Auditory-visual interaction in cat orbital-insular cortex

Effects of neonatal deafness on resting-state functional network connectivity

Normal brain development depends on early sensory experience. Behavioral consequences of brain maturation in the absence of sensory input early in life are well documented. For example, experiments with mature, neonatally deaf human or animal subjects have revealed improved peripheral visual motion detection and spatial localization abilities. Such supranormal behavioral abilities in the nondeprived sensory modality are evidence of compensatory plasticity occurring in deprived brain regions at some point or throughout development. Sensory deprived brain regions may simply become unused neural real-estate resulting in a loss of function. Compensatory plasticity and loss of function are likely reflected in the differences in correlations between brain networks in deaf compared with hearing subjects. To address this, we used resting-state functional magnetic resonance imaging (fMRI) in lightly anesthetized hearing and neonatally deafened cats. Group independent component analysis (ICA) was used to identify 20 spatially distinct brain networks across all animals including auditory, visual, somatosensory, cingulate, insular, cerebellar, and subcortical networks. The resulting group ICA components were back-reconstructed to individual animal brains. The maximum correlations between the time-courses associated with each spatial component were computed using functional network connectivity (FNC). While no significant differences in the delay to peak correlations were identified between hearing and deaf cats, we observed 10 (of 190) significant differences in the amplitudes of between-network correlations. Six of the significant differences involved auditory-related networks and four involved visual, cingulate, or somatosensory networks. The results are discussed in context of known behavioral, electrophysiological, and anatomical differences following neonatal deafness. Furthermore, these results identify novel targets for future investigations at the neuronal level.

Reorganization of the connectivity of cortical field DZ in congenitally deaf cat

Psychophysics and brain imaging studies in deaf patients have revealed a functional crossmodal reorganization that affects the remaining sensory modalities. Similarly, the congenital deaf cat (CDC) shows supra-normal visual skills that are supported by specific auditory fields (DZ-dorsal zone and P-posterior auditory cortex) but not the primary auditory cortex (A1). To assess the functional reorganization observed in deafness we analyzed the connectivity pattern of the auditory cortex by means of injections of anatomical tracers in DZ and A1 in both congenital deaf and normally hearing cats. A quantitative analysis of the distribution of the projecting neurons revealed the presence of non-auditory inputs to both A1 and DZ of the CDC which were not observed in the hearing cats. Firstly, some visual (areas 19/20) and somatosensory (SIV) areas were projecting toward DZ of the CDC but not in the control. Secondly, A1 of the deaf cat received a weak projection from the visual lateral posterior nuclei (LP). Most of these abnormal projections to A1 and DZ represent only a small fraction of the normal inputs to these areas. In addition, most of the afferents to DZ and A1 appeared normal in terms of areal specificity and strength of projection, with preserved but smeared nucleotopic gradient of A1 in CDCs. In conclusion, while the abnormal projections revealed in the CDC can participate in the crossmodal compensatory mechanisms, the observation of a limited reorganization of the connectivity pattern of the CDC implies that functional reorganization in congenital deafness is further supported also by normal cortico-cortical connectivity.

Event-related potentials to visual, auditory, and bimodal (combined auditory-visual) stimuli

The purpose of this study was to investigate the response properties of event related potentials to unimodal and bimodal stimulations. The amplitudes of N1 and P2 were larger during bimodal evoked potentials (BEPs) than auditory evoked potentials (AEPs) in the anterior sites and the amplitudes of P1 were larger during BEPs than VEPs especially at the parieto-occipital locations. Responses to bimodal stimulation had longer latencies than responses to unimodal stimulation. The N1 and P2 components were larger in amplitude and longer in latency during the bimodal paradigm and predominantly occurred at the anterior sites. Therefore, the current bimodal paradigm can be used to investigate the involvement and location of specific neural generators that contribute to higher processing of sensory information. Moreover, this paradigm may be a useful tool to investigate the level of sensory dysfunctions in clinical samples.

Focal projections of cat auditory cortex to the pontine nuclei

The pontine nuclei (PN) receive projections from the auditory cortex (AC) and they are a major source of mossy fibers to the cerebellum. However, they have not been studied in detail using sensitive neuroanatomical tracers, and whether all AC areas contribute to the corticopontine (CP) system is unknown. We characterized the projection patterns of 11 AC areas with WGA-HRP. We also compared them with their corticothalamic and corticocollicular counterparts. A third objective was to analyze the structure of the CP axons and their terminals with BDA. Both tracers confirm that all AC areas projected to lateral, central, and medial ipsilateral pontine divisions. The strongest CP projections were from nontonotopic and polymodal association areas. Preterminal fibers formed single terminal fields having many boutons en passant as well as terminal endings, and there was a specific morphological pattern for each pontine target, irrespective of their areal origin. Thus, axons in the medial division had a simpler terminal architecture (type 1 terminal plexus); both the central and lateral pons received more complex endings (type 2 terminal plexus). Auditory CP topographical distribution resembled visual and somatosensory CP projections, which preserve retinotopy and somatotopy in the pons, respectively. However, the absence of pontine tonotopy suggests that the AC projection topography is unrelated to tonotopy. CP input to the medial and central pons coincides with the somatosensory and visual cortical inputs, respectively, and such overlap might subserve convergence in the cerebellum. In contrast, lateral pontine input may be exclusively auditory.

Cross-modal perceptual integration of spatially and temporally disparate auditory and visual stimuli

Under certain conditions, auditory and visual information are integrated into a single unified percept even when they originate in different locations in space. The present study shows how this illusion, known as the ventriloquism effect, depends on spatial, temporal and cognitive factors. A method of psychophysical scaling was employed in combination with simple auditory-visual stimuli (tone bursts and flashing light spots) that were presented with various spatiotemporal disparities. Participants either judged their impression of the likelihood of a common cause (Experiment 1) or spatial alignment (Experiment 2) or synchrony of sound and light (Experiment 3). In all three experiments the participants' judgements depended significantly on temporal disparity whereas influences of spatial disparity were significant in Experiments 1 and 2. Optimum scores were always obtained when auditory stimuli were presented with a delay of 50-100 ms after the visual stimuli. These results demonstrate that both temporal and spatial proximity of the two stimuli are critical for the experience of phenomenal causality. On the other hand, spatio-temporal ranges for optimal perception of phenomenal causality in Experiment 1 were significantly larger than predicted by simultaneous detection of spatial and temporal disparities. This finding suggests that auditory-visual binding was further facilitated by additional, cognitive, factors, associated with the specific instruction to judge the likelihood of a common cause. Obviously, these instructional influences may reflect similar perceptual effects, as have been shown previously by increasing the complexity or cognitive compellingness of auditory-visual stimuli.

Rapid adaptation to auditory-visual spatial disparity

The so-called ventriloquism aftereffect is a remarkable example of rapid adaptative changes in spatial localization caused by visual stimuli. After exposure to a consistent spatial disparity of auditory and visual stimuli, localization of sound sources is systematically shifted to correct for the deviation of the sound from visual positions during the previous adaptation period. In the present study, this aftereffect was induced by presenting, within 17 min, 1800 repetitive noise or pure-tone bursts in combination with synchronized, and 20 degrees disparate flashing light spots, in total darkness. Post-adaptive sound localization, measured by a method of manual pointing, was significantly shifted 2.4 degrees (noise), 3.1 degrees (1 kHz tones), or 5.8 degrees (4 kHz tones) compared with the pre-adaptation condition. There was no transfer across frequencies; that is, shifts in localization were insignificant when the frequencies used for adaptation and the post-adaptation localization test were different. It is hypothesized that these aftereffects may rely on shifts in neural representations of auditory space with respect to those of visual space, induced by intersensory spatial disparity, and may thus reflect a phenomenon of neural short-term plasticity.

On the neuronal basis for multisensory convergence: a brief overview

For multisensory stimulation to effect perceptual and behavioral responses, information from the different sensory systems must converge on individual neurons. A great deal is already known regarding processing within the separate sensory systems, as well as about many of the integrative and perceptual/behavioral effects of multisensory processing. However, virtually nothing is known about the functional architecture that underlies multisensory convergence even though it is an integral step to this processing sequence. This paper seeks to summarize the findings pertinent to multisensory convergence, and to initiate the identification of specific convergence patterns that may underlie different multisensory perceptual and behavioral effects.

Connections of the auditory cortex with the claustrum and the endopiriform nucleus in the cat

We studied the connections of eleven auditory cortical areas with the claustrum and the endopiriform nucleus in the cat, by means of cortical injections of either wheat germ agglutinin conjugated to horseradish peroxidase, or biotinylated dextran amines. Unlike previously accepted reports, all auditory areas have reciprocal connections with the ipsi- and contralateral claustrum, though they differ in strength and/or topography. The areas that send the strongest projections are the intermediate region of the posterior ectosylvian gyrus and the insular cortex, followed by the primary auditory cortex and the dorsal portion of the posterior ectosylvian gyrus. The high degree of convergence of cortical axons in the intermediate region of the claustrum, arising from tonotopic and nontonotopic areas, suggests that claustral neurons are unlikely to be well tuned to the frequency of the acoustic stimulus. Corticoclaustral axons from any given area cover territories largely overlapping with those occupied by the claustrocortical neurons projecting back to the same area. The location of cortically projecting neurons in the claustrum matches the position of the target cortical area in the cerebral hemisphere, both rostrocaudally and dorsoventrally. These findings suggest that the intermediate region of the claustrum integrates inputs from all auditory cortical areas, and then sends the result of such processing back to every auditory cortical field. On the other hand, the endopiriform nucleus, a limbic-related structure thought to play a role in the acquisition of conditioned fear, would process mostly polymodal information, since it only receives projections from the insular and temporal cortices.

Connections of the insular cortex in kittens: an anatomical demonstration with wheatgerm agglutinin conjugated to horseradish peroxidase technique

The postnatal development of the afferent and efferent connections of the feline insular cortex was investigated using wheat germ agglutinin conjugated to horseradish peroxidase method. The overall pattern of connections was found to be substantially the same as that observed in adults. In the cortex, anterograde and retrograde labeling was found in the presylvian sulcus, cingulate gyrus, cruciate sulcus, medial prefrontal area, lateral suprasylvian cortex, and posterior rhinal sulcus. Subcortical regions containing label included the lateralis medialis-suprageniculate nuclear complex, ventral medial thalamic nucleus, claustrum, lateral amygdaloid nucleus, hypothalamus, and raphe nuclei. In addition, axonal labeling was seen in the striatum, superior colliculus, and pontine nuclei. In contrast, the insular associational projections in kittens differed markedly from those found in adults. In the early postnatal period (1-14 days), dense terminal labeling was found in the cortical layers I, V and VI as well as in the underlying white matter, whereas only moderate to sparse labeling was observed in layers II, III and IV. By four-weeks, an adult-like distribution of terminals was present in each cortical areas: labeling was found mainly in layers I, II, III and IV, with less in layers V and VI. The present results suggest that the basic framework of the insular connections is formed prenatally and that the fine tuning of the axonal terminals and the formation of synapses occurs mainly during the first four postnatal weeks.

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.

The anterior ectosylvian sulcal auditory field in the cat: I. An electrophysiological study of its relationship to surrounding auditory cortical fields

The extent of a region containing acoustically responsive neurons within the anterior ectosylvian sulcus and its relationship to surrounding gyral auditory cortical fields was examined in chloralose-anaesthetized cats. Multiple microelectrode penetrations were made orthogonal to the middle and anterior ectosylvian gyral surfaces, and longer penetrations were made into the dorsal and ventral banks and fundus of the anterior ectosylvian sulcus. The quantitative and qualitative auditory response characteristics of neurons and neuron clusters in the sulcal banks and surrounding regions were mapped in detail, and the degree of overlap of auditory and visual neurons within the sulcus was determined by routinely testing for responsiveness to a gross light flash. The detailed results from three animals and a summary of all penetrations into the sulcus are presented. The anterior ectosylvian sulcal field (Field AES) lay deep within the banks and fundus of the posterior three quarters of the sulcus. A combination of changes in the auditory response characteristics of neurons (i.e., in optimal stimulus, latency, and frequency tuning), and the presence of visually responsive cells, distinguished this field from surrounding fields. The distinction between the anterior ectosylvian field and extensions of the nearby tonotopic fields (i.e., primary and anterior auditory fields) into the dorsal and ventral banks of the dorsoposterior sector of the sulcus was readily made on the basis of these characteristics. The distinction between the anterior ectosylvian field and extensions of the second auditory field into the ventral bank of the middle sector of the sulcus was more difficult and there were differences between animals in the transition between these fields. Anterior ectosylvian sulcal field responses did not extend into the dorsal bank in anterior parts of the sulcus but were restricted to fundal regions, an observation consistent with the presence of the fourth somatosensory field in the dorsal bank of this sector of the sulcus. The majority of penetrations into the sulcus revealed coextensive auditory and visual activity, an observation apparently at variance with the identification of a purely visual field in this region. Barbiturate anaesthesia, which has been used in experiments demonstrating an anterior ectosylvian visual area, was found to have a depressing effect on auditory responses within the anterior ectosylvian sulcal field.

Cortical connections of electrophysiologically and architectonically defined subdivisions of auditory cortex in squirrels

Multiunit recordings with microelectrodes were used to identify and delimit subdivision of auditory cortex in squirrels. In the same animals, cortical connections of subdivisions of auditory cortex were determined by placing injections of the tracer wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) into electrophysiologically defined locations. The electrophysiological results and patterns of connections were later related to myeloarchitectonic distinctions in brain sections cut parallel to the surface of the artificially flattened cortex. As previously described (Merzenich et al.: J. Comp. Neurol. 166:387-402, '76), a primary auditory field, A-I, was characterized by (1) neurons narrowly tuned to tone frequency; (2) a tonotopic map with high frequencies, which represented caudal to low frequencies; and (3) dense myelination. A-I was reciprocally connected with a rostral field, R, a parietal ventral somatosensory representation, PV, cortex ventral to A-I, and other nearby regions of cortex of the same hemisphere. Callosal connections of A-I were with A-I, R, and two or more other regions of temporal cortex. The less densely myelinated rostral field, R, also had neurons that were frequency tuned, but the neurons were often less securely driven. R appeared to have a tonotopic organization that roughly mirrored that of A-I. Ipsilateral connections of R included A-I, PV, and cortex ventral and caudal to R. Callosal connections were with R, A-I, PV, and cortex ventral and caudal to R. Callosal connections were with R, A-I, PV, and other locations in temporal cortex. Cortex in caudal PV, ventral to A-I, and ventral to R was responsive to auditory stimuli, but responses to pure tones were weak and inconsistent, and habituation to a repeated stimulus was rapid. The cortex responsive to auditory stimuli included some but not all of the cortex connected with A-I and R. The results lead to the conclusion that auditory cortex of squirrels contains at least two tonotopically organized fields, possibly as many as five or more auditory fields, and at least two auditory-somatosensory fields.

Organization of cortical and subcortical projections to area 6m of the cat

By analyzing regional variations of afferent connectivity, we have identified a medial subdivision of feline area 6 (area 6m) which differs from all surrounding sectors of the frontal lobe in its pattern of inputs. Area 6m is located in the ventral bank of the cruciate sulcus and on the adjacent medial face of the frontal lobe and is partially coextensive with the medial frontal eye field as identified previously in electrophysiological experiments. Area 6m is innervated by axons from visual, association, and oculomotor areas and does not receive projections from somesthetic or somatomotor areas. Cortical sources of input to area 6m include several retinotopically organized extrastriate visual areas (AMLS, ALLS, and PLLS), association areas with strong links to the visual system (area 7, granular insula, posterior ectosylvian gyrus, and cingulate gyrus), and a lateral division of area 6 (area 61) with oculomotor functions. Thalamic afferents of area 6m derive from the paralamellar ventral anterior nucleus, from a dorsolateral division of the mediodorsal nucleus, and from the rostral intralaminar nuclei. The claustrum and the basolateral nucleus of the amygdala project to area 6m. Projections from area 7, the posterior cingulate area, the ventral anterior nucleus, and the mediodorsal nucleus are spatially ordered in a pattern such that parts of area 6 close to the fundus of the cruciate sulcus receive input from neurons positioned anteriorly in the cortical areas, dorsolaterally in the ventral anterior nucleus, and ventrolaterally in the mediodorsal nucleus. Our results indicate that area 6m probably is involved in the voluntary control of gaze and attention rather than in skeletomotor functions.

Ectosylvian visual area of the cat: location, retinotopic organization, and connections

We have mapped out the ectosylvian visual area (EVA) of the cat in a series of single- and multiunit recording studies. EVA occupies 10-20 mm2 of cortex at the posterior end of the horizontal limb of the anterior ectosylvian sulcus. EVA borders on somatosensory cortex anteriorly, auditory cortex posteriorly, and nonresponsive cortex laterally. EVA exhibits limited retinotopic organization, as indicated by the fact that receptive fields shift gradually with tangential travel of the microelectrode through cortex. However, a point-to-point representation of the complete visual hemifield is not present. We have characterized the afferent and efferent connections of EVA by placing retrograde and anterograde tracer deposits in EVA and in other cortical visual areas. The strongest transcortical fiber projection to EVA arises in the lateral suprasylvian visual areas. Area 20, the granular insula, and perirhinal cortex provide additional sparse afferents. The projection from lateral suprasylvian cortex to EVA arises predominantly in layer 3 and terminates in layer 4. EVA projects reciprocally to all cortical areas from which it receives input. The projection from EVA to the lateral suprasylvian areas arises predominantly in layers 5 and 6 and terminates in layer 1. EVA is linked reciprocally to a thalamic zone encompassing the lateromedial-suprageniculate complex and the adjacent medial subdivision of the latero-posterior nucleus. We conclude that EVA is an exclusively visual area confined to the anterior ectosylvian sulcus and bounded by nonvisual cortex. EVA is distinguished from other visual areas by its physical isolation from those areas, by its lack of consistent global retinotopic organization, and by its placement at the end of a chain of areas through which information flows outward from the primary visual cortex.

Cortical and subcortical afferent connections of a posterior division of feline area 7 (area 7p)

Area 7 of the cat, as identified cytoarchitecturally, includes cortex both on the middle suprasylvian gyrus and on the anterior lateral gyrus. The aim of the experiments reported here was to determine whether within this zone there are subdivisions with qualitatively different patterns of afferent connectivity. Deposits of distinguishable retrograde tracers were placed at 29 sites in and around area 7 of 15 cats; cortical and subcortical telencephalic structures were then scanned for retrograde labeling. Our results indicate that cortex on the anterior lateral gyrus, although often included in area 7, is indistinguishable on connectional grounds from adjacent somesthetic cortex (area 5b). Cortex with strong links to visual, oculomotor, and association areas is confined to the middle suprasylvian gyrus and the adjacent lateral bank of the lateral sulcus. We refer to this discrete, connectionally defined zone as posterior area 7 (area 7p). Area 7p receives input from visual areas 19, 20a, 20b, 21a, 21b, AMLS, ALLS, and PLLS; from frontal oculomotor cortex (areas 6m and 6l); and from cortical association areas (posterior cingulate cortex, the granular insula, the posterior ectosylvian gyrus, and posterior area 35). Thalamic projections to area 7p arise from three specific nuclei (pulvinar; nucleus lateralis intermedius, pars caudalis; nucleus ventralis anterior) and from the intralaminar complex (nuclei centralis lateralis, paracentralis and centralis medialis). Neurons in a division of the claustrum immediately beneath the somatosensory and visual zones project to area 7p. Within area 7p, anterior-posterior regional differentiation is present, as indicated by the spatial ordering of projections from cingulate and frontal cortex, the thalamus, and the claustrum. Area 7p, as delineated by connectional analysis in this study, resembles cortex of the primate inferior parietal lobule both in its location relative to other cortical districts and in its pattern of neural connectivity.

Auditory response properties of neurons in the anterior ectosylvian sulcus of the cat

The auditory response properties of single neurons in the fundus and banks of the anterior ectosylvian sulcus (AES) were studied with simple dichotic stimuli (viz. noise- and tone-bursts) in cats anaesthetized with alpha-chloralose. Neurons within AES showed simple onset responses, were most commonly excited by stimulation of both ears, and showed either broad tuning or multiple high best frequencies. Some neurons were also tested for visual responsiveness and it was found that auditory cells and visual cells were intermingled within the sulcus. A small percentage of cells responded to both auditory and visual stimulation. Overall, the response properties of AES neurons differed from those of nearby auditory cortical fields. The region of AES studied appears to be outside the recently defined fourth somatosensory area (SIV), but overlaps para-SIV found deeper in the sulcus. It appears that deep within the sulcus and along most of its length there is a population of auditory, somatosensory and visual cells; to delineate this auditory population from the surrounding auditory cortical fields this region has been designated Field AES.

Visual localization and discrimination after ibotenic lesion of the cat orbito-insular cortex

Behavioral tasks were used to investigate how the orbito-insular cortex (OIC) of the cat is involved in complex operations such as the orienting reaction towards a novel stimulus. Six cats were trained preoperatively on a perimetry test to assess their ability to orient the head and eyes to objects presented in restricted regions of the visual field, and on brightness, pattern and form discrimination tasks for food reward in a two-choice discrimination apparatus. Two animals then underwent unilateral chemical lesion of the OIC using injections of ibotenic acid, two others received bilateral lesions of this same area, and the remaining two cats were used as normal controls. Postoperative performance of brightness, pattern and form discrimination was normal following OIC lesions, and no lack of retention was observed. In contrast, the cats with OIC lesions had significant deficits in their visually guided behavior. The cats ignored objects presented in the monocular segment of both sides of the visual field, even after unilateral lesion, and there was an effect on the ability to attend and fixate the central preconditioned stimulus.

Connections of the parahippocampal cortex in the cat. III. Cortical and thalamic efferents

To study the distribution of the cortical and thalamic efferent projections from the parahippocampal cortex in the cat, a series of injections of anterogradely transported radioactively labeled amino acids were placed in different parts of the entorhinal and perirhinal cortices. Subsequently, some of the identified cortical and thalamic target areas were injected with retrograde tracers such as wheat germ-agglutinin conjugated with horseradish peroxidase (WGA-HRP) or with a fluorescent tracer--fast blue or nuclear yellow--in order to disclose the laminar origin of the parahippocampal efferent projections. The results indicate that the parahippocampal cortex gives rise to widespread projections to the association cortex, and, to a lesser extent, sends fibers to the limbic cortex and the primary sensory cortex. These projections arise mainly from the deep layers of the parahippocampal cortex and terminate predominantly in superficial layers of the cortex, with a preference for layer I. Within the cortical projections a medial-to-lateral topography could be observed such that the entorhinal cortex projects predominantly to the allocortical and periallocortical limbic areas, including parts of the subicular complex, the ventral retrosplenial and the infralimbic cortices, and olfactory related areas--i.e., the olfactory bulb, the anterior olfactory nucleus, the prepiriform cortex, and the ventral tenia tecta. The more lateral parts of the parahippocampal cortex, which surround the posterior rhinal sulcus, project in addition to extensive parts of the paralimbic association cortex that include the proisocortical cingular, prelimbic, orbitofrontal, and agranular and granular insular cortices. The most lateral portion of the parahippocampal cortex, the perirhinal cortex, furthermore issues projections to widespread neocortical areas on the lateral and medial aspects of the hemisphere that constitute part of the parasensory association cortex. Weak-to-moderate projections are found to the cortex of the middle suprasylvian and anterior ectosylvian sulci, as well as the cruciate and splenial sulci, all of which have been reported to constitute sensory convergence areas. The most marked projections from the perirhinal cortex reach a zone of neocortex directly lateral to the perirhinal cortex including ventral parts of the posterior sylvian, posterior ectosylvian, posterior suprasylvian, and lateral gyri. These projections appear to be topographically organized such that rostral parts of the perirhinal cortex project more rostrally, and more caudal parts of the perirhinal cortex project to more caudal parts of this cortical zone.(ABSTRACT TRUNCATED AT 400 WORDS)

Origins of afferents to visual suprageniculate nucleus of the cat

Small iontophoretic ejections of horseradish peroxidase (HRP) were made from recording-multibarrel micropipette assemblies in areas of the cat's suprageniculate nucleus (SGn) that contained visually responsive neurones. The sources of afferents of the SGn were determined by locating the labeled cell bodies of neurones that were presumed to send their axons to the area of the SGn containing the light-sensitive cells. The greatest concentration of labeled cell bodies was found in the granular insular cortex and the adjacent area of agranular insula. Most cells projecting to SGn from these areas were distributed in the middle and lower laminae. A second intensely labeled region was found in stratum opticum and stratum griseum intermediate of the superior colliculus. Other areas containing labeled cells that were distributed with intermediate density included the ventral thalamic nuclear complex (basal, medial, and lateral divisions), periaqueductal gray (PAG), zona incerta, and pretectal nuclei (posterior, medial, and anterior divisions). Sparsely labeled sites included the fields of Forel, substantia nigra (pars reticulata), peri-insular cortex, superior colliculus (profundum), lateral suprasylvian cortex (posterolateral lateral suprasylvian, PLLS and posteromedial lateral suprasylvian, PMLS), anterior ectosylvian cortex, thalamic reticular complex, nucleus of the optic tract, basal part of the ventromedial hypothalamic nucleus, and the pontine reticular nucleus (oralis) and adjacent reticular formation. Together with previous electrophysiological and neuroanatomical studies, the findings suggest that the SGn provides an integrating link between limbic structures and certain modalities of sensory information.

Physiological properties of visually responsive neurones in the insular cortex of the cat

Extracellularly recorded responses of neurones in the insula and the adjoining rostroventral bank of the anterior ectosylvian sulcus to moving bars of light and to electrical stimulation of the superior colliculus and suprageniculate nucleus were recorded in barbiturate-anaesthetized cats. Insular cortical neurones had extremely large receptive fields, exhibited a high incidence of directional selectivity, responded best to high or medium velocity movements of the stimulus and some displayed fairly powerful end-inhibitions. Orthodromically elicited responses from the superior colliculus and suprageniculate nucleus were obtained at latencies of 5-6.5 ms and 3.0-6.0 ms, respectively. No polysensory responses were obtained from visually sensitive neurones. These data provide evidence that a population of neurones in the dorsal rim of the insula comprise a visual area which may be closely related anatomically and functionally to the recently described anterior ectosylvian visual area.

Cortical projections originating from the cat's insular area and remarks on claustrocortical connections

The cortical projections originating in the cat's insular cortex and claustrum were investigated with the aid of the horseradish peroxidase retrograde tracing technique. Twenty small injections of horseradish peroxidase were distributed along lateral and medial regions of the hemisphere. Labeling in the insular cortex occurred following all injections except those six situated along the lateral gyrus--that is, within the visual cortex. In the claustrum labeled neurons were found following all injections, except following the injection situated in the posterior temporal area. Claustral labeling was frequently more intense than insular labeling. The injections into the occipital cortex that revealed no insular innervation nevertheless received a considerable number of claustral projections. As the insular cortex itself receives at most a minor projection from the claustrum the differing cortical projection patterns of insula and claustrum have to be considered unrelated. Our findings confirm the view that the claustrum projects to most regions of the cerebral cortex; these projections are at least in part topographically organized. A topographical pattern can also be constructed for the insular cortex, though it is less stringent than for the claustrocortical connections. Both the afferent and efferent connections of the insula show similarities to those of the prefrontal cortex. Nevertheless, the insula differs in that it receives strong input from the sensory associative nuclei of the thalamus. Consequently, and in line with behavioral observations following its ablation, we consider the insula as involved in the temporal structuring of perceived patterns.

Afferents to a midbrain periaqueductal grey region involved in the 'defence reaction' in the cat as revealed by horseradish peroxidase. I. The telencephalon

Horseradish peroxidase injections were made at sites, within the midcollicular portion of the midbrain periaqueductal grey region (PAG), at which both electrical stimulation and subsequent microinjections of excitatory amino acids elicited defensive behaviour. Since excitatory amino acids depolarize cell bodies and dendrites located in the vicinity of the injection site but not axons of passage, the injections were centred within a PAG region known to contain neurones whose excitation elicited defensive behaviour. The telencephalic afferents to these sites were then determined. Sixty percent of the labelled telencephalic neurones were found in the frontal cortex, specifically in the medial frontal cortex along the banks of the rostral two-thirds of the cruciate sulcus, primarily area 6 and area 4, and the medial frontal cortex ventral to area 6 (area 32). Twenty-five percent of the labelled telencephalic neurones were found in the orbito-insular cortex while 8% were found in the parietal cortex surrounding the anterior ectosylvian sulcus. Although the functional significance of these projections remains to be established, available data suggest that these projections to the PAG arise from frontal 'oculomotor' and motor cortices, a polysensory insular cortical region and somatosensory, visual and auditory parietal cortical areas.

Cortical and thalamic afferent connections of the insular and adjacent cortex of the cat

The thalamo-cortical and cortico-cortical afferents of the cat's insular cortex were investigated with the retrograde horseradish peroxidase technique. The most prominent loci of thalamic labeling were the suprageniculate nucleus and parts of the posterolateral nucleus. Injections into the anterior part of the insular cortex also resulted in labeled cells in the ventromedial posterior nucleus and in the intralaminar nuclei, while injections into posterior parts revealed projections from the medial and dorsal parts of the medial geniculate nucleus. Only the anterior and most ventral parts of the insular cortex overlying the anterior rhinal sulcus were connected with the mediodorsal nucleus of the thalamus. All injections into the gyrus sylvius anterior showed a specific pattern of cortical afferents: With the exception of the labeling in the prefrontal cortex and the inferotemporal region, the labeled cells were very narrowly restricted to the presylvian, the suprasylvian, and the splenial sulcus. The thalamic neurons projecting to the cortex were generally organized in a bandlike pattern which crossed nuclear borders. The majority of the cortico-cortical connections originated from sulcal areas next to the prefrontal, parietal, and cingulate cortex, that is, next to so-called association cortices. In the light of the present results the role of the insular cortex as a multifunctional association area is discussed, as well as its relation to other cortical centers.

Visual properties of neurons in the suprageniculate nucleus of the cat

Visual response properties were studied in 83 single units recorded in the cat suprageniculate nucleus (Sg). About 70% of the cells were visually driven preferentially by the contralateral eye and triggered by moving stimuli without directional selectivity. Receptive fields were usually of a large size (greater than 20 degrees) and for half the cells, extended into both contralateral and ipsilateral fields of vision. No retinotopy nor functional clustering within the nucleus could be demonstrated. These properties of the visual neurons in the Sg nucleus are discussed in relation to the afferent and efferent connections of this nucleus.

An anatomical investigation of visual inputs to the cat orbital cortex

The orbital region of the cortex in the cat is known to be involved in multimodal sensory functions. Visual afferent connections of this area have been investigated using the horseradish peroxidase technique. Our results demonstrate the existence of projections from ipsilateral thalamic nuclei essentially at the suprageniculate and magnocellular medial geniculate junction, well known to have multimodal physiological properties.

Cortical and thalamic afferent connections of the insular and adjacent cortex of the rat

Thalamic and cortical afferents to the insular and perirhinal cortex of the rat were investigated. Unilateral injections of horseradish peroxidase (HRP) were made iontophoretically along the rhinal sulcus. HRP injections covered or invaded areas along the rhinal fissure from about the level of the middle cerebral artery to the posterior end of the fissure. The most anterior injection labeled a few cells in the mediodorsal nucleus. More posterior injections labeled neurons in the basal portion of the nucleus ventralis medialis, thus suggesting that this cortical region constitutes the rat's gustatory (insular) cortex. We consider the cortex situated posterior to the gustatory cortex in and above the rhinal sulcus as the core region of the rat's (associative) insular cortex, as this cortex receives afferents from the regions of and between the nuclei suprageniculatus and geniculatus medialis, pars magnocellularis. It includes parts of the cortex termed perirhinal in other studies. The cortex dorsal and posterior to the insular cortex we consider auditory cortex, as it receives afferents from the principal part of the medial geniculate nucleus, and the cortex ventral to the insular cortex (below the fundus of the rhinal sulcus) we consider to constitute the prepiriform cortex, which is athalamic. The posterior part of the perirhinal cortex (area 35) receives afferents from nonspecific thalamic nuclei (midline nuclei). Cortical afferents to the injection loci arise from a number of regions, above all from regions of the medial and sulcal prefrontal cortex. Those injections confined to the projection cortex of the suprageniculate-magnocellular medial geniculate nuclear complex also led to labeling in contralateral prefrontal regions, particularly in area 25 (infralimbic region). A comparison of our results with those on the insular cortex of cats and monkeys suggests that on the basis of thalamocortical connections, topographical relations, and involvements of neurons in information processing and overt behavior, the insular cortex has to be regarded as a heterogeneous region which may be separated into prefrontal insular, gustatory (somatosensory) insular, and associative insular portions.

Prefrontal cortex of the cat: evidence for an additional area

Direct projections from the mediodorsal nucleus of the thalamus to ventral parts of the insular region of the cat's cortex were demonstrated by using the horseradish peroxidase technique.