in more spikes.

Subthreshold multisensory processing in cat auditory cortex

For multisensory processing to occur, inputs from different sensory modalities must converge onto individual neurons. Traditionally, multisensory neurons have been identified as those which were independently activated by more than one sensory modality. Recently, a different multisensory search paradigm revealed neurons in somatosensory and visual cortex that were activated by only a single modality, whose responses were significantly affected by the presence of a second modality cue. The present experiments recorded neurons in the cat auditory field of the anterior ectosylvian sulcus that also met this criterion, suggesting that subthreshold forms of multisensory processing may represent a general feature of multisensory systems.

Adaptation in multisensory neurons: Impact on cross-modal enhancement

Adaptation is a ubiquitous property of sensory neurons. Multisensory neurons, receiving convergent input from different sensory modalities, also likely exhibit adaptation. The responses of multisensory superior colliculus neurons have been extensively studied, but the impact of adaptation on these responses has not been examined. Multisensory neurons in the superior colliculus exhibit cross-modal enhancement, an often non-linear and non-additive increase in response when a stimulus in one modality is paired with a stimulus in a different modality. We examine the possible impact of adaptation on cross-modal enhancement within the framework of a simple model of adaptation for a neuron employing a saturating, logistic response function. We consider how adaptation to an input's mean and standard deviation affects cross-modal enhancement, and also how the statistical correlations between two different modalities influence cross-modal enhancement. We determine the optimal bimodal stimuli to present a bimodal neuron that evoke the largest changes in cross-modal enhancement under adaptation to input statistics. The model requires separate gains for each modality, unless the statistics specific to each modality have been standardised by prior adaptation in earlier, unisensory neurons. The model also predicts that increasing the correlation coefficient between two modalities reduces a multisensory neuron's overall gain.

Audiovisual integration in human superior temporal sulcus: Inverse effectiveness and the neural processing of speech and object recognition

The superior temporal sulcus (STS) is a region involved in audiovisual integration. In non-human primates, multisensory neurons in STS display inverse effectiveness. In two fMRI studies using multisensory tool and speech stimuli presented at parametrically varied levels of signal strength, we show that the pattern of neural activation in human STS is also inversely effective. Although multisensory tool-defined and speech-defined regions of interest were non-overlapping, the pattern of inverse effectiveness was the same for tools and speech across regions. The findings suggest that, even though there are sub-regions in STS that are speech-selective, the manner in which visual and auditory signals are integrated in multisensory STS is not specific to speech.

Ubiquitous Crossmodal Stochastic Resonance in Humans: Auditory Noise Facilitates Tactile, Visual and Proprioceptive Sensations

BackgroundStochastic resonance is a nonlinear phenomenon whereby the addition of noise can improve the detection of weak stimuli. An optimal amount of added noise results in the maximum enhancement, whereas further increases in noise intensity only degrade detection or information content. The phenomenon does not occur in linear systems, where the addition of noise to either the system or the stimulus only degrades the signal quality. Stochastic Resonance (SR) has been extensively studied in different physical systems. It has been extended to human sensory systems where it can be classified as unimodal, central, behavioral and recently crossmodal. However what has not been explored is the extension of this crossmodal SR in humans. For instance, if under the same auditory noise conditions the crossmodal SR persists among different sensory systems.Methodology/Principal FindingsUsing physiological and psychophysical techniques we demonstrate that the same auditory noise can enhance the sensitivity of tactile, visual and propioceptive system responses to weak signals. Specifically, we show that the effective auditory noise significantly increased tactile sensations of the finger, decreased luminance and contrast visual thresholds and significantly changed EMG recordings of the leg muscles during posture maintenance.Conclusions/SignificanceWe conclude that crossmodal SR is a ubiquitous phenomenon in humans that can be interpreted within an energy and frequency model of multisensory neurons spontaneous activity. Initially the energy and frequency content of the multisensory neurons' activity (supplied by the weak signals) is not enough to be detected but when the auditory noise enters the brain, it generates a general activation among multisensory neurons of different regions, modifying their original activity. The result is an integrated activation that promotes sensitivity transitions and the signals are then perceived. A physiologically plausible model for crossmodal stochastic resonance is presented.

Multisensory Integration in Macaque Visual Cortex Depends on Cue Reliability

Responses of multisensory neurons to combinations of sensory cues are generally enhanced or depressed relative to single cues presented alone, but the rules that govern these interactions have remained unclear. We examined integration of visual and vestibular self-motion cues in macaque area MSTd in response to unimodal as well as congruent and conflicting bimodal stimuli in order to evaluate hypothetical combination rules employed by multisensory neurons. Bimodal responses were well fit by weighted linear sums of unimodal responses, with weights typically less than one (subadditive). Surprisingly, our results indicate that weights change with the relative reliabilities of the two cues: visual weights decrease and vestibular weights increase when visual stimuli are degraded. Moreover, both modulation depth and neuronal discrimination thresholds improve for matched bimodal compared to unimodal stimuli, which might allow for increased neural sensitivity during multisensory stimulation. These findings establish important new constraints for neural models of cue integration.

Subthreshold auditory inputs to extrastriate visual neurons are responsive to parametric changes in stimulus quality: Sensory-specific versus non-specific coding

A new subthreshold form of multisensory processing has been recently identified that results from the convergence of suprathreshold excitatory inputs from one modality with subthreshold inputs from another. Because of the subthreshold nature of the second modality, descriptive measures of sensory features such as receptive field properties or location are not directly apparent as they are for traditional bimodal neurons. This raises the question of whether or not subthreshold signals actually convey sensory-specific receptive field information as seen in their bimodal counterparts, or if they represent non-specific effects such as arousal. The present experiment addressed this issue in visually-responsive neurons from the cat posterolateral lateral suprasylvian cortex (PLLS). Single-unit electrophysiological techniques were used to record neuronal responses to visual, auditory and combined visual–auditory stimuli while the intensity of stimulation in the subthreshold auditory modality was systematically altered. The results showed that subthreshold multisensory neurons were sensitive to changes in auditory stimulus intensity. These receptive field sensitivities are similar to those observed in bimodal neurons and thereby represent sensory-specific, not arousal-related responses. In addition, these results provide further support for the notion that multisensory processing occurs along a dynamic continuum of neuronal convergence patterns from bimodal to purely sensory-specific.

Spatial Heterogeneity of Cortical Receptive Fields and Its Impact on Multisensory Interactions

Investigations of multisensory processing at the level of the single neuron have illustrated the importance of the spatial and temporal relationship of the paired stimuli and their relative effectiveness in determining the product of the resultant interaction. Although these principles provide a good first-order description of the interactive process, they were derived by treating space, time, and effectiveness as independent factors. In the anterior ectosylvian sulcus (AES) of the cat, previous work hinted that the spatial receptive field (SRF) architecture of multisensory neurons might play an important role in multisensory processing due to differences in the vigor of responses to identical stimuli placed at different locations within the SRF. In this study the impact of SRF architecture on cortical multisensory processing was investigated using semichronic single-unit electrophysiological experiments targeting a multisensory domain of the cat AES. The visual and auditory SRFs of AES multisensory neurons exhibited striking response heterogeneity, with SRF architecture appearing to play a major role in the multisensory interactions. The deterministic role of SRF architecture was tightly coupled to the manner in which stimulus location modulated the responsiveness of the neuron. Thus multisensory stimulus combinations at weakly effective locations within the SRF resulted in large (often superadditive) response enhancements, whereas combinations at more effective spatial locations resulted in smaller (additive/subadditive) interactions. These results provide important insights into the spatial organization and processing capabilities of cortical multisensory neurons, features that may provide important clues as to the functional roles played by this area in spatially directed perceptual processes.

Spatiotemporal architecture of cortical multisensory interactions: New insights into old rules

Event Abstract Back to Event Spatiotemporal architecture of cortical multisensory interactions: New insights into old rules Mark Wallace1, 2, 3* 1 Vanderbilt University, Department of Hearing & Speech Sciences, United States 2 Vanderbilt University, Department of Psychology, United States 3 Vanderbilt University, Vanderbilt Kennedy Center, United States Neurons that receive convergent input from multiple sensory modalities are found at virtually all levels of the neuraxis. Rather than simply serving as passive filters, these multisensory neurons actively transform their different sensory inputs to give rise to responses that often differ dramatically from the individual unisensory responses (or their simple summation). Such changes in neural encoding have been suggested to form the substrate for the behavioral and perceptual alterations that can be observed under multisensory conditions. Prior work has established that both the sign (i.e., enhancement vs. depression) and magnitude of the interaction that results from a multisensory combination is critically dependent on the spatial and temporal relationship of the paired stimuli, as well as on their relative effectiveness in eliciting a response. However, despite the profound insights that they offer, recent work has shown that the spatial, temporal and inverse effectiveness “principles” of multisensory integration often fail to predict or capture the complete interactive profile of the neuron under study. Recent observations from our laboratory have suggested that a solution to this problem may lie in the receptive field architecture of multisensory neurons, and in the way that space, time and effectiveness interact to shape the resultant multisensory interaction. Single-unit recordings from individual multisensory neurons in both the brainstem (i.e., the superior colliculus) and cortex (i.e., the anterior ectosylvian sulcus) of the cat have revealed a complex and heterogeneous spatial architecture to the receptive fields of multisensory neurons, often with areas of robust response being surrounded by areas of weak response. Furthermore, this heterogeneity can be shared across different sensory modalities (e.g., similar visual and auditory spatial receptive fields [SRFs]), or it can differ dramatically between these modalities. Perhaps most importantly, because of its effects on the magnitude of the unisensory responses, SRF heterogeneity appeared to be a major determinant of the resultant multisensory interaction. For example, pairings at locations of minimal efficiacy resulted in large superadditive interactions, whereas those at more effective locations gave rise to additive or subadditive interactions. Such a result makes the provocative suggestion that space matters in multisensory interactions only as a result of its effects on the vigor of the evoked responses. Furthermore, when examined in the temporal dimension, the creation of spatiotemporal receptive fields (STRFs) for subcortical and cortical multisensory neurons illustrate a previously unrecognized dynamism to multisensory interactions that can dramatically alter both the latency and duration of the evoked discharges. Taken together, these results reveal a previously unexplored level of complexity to multisensory processes that is likely to provide important insights into the transformation and encoding of multisensory information and its role in guiding behavior and shaping perception. Supported by NIH MH63861 and the Vanderbilt Kennedy Center Conference: 10th International Conference on Cognitive Neuroscience, Bodrum, Turkey, 1 Sep - 5 Sep, 2008. Presentation Type: Oral Presentation Topic: Symposium 3: Neural basis of multisensory integration Citation: Wallace M (2008). Spatiotemporal architecture of cortical multisensory interactions: New insights into old rules. Conference Abstract: 10th International Conference on Cognitive Neuroscience. doi: 10.3389/conf.neuro.09.2009.01.021 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 27 Nov 2008; Published Online: 27 Nov 2008. * Correspondence: Mark Wallace, Vanderbilt University, Department of Hearing & Speech Sciences, Nashville,TN, United States, mark.wallace@vanderbilt.edu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Mark Wallace Google Mark Wallace Google Scholar Mark Wallace PubMed Mark Wallace Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. 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Dorsal cochlear nucleus responses to somatosensory stimulation are enhanced after noise‐induced hearing loss

Multisensory neurons in the dorsal cochlear nucleus (DCN) achieve their bimodal response properties [Shore (2005) Eur. J. Neurosci., 21, 3334-3348] by integrating auditory input via VIIIth nerve fibers with somatosensory input via the axons of cochlear nucleus granule cells [Shore et al. (2000) J. Comp. Neurol., 419, 271-285; Zhou & Shore (2004)J. Neurosci. Res., 78, 901-907]. A unique feature of multisensory neurons is their propensity for receiving cross-modal compensation following sensory deprivation. Thus, we investigated the possibility that reduction of VIIIth nerve input to the cochlear nucleus results in trigeminal system compensation for the loss of auditory inputs. Responses of DCN neurons to trigeminal and bimodal (trigeminal plus acoustic) stimulation were compared in normal and noise-damaged guinea pigs. The guinea pigs with noise-induced hearing loss had significantly lower thresholds, shorter latencies and durations, and increased amplitudes of response to trigeminal stimulation than normal animals. Noise-damaged animals also showed a greater proportion of inhibitory and a smaller proportion of excitatory responses compared with normal. The number of cells exhibiting bimodal integration, as well as the degree of integration, was enhanced after noise damage. In accordance with the greater proportion of inhibitory responses, bimodal integration was entirely suppressive in the noise-damaged animals with no indication of the bimodal enhancement observed in a sub-set of normal DCN neurons. These results suggest that projections from the trigeminal system to the cochlear nucleus are increased and/or redistributed after hearing loss. Furthermore, the finding that only neurons activated by trigeminal stimulation showed increased spontaneous rates after cochlear damage suggests that somatosensory neurons may play a role in the pathogenesis of tinnitus.

Cortex Mediates Multisensory But Not Unisensory Integration in Superior Colliculus

Converging cortical influences from the anterior ectosylvian sulcus and the rostral lateral suprasylvian sulcus were shown to have a multisensory-specific role in the integration of sensory information in superior colliculus (SC) neurons. These observations were based on changes induced by cryogenic deactivation of these cortico-SC projections. Thus, although the results indicated that they played a critical role in integrating SC responses to stimuli derived from different senses (i.e., visual-auditory), they played no role in synthesizing its responses to stimuli derived from within the same sense (visual-visual). This was evident even in the same multisensory neurons. The results suggest that very different neural circuits have evolved to code combinations of cross-modal and within-modal stimuli in the SC, and that the differences in multisensory and unisensory integration are likely caused by differences in the configuration of each neuron's functional inputs rather than to any inherent differences among the neurons themselves. The specificity of these descending influences was also apparent in the very different ways in which they affected responses to the component cross-modal stimuli and their actual integration. Furthermore, they appeared to target only multisensory neurons and not their unisensory neighbors.

Visual Deprivation Alters the Development of Cortical Multisensory Integration

It has recently been demonstrated that the maturation of normal multisensory circuits in the cortex of the cat takes place over an extended period of postnatal life. Such a finding suggests that the sensory experiences received during this time may play an important role in this developmental process. To test the necessity of sensory experience for normal cortical multisensory development, cats were raised in the absence of visual experience from birth until adulthood, effectively precluding all visual and visual-nonvisual multisensory experiences. As adults, semichronic single-unit recording experiments targeting the anterior ectosylvian sulcus (AES), a well-defined multisensory cortical area in the cat, were initiated and continued at weekly intervals in anesthetized animals. Despite having very little impact on the overall sensory representations in AES, dark-rearing had a substantial impact on the integrative capabilities of multisensory AES neurons. A significant increase was seen in the proportion of multisensory neurons that were modulated by, rather than driven by, a second sensory modality. More important, perhaps, there was a dramatic shift in the percentage of these modulated neurons in which the pairing of weakly effective and spatially and temporally coincident stimuli resulted in response depressions. In normally reared animals such combinations typically give rise to robust response enhancements. These results illustrate the important role sensory experience plays in shaping the development of mature multisensory cortical circuits and suggest that dark-rearing shifts the relative balance of excitation and inhibition in these circuits.

Sensory and multisensory representations within the cat rostral suprasylvian cortex

Because the posterior limb of the rostral suprasylvian sulcus (RSp) of the cat resides in close proximity to representations of the somatosensory, auditory, and visual modalities, the surrounding cortices would be expected to be a region where a high degree of multisensory convergence and integration is found. The present experiments tested this notion by using anatomical and electrophysiological methods. Tracer injections into somatosensory, auditory, and visual cortical areas almost all produced terminal labeling within the RSp, albeit at different locations and in different proportions. Inputs from somatosensory cortices primarily targeted the inner portion of the anterior RSp; inputs from auditory cortices generally filled the outer portion of the middle and posterior RSp; inputs from visual cortices terminated in the inner portion of the posterior RSp. These projections did not have sharp borders but often overlapped one another, thereby providing a substrate for multisensory convergence. Electrophysiological recordings confirmed this anatomical organization as well as identifying the presence of multisensory (bimodal) neurons in the areas of overlap between representations. Curiously, however, the proportion of bimodal neurons was only 24% of the neurons sampled in this region, and the majority of these did not show multisensory interactions when combined-modality stimuli were presented. In summary, these experiments indicate that the RSp is primarily auditory in nature, but this representation could be further subdivided into an outer sulcal anterior auditory field (sAAF) and an inner field of the rostral suprasylvian sulcus (FRS).