Dendritic spine density in multisensory versus primary sensory cortex

In sensory areas, neuronal dendritic spines receive sensory‐specific inputs whose net activity drives neuronal spiking responses to effective external stimuli. Previous studies indicate that neurons in primary sensory cortical areas, which largely receive inputs from a single sensory modality, exhibit an average of 0.5–1.4 dendritic spines/μm, depending on species. In higher‐order, associational cortices, inputs converge from multiple sensory sources onto individual, multisensory neurons. This raises the question: when inputs from two different modalities converge onto individual neurons, how are the dendritic spines apportioned to subserve the generation of robust spiking responses to each modality? As inputs arrive from two different sensory sources, it might be expected that neurons in multisensory areas exhibit approximately double the spine density of neurons in areas that receive just one sensory input. The present study examined this possibility in Golgi‐stained neurons from ferret primary auditory (A1) and somatosensory (S1) cortices, as well as from regions in which inputs from two different sensory modalities converge: the lateral rostral suprasylvian sulcus (LRSS) and the rostral posterior parietal (PPr) areas. Dendritic spine density (spines/μm) was measured for pyramidal neurons in layers 2–3 and layers 5–6 for each cortical area from three animals using light microscopy. Primary sensory regions A1 and S1 showed remarkably similar average spine densities (A1 = 1.27 spines/μm ± 0.3 s.d.; S1 = 1.14 spines/μm ± 0.3), but average spine densities from the multisensory areas were lower (LRSS = 0.98 ± 0.3; PPr = 1.04 ± 0.3). Thus, for a given cortical area, dendritic spine density appears to be determined by factors other than the levels of sensory modality convergence. Synapse, 2012. © 2012 Wiley Periodicals, Inc.     

The timing and laminar profile of converging inputs to multisensory areas of the macaque neocortex

Two fundamental requirements for multisensory integration are convergence of unisensory (e.g. visual and auditory) inputs and temporal alignment of the neural responses to convergent inputs. We investigated the anatomic mechanisms of multisensory convergence by examining three areas in which convergence occurs, posterior auditory association cortex, superior temporal polysensory area (STP) and ventral intraparietal sulcus area (VIP). The first of these was recently shown to be a site of multisensory convergence and the latter two are more well known as 'classic' multisensory regions. In each case, we focused on defining the laminar profile of response to the unisensory inputs. This information is useful because two major types of connection, feedforward and feedback, have characteristic differences in laminar termination patterns, which manifest physiologically. In the same multisensory convergence areas we also examined the timing of the unisensory inputs using the same standardized stimuli across all recordings. Our findings indicate that: (1) like somatosensory input [J. Neurophysiol., 85 (2001) 1322], visual input is available at very early stages of auditory processing, (2) convergence occurs through feedback, as well as feedforward anatomical projections and (3) input timing may be an asset, as well as a constraint in multisensory processing.     

Multiple sensory afferents to ferret pseudosylvian sulcal cortex

While the ferret cerebral cortex is being used with increasing frequency in studies of neural processing and development, little is known regarding the organization of its associational sensory and multisensory regions. Therefore, the present investigation used neuroanatomical methods to identify non-primary visual and somatosensory representations and their potential for multisensory convergence. Tracer injections made into V1 or SI cortex labeled axon terminals within the pseudosylvian sulcal cortex (PSSC). These inputs were distributed according to modality, with visual inputs identified in the lateral aspects of the posterior dorsal bank, and somatosensory inputs found anterior along the dorsal bank, fundus and ventral bank. Somatosensory inputs showed a topographic arrangement, with inputs representing face found more anteriorly than those representing trunk regions. Overlap between these different sensory projections occurred posteriorly in the PSSC and may represent a zone of multisensory convergence. These data are consistent with the presence of associational visual, somatosensory, and multisensory areas within the PSSC.     

Tonotopic organization in auditory cortex of the cat

Microelectrode mapping techniques were employed in the cat's auditory cortex to relate the best frequencies of a large population of neurons with their spatial loci. Based upon the best-frequency distribution, the auditory region was divided into four complete and orderly tonotopic representations and a surrounding belt of cortex in which the tonotopic organization was more complex. The four auditory fields occupy a crescent-shaped band of tissue which comprises portions of both the exposed gyral surfaces and sulcal banks of the ectosylvian cortex. The anterior auditory field (A) is situated most rostrally upon the anterior ectosylvian gyrus. It extends upon the ventral bank of the suprasylvian sulcus and upon the banks of the anterior ectosylvian sulcus. Adjoining field A caudally is the primary auditory field (AI), which extends across the middle ectosylvian gyrus and portions of both banks of the posterior ectosylvian sulcus. The representations of the highest best frequencies in fields A and AI are contiguous. Caudal and ventral to AI are located the posterior (P) and ventroposterior (VP) auditory fields. They lie mainly upon the caudal bank of the posterior ectosylvian sulcus but also extend upon the rostral bank and upon the posterior ectosylvian gyrus. The low best-frequency representations of fields AI and P are contiguous, whereas the low best-frequency representation of field VP lies near the ventral end of the posterior ectosylvian sulcus. Fields P and VP are joined along their middle and high best-frequency representations. Within each auditory field isofrequency lines defined by the spatial loci of neurons with similar best frequencies are oriented orthogonal to the low-to-high best-frequency gradients.     

Cerebral areas mediating visual redirection of gaze: cooling deactivation of 15 loci in the cat

In humans, damage to posterior parietal or frontal cortices often induces a severe impairment of the ability to redirect gaze to visual targets introduced into the contralateral field. In cats, unilateral deactivation of the posterior middle suprasylvian (pMS) sulcus in the posterior inferior parietal region also results in an equally severe impairment of visually mediated redirection of gaze. In this study we tested the contributions of the pMS cortex and 14 other cortical regions in mediating redirection of gaze to visual targets in 31 adult cats. Unilateral cooling deactivation of three adjacent regions along the posterior bend of the suprasylvian sulcus (posterior middle suprasylvian sulcus, posterior suprasylvian sulcus, and dorsal posterior ectosylvian gyrus at the confluence of the occipital, parietal, and temporal cortices) eliminated visually mediated redirection of gaze towards stimuli introduced into the contralateral hemifield, while the redirection of gaze toward the ipsilateral hemifield remained highly proficient. Additional cortical loci critical for visually mediated redirection of gaze include the anterior suprasylvian gyrus (lateral area 5, anterior inferior parietal cortex) and medial area 6 in the frontal region. Cooling deactivation of: 1) dorsal or 2) ventral posterior suprasylvian gyrus; 3) ventral posterior ectosylvian gyrus, 4) middle ectosylvian gyrus; 5) anterior or 6) posterior middle suprasylvian gyrus (area 7); 7) anterior middle suprasylvian sulcus; 8) medial area 5; 9) the visual portion of the anterior ectosylvian sulcus (AES); 10) or lateral area 6 were all without impact on the ability to redirect gaze. In summary, we identified a prominent field of cortex at the junction of the temporo-occipito-parietal cortices (regions pMS, dPE, PS), an anterior inferior parietal field (region 5L), and a frontal field (region 6M) that all contribute critically to the ability to redirect gaze to novel stimuli introduced into the visual field during fixation. These loci have several features in common with cortical fields in monkey and human brains that contribute to the visually guided redirection of the head and eyes.     

Two corticotectal areas facilitate multisensory orientation behavior

It had previously been shown that influences from two cortical areas, the anterior ectosylvian sulcus (AES) and the rostral lateral suprasylvian sulcus (rLS), play critical roles in rendering superior colliculus (SC) neurons capable of synthesizing their cross-modal inputs. The present studies examined the consequences of selectively eliminating these cortical influences on SC-mediated orientation responses to cross-modal stimuli. Cats were trained to orient to a low-intensity modality-specific cue (visual) in the presence or absence of a neutral cue from another modality (auditory). The visual target could appear at various locations within 45 degrees of the midline, and the stimulus effectiveness was varied to yield an average of correct orientation responses of approximately 45%. Response enhancement and depression were observed when the auditory cue was coupled with the target stimulus: A substantially enhanced probability in correct responses was evident when the cross-modal stimuli were spatially coincident, and a substantially decreased response probability was obtained when the stimuli were spatially disparate. Cryogenic blockade of either AES or rLS disrupted these behavioral effects, thereby eliminating the enhanced performance in response to spatially coincident cross-modal cues and degrading the depressed performance in response to spatially disparate cross-modal cues. These disruptive effects on targets contralateral to the deactivated cortex were restricted to multisensory interactive processes. Orientation to modality-specific targets was unchanged. Furthermore, the pattern of orientation errors was unaffected by cortical deactivation. These data bear striking similarities to the effects of AES and rLS deactivation on multisensory integration at the level of individual SC neurons. Presumably, eliminating the critical influences from AES or rLS cortex disrupts SC multisensory synthesis that, in turn, disables SC-mediated multisensory orientation behaviors.     

Auditory cortical projection from the anterior ectosylvian sulcus (Field AES) to the superior colliculus in the cat: an anatomical and electrophysiological study

Sensory activity in the deep layers of the superior colliculus (SC) is strongly influenced by descending cortical inputs. Elimination (permanent or reversible) of specific regions of visual or somatosensory cortex, known to have direct access to the SC, abolishes or dramatically reduces SC responses to stimuli from those modalities. While many SC neurons are also responsive to auditory cues, the origin of auditory corticotectal connections is not clear at present and their affect on activity in the SC is unknown. Therefore, the present study was undertaken to determine the origin, organization, and functional characteristics of auditory corticotectal projections. Of the auditory cortices (AI; AII; Fields A, P, and VP), only the auditory subregion of the banks of the anterior ectosylvian sulcus (Field AES) showed a robust anatomical projection to the SC. These data were confirmed physiologically: auditory neurons in Field AES projected to the SC and auditory SC neurons responded to stimulation of the Field AES. However, neither anatomical nor physiological techniques revealed a clear topographic relationship between the Field AES and the SC but suggested instead a diffuse and extremely divergent/convergent projection. Stimulation and cryoblockade of Field AES demonstrated the excitatory nature of this corticotectal pathway, whose influence was most evident on SC responses to stimuli of reduced intensity. Given the short latency of this ear-cortex-SC circuit and its excitatory influence on unimodal as well as on multisensory auditory neurons, it seems likely that Field AES plays a significant role in facilitating SC responses to auditory stimuli.     

Topographical organization of the cortical afferent connections of the prefrontal cortex in the cat

The topographical distribution of the cortical afferent connections of the prefrontal cortex (PFC) in adult cats was studied by using the retrograde axonal transport of horseradish peroxidase technique. Small single injections of the enzyme were made in different locations of the PFC, and the areal location and density of the subsequent neuronal labeling in neocortex and allocortex were evaluated in each case. The comparison of the results obtained in the various cases revealed that four prefrontal sectors (rostral, dorsolateral, ventral, and dorsomedial) can be distinguished, each exhibiting a particular pattern of cortical afferents. All PFC sectors receive projections from the ipsilateral insular (agranular and granular subdivisions) and limbic (infralimbic, prelimbic, anterior limbic, cingular, and retrosplenial areas) cortices. These cortices provide the most abundant cortical projections to the PFC, and their various subdivisions have different preferential targets within the PFC. The premotor cortex and the following neocortical sensory association areas project differentially upon the various ipsilateral PFC sectors: the portion of the somatosensory area SIV in the upper bank of the anterior ectosylvian sulcus, the visual area in the lower bank of the same sulcus, the auditory area AII, the temporal area, the perirhinal cortex, the posterior suprasylvian area, area 20, the posterior ectosylvian area, the suprasylvian fringe, the lateral suprasylvian area (anterolateral and posterolateral subdivisions), area 5, and area 7. The olfactory peduncle, the prepiriform cortex, the cortico-amygdaloid transition area, the entorhinal cortex, the subiculum (ventral, posteroventral, and posterodorsal sectors), the caudomedial band of the hippocampal formation and the postsubiculum are the allocortical sources of afferents to the PFC. The dorsolateral PFC sector is the target of the largest insular, limbic, and neocortical sensory association projections. The dorsomedial and rostral sectors receive notably less abundant cortical afferents than the dorsolateral sector. Those to the dorsomedial sector arise from the same areas that project to the dorsolateral sector and are more abundant to the dorsal part, where the medial frontal eye field cortex is located. The rostral sector receives projections principally from all other PFC sectors, and from the limbic and insular cortices. The projections from the allocortex reach preferentially the ventral PFC sector. Intraprefrontal connections are most abundant within each PFC sector. Commissural interprefrontal connections are largest from the site homotopic to the HRP injection.(ABSTRACT TRUNCATED AT 400 WORDS)     

Reciprocal connections between olfactory structures and the cortex of the rostral superior temporal sulcus in the Macaca fascicularis monkey

Convergence of sensory modalities in the nonhuman primate cerebral cortex is still poorly understood. We present an anatomical tracing study in which polysensory association cortex located at the fundus and upper bank of the rostral superior temporal sulcus presents reciprocal connections with primary olfactory structures. At the same time, projections from this polysensory area reach multiple primary olfactory centres. Retrograde (Fast Blue) and anterograde (biotinylated dextran-amine and 3H-amino acids) tracers were injected into primary olfactory structures and rostral superior temporal sulcus. Retrograde tracers restricted to the anterior olfactory nucleus resulted in labelled neurons in the rostral portion of the upper bank and fundus of superior temporal sulcus. Injections of biotinylated dextran-amine at the fundus and upper bank of the superior temporal sulcus confirmed this projection by labelling axons in the dorsal and lateral portions of the anterior olfactory nucleus, as well as piriform, periamygdaloid and entorhinal cortices. Retrograde tracer injections at the rostral superior temporal sulcus resulted in neuronal labelling in the anterior olfactory nucleus, piriform, periamygdaloid and entorhinal cortices, thus providing confirmation of the reciprocity between primary olfactory structures and the cortex at the rostral superior temporal sulcus. The reciprocal connections between the rostral part of superior temporal sulcus and primary olfactory structures represent a convergence for olfactory and other sensory modalities at the cortex of the rostral temporal lobe.     

Multisensory convergence in auditory cortex, I. Cortical connections of the caudal superior temporal plane in macaque monkeys

The caudal medial auditory area (CM) has anatomical and physiological features consistent with its role as a first-stage (or "belt") auditory association cortex. It is also a site of multisensory convergence, with robust somatosensory and auditory responses. In this study, we investigated the cerebral cortical sources of somatosensory and auditory inputs to CM by injecting retrograde tracers in macaque monkeys. A companion paper describes the thalamic connections of CM (Hackett et al., J. Comp. Neurol. [this issue]). The likely cortical sources of somatosensory input to CM were the adjacent retroinsular cortex (area Ri) and granular insula (Ig). In addition, CM had reliable connections with areas Tpt and TPO, which are sites of multisensory integration. CM also had topographic connections with other auditory areas. As expected, connections with adjacent caudal auditory areas were stronger than connections with rostral areas. Surprisingly, the connections with the core were concentrated along its medial side, suggesting that there may be a medial-lateral division of function within the core. Additional injections into caudal lateral auditory area (CL) and Tpt showed similar connections with Ri, Ig, and TPO. In contrast to CM injections, these lateral injections had inputs from parietal area 7a and had a preferential connection with the lateral (gyral) part of Tpt. Taken together, the findings indicate that CM may receive somatosensory input from nearby areas along the fundus of the lateral sulcus. The differential connections of CM compared with adjacent areas provide additional evidence for the functional specialization of the individual auditory belt areas.     

Parcellation of cortical afferents to three distinct sectors in the parahippocampal gyrus of the rhesus monkey: an anatomical and neurophysiological study

The posterior parahippocampal gyrus (PHG) of the rhesus monkey (Macaca mulatta) is comprised of three distinct cortical areas based on cytoarchitecture, connectivity, and neurophysiological response properties. Fluorescent retrograde tracers placed in each PHG area demonstrated unique patterns of cortical afferent input to areas TH, TL, and TF. Area TF receives inputs from the multimodal cortices of the superior temporal sulcus including areas PGa, TPO, and MST, from the visuospatial parietal area PG-Opt, and from visual areas V3A and dorsal V4. Area TL receives afferents from the inferotemporal region including visual areas TE1 and TE2 as well as from areas TEa, IPa, and FST in the lower bank and depth of the superior temporal sulcus. In contrast, the input to area TH is from the rostral part of superior temporal gyrus, including the auditory association areas TS1-3, and from the middle sector of area TPO in the superior temporal sulcus. Frontal and cingulate areas also project to the PHG in largely differential patterns. To further investigate this a correlative electrophysiological study of the three PHG areas resulted in a confirmation of these differential cortical inputs such that visually responsive neurons were found in areas TF and TL, auditory responsive neurons or bimodal auditory/visual-responsive neurons in area TH, and somatosensory-responsive neurons at the TF/TL border. Since each PHG area also receives differential hippocampal input, these data suggest that the processing of unimodal or multimodal information may be related to memory processing functions that are largely segregated within areas TH, TL, and TF.     

Heteromodal connections supporting multisensory integration at low levels of cortical processing in the monkey

While multisensory integration is thought to occur in higher hierarchical cortical areas, recent studies in man and monkey have revealed plurisensory modulations of activity in areas previously thought to be unimodal. To determine the cortical network involved in multisensory interactions, we performed multiple injections of different retrograde tracers in unimodal auditory (core), somatosensory (1/3b) and visual (V2 and MT) cortical areas of the marmoset. We found three types of heteromodal connections linking unimodal sensory areas. Visuo-somatosensory projections were observed originating from visual areas [probably the ventral and dorsal fundus of the superior temporal area (FSTv and FSTd), and middle temporal crescent (MTc)] toward areas 1/3b. Somatosensory projections to the auditory cortex were present from S2 and the anterior bank of the lateral sulcus. Finally, a visuo-auditory projection arises from an area anterior to the superior temporal sulcus (STS) toward the auditory core. Injections in different sensory regions allow us to define the frontal convexity and the temporal opercular caudal cortex as putative polysensory areas. A quantitative analysis of the laminar distribution of projecting neurons showed that heteromodal connections could be either feedback or feedforward. Taken together, our results provide the anatomical pathway for multisensory integration at low levels of information processing in the primate and argue against a strict hierarchical model.     

The anterior ectosylvian sulcal auditory field in the cat: II. A horseradish peroxidase study of its thalamic and cortical connections.

The thalamic and cortical projections to acoustically responsive regions of the anterior ectosylvian sulcus were determined by identifying retrogradely labelled cells after physiologically guided iontophoretic injections of horseradish peroxidase. The medial division of the medial geniculate nucleus, the intermediate division of the posterior nuclear group, the principal division of the ventromedial nucleus, and the lateroposterior complex were consistently labelled after these injections, although each animal showed slightly different patterns of labelling. The suprageniculate nucleus and the lateral and medial divisions of the posterior nuclear group were also labelled in most experiments. The cortex of the suprasylvian sulcus was the most consistently and densely labelled cortical region; each experiment showed a slightly different pattern of labelling throughout the suprasylvian sulcus, with an overall tendency for greater labelling in the ventral (lateral) bank of the middle region of the sulcus. Other cortical regions labelled less consistently included the anterior ectosylvian sulcus itself, the insular cortex of the anterior sylvian gyrus, and the posterior rhinal sulcus. In three experiments the contralateral cortex was examined and a small number of labelled cells was located in the anterior ectosylvian and suprasylvian sulci. Input from extralemniscal auditory thalamus is compatible with previously described auditory response properties of anterior ectosylvian sulcus neurons. The results also confirm the presence of input from visual and multimodal regions of thalamus and cortex, and therefore support claims of overlap of modalities within the sulcus. This overlap, as well as input from motor regions, suggests that the anterior ectosylvian sulcal field serves a sensorimotor role.     

Anterior ectosylvian cortical projections to the rostral suprasylvian multisensory zone in cat

Recent studies have shown that the anterior ectosylvian sulcal cortex (AESc) and the rostral suprasylvian sulcal cortex (RSSSc) of the cat play integral roles in behavioral and collicular responses to multisensory stimuli. However, substantially more multisensory superior colliculus (SC) neurons are affected by blockade of the AESc than the RSSSc. Although both cortical regions project directly to the SC, a possible explanation for this differential effect is that the AESc may also relay an indirect corticotectal signal via the RSSSc that is reduced when the AESc is deactivated. This possibility was examined by placing orthograde tracer in the auditory field AES (FAES), visual AEV, or between these two regions of the AESc. FAES injections produced labeled boutons in the posterior-lateral bank of the RSSSc, while those placed in AEV failed to label the RSSSc. However, injections between the FAES and AEV regions revealed terminal label in both the posterior lateral bank and fundus. These observations and other studies showing connections between somatosensory portions of the AESc and RSSSc are consistent with the hypothesis that signals from the AESc can take both direct and indirect (through the RSSSc) corticotectal routes to influence processing in the SC.     

Response amplification in sensory-specific cortices during crossmodal binding

Integrating information across the senses can enhance our ability to detect and classify stimuli in the environment. For example, auditory speech perception is substantially improved when the speaker's face is visible. In an fMRI study designed to investigate the neural mechanisms underlying these crossmodal behavioural gains, bimodal (audio-visual) speech was contrasted against both unimodal (auditory and visual) components. Significant response enhancements in auditory (BA 41/42) and visual (V5) cortices were detected during bimodal stimulation. This effect was found to be specific to semantically congruent crossmodal inputs. These data suggest that the perceptual improvements effected by synthesizing matched multisensory inputs are realised by reciprocal amplification of the signal intensity in participating unimodal cortices.     

Visual and somatosensory integration in the anterior ectosylvian cortex of the cat

We recorded from single neurons in both banks of the posterior two-thirds of the anterior ectosylvian sulcus. All neurons were tested with visual and tactile stimulations. In each bank of the anterior ectosylvian sulcus the majority of neurons were bimodal, i.e. responded to both visual and tactile stimuli (B cells); the remaining population was strictly unimodal, responding either to visual (V cells) or to somatosensory (T cells) stimulation. Bimodal and unimodal neurons were recorded at all explored cortical sites and were consistently intermixed. Unlike bimodal neurons, unimodal neurons showed an asymmetric localization: the V cells were significantly more numerous in the ventral bank while the T neurons were preferentially found in the dorsal bank of the sulcus. We could not detect an orderly somatotopic or visuotopic representation, nor was it possible to find a systematic spatial correspondence between somatic and visual receptive fields. The functional organization of the anterior ectosylvian cortex is discussed in terms of a hierarchical processing of sensory information.     

Ipsilateral cortical connections of motor, premotor, frontal eye, and posterior parietal fields in a prosimian primate, Otolemur garnetti

The ipsilateral connections of motor areas of galagos were determined by injecting tracers into primary motor cortex (M1), dorsal premotor area (PMD), ventral premotor area (PMV), supplementary motor area (SMA), and frontal eye field (FEF). Other injections were placed in frontal cortex and in posterior parietal cortex to define the connections of motor areas further. Intracortical microstimulation was used to identify injection sites and map motor areas in the same cases. The major connections of M1 were with premotor cortex, SMA, cingulate motor cortex, somatosensory areas 3a and 1, and the rostral half of posterior parietal cortex. Less dense connections were with the second (S2) and parietal ventral (PV) somatosensory areas. Injections in PMD labeled neurons across a mediolateral belt of posterior parietal cortex extending from the medial wall to lateral to the intraparietal sulcus. Other inputs came from SMA, M1, PMV, and adjoining frontal cortex. PMV injections labeled neurons across a large zone of posterior parietal cortex, overlapping the region projecting to PMD but centered more laterally. Other connections were with M1, PMD, and frontal cortex and sparsely with somatosensory areas 3a, 1-2, S2, and PV. SMA connections were with medial posterior parietal cortex, cingulate motor cortex, PMD, and PMV. An FEF injection labeled neurons in the intraparietal sulcus. Injections in posterior parietal cortex revealed that the rostral half receives somatosensory inputs, whereas the caudal half receives visual inputs. Thus, posterior parietal cortex links visual and somatosensory areas with motor fields of frontal cortex.     

The Effects of Early Postnatal Modification of Body Shape on the Somatosensory—Visual Organization in Mouse Superior Colliculus

Different regions of the body of an animal have their own shape and location within visual space. Accordingly, in the superior colliculus there are somatosensory-visual bimodal neurons receiving tactile and visual input from the same region of space. In newborn mice, we changed the position of some body parts within visual space in order to see what happened to the alignment of the somatosensory and visual receptive fields of superior colliculus bimodal neurons. To do this, we modified the shape of the head by displacing the superior vibrissae and the ears, normally in the superior portion of visual space, into the inferior visual space. Analogously, we bent the inferior vibrissae into the superior visual space. At the sixth postnatal week we recorded from somatosensory-visual bimodal neurons of the deep layers of the superior colliculus and found that the tactile and visual receptive fields were aligned. Neurons receiving tactile input from the downward-displaced superior vibrissae and ears showed visual receptive fields in the inferior portion of visual space, whereas neurons receiving input from the upward-displaced inferior vibrissae showed visual receptive fields in the superior visual space. These results show that an experience-dependent interaction between visual and somatosensory inputs occurs during development, and that early exposure to abnormal visual-somatosensory experience modifies the organization of multisensory neurons in the superior colliculus.     

Proprioceptive pathways to posterior parietal areas MIP and LIPv from the dorsal column nuclei and the postcentral somatosensory cortex

The posterior parietal cortex (PPC) serves as an interface between sensory and motor cortices by integrating multisensory signals with motor-related information. Sensorimotor transformation of somatosensory signals is crucial for the generation and updating of body representations and movement plans. Using retrograde transneuronal transfer of rabies virus in combination with a conventional tracer, we identified direct and polysynaptic somatosensory pathways to two posterior parietal areas, the ventral lateral intraparietal area (LIPv) and the rostral part of the medial intraparietal area (MIP) in macaque monkeys. In addition to direct projections from somatosensory areas 2v and 3a, respectively, we found that LIPv and MIP receive disynaptic inputs from the dorsal column nuclei as directly as these somatosensory areas, via a parallel channel. LIPv is the target of minor neck muscle-related projections from the cuneate (Cu) and the external cuneate nuclei (ECu), and direct projections from area 2v, that likely carry kinesthetic/vestibular/optokinetic-related signals. In contrast, MIP receives major arm and shoulder proprioceptive inputs disynaptically from the rostral Cu and ECu, and trisynaptically (via area 3a) from caudal portions of these nuclei. These findings have important implications for the understanding of the influence of proprioceptive information on movement control operations of the PPC and the formation of body representations. They also contribute to explain the specific deficits of proprioceptive guidance of movement associated to optic ataxia.     

Distributed population coding of multisensory spatial information in the associative cortex

This study describes a possible mechanism of coding of multisensory information in the anterior ectosylvian visual area of the feline cortex. Extracellular microelectrode recordings on 168 cells were carried out in the anterior ectosylvian sulcal region of halothane-anaesthetized, immobilized, artificially ventilated cats. Ninety-five neurons were found to respond to visual stimuli, 96 responded to auditory stimuli and 45 were bimodal, reacting to both visual and auditory modalities. A large proportion of the neurons exhibited significantly different responses to stimuli appearing in different regions of their huge receptive field. These neurons have the ability to provide information via their discharge rate on the site of the stimulus within their receptive field. This suggests that they may serve as panoramic localizers. The ability of the bimodal neurons to localize bimodal stimulus sources is better than any of the unimodal localizing functions. Further, the sites of maximal responsivity of the visual, auditory and bimodal neurons are distributed over the whole extent of the large receptive fields. Thus, a large population of such panoramic visual, auditory and multisensory neurons could accurately code the locations of the sensory stimuli. Our findings support the notion that there is a distributed population code of multisensory information in the feline associative cortex.