The general organization of somatotopic projections to SII cerebral neocortex in the cat

Colorectal distension-evoked potentials in awake rats: a novel method for studies of visceral sensitivity

BACKGROUND

Quantification of the visceromotor response induced by colorectal distension (CRD) in rodents is commonly used for preclinical studies of visceral pain. The model is well established but does not fully assess the central response to stimulation. The aim of this study was to establish a novel model assessing cerebral evoked potentials (CEPs) in response to CRD in awake rats.

METHODS

Epidural recording electrodes were chronically implanted in the skull of female Sprague-Dawley rats. Colorectal distension-induced CEPs were recorded using either rapid balloon distensions (100 ms, 20-80 mmHg) or electric stimulation (1 ms, 1-4 mA) using stimulation probes placed in the distal colon.

KEY RESULTS

Colorectal distension-induced CEPs were separated in three partly temporally overlapping components consisting of five prominent peaks. Peak latencies at 80 mmHg were (P1, N1) 23 ± 1 and 55 ± 4 ms, (N2, P2a, P2b) 91 ± 3, 143 ± 5 and 174 ± 3 ms, and (P3) 297 ± 3 ms. Amplitudes and latencies were, except for the early component, intensity dependent. Intrarectal administration of lidocaine significantly reduced the amplitude of N2 (by 42 ± 6%, P < 0.001) and P2 (by 34 ± 6%, P < 0.001). Electrically induced CEPs were intensity dependent and had similar topography and latencies as the mechanical evoked potentials (P1: 26 ± 2 ms; N1: 61 ± 1 ms; P2: 84 ± 6 ms; N2: 154 ± 6 ms; P3: 326 ± 10 ms), but there were large variations in amplitudes in between repeated electrical stimulations.

CONCLUSIONS & INFERENCES

Colorectal distension-induced CEPs can be recorded reliably in awake rats and may serve as a surrogate marker of colonic sensation and be a useful parameter in studies of visceral sensitivity.

An examination of somatosensory area SIII in ferret cortex

A somatotopically organized region on the suprasylvian gyrus of the ferret was examined using multiunit recordings and anatomical tracer injections. This area, which contains a representation of the face, was bordered by the primary somatosensory area (SI), anteriorly, and by the visually responsive rostral posterior parietal cortex (PPr), posteriorly. Anatomical tracers revealed connections to this region from cortical areas MI, SI, MRSS, PPr, and the thalamic posterior nucleus. These results are consistent with previous work in ferrets as well as with the location, physiology, and connectivity of area SIII in cats. Given its associations, functional properties, location, and homology, it is proposed that this region represents the third cortical somatosensory area (SIII) in ferrets.

The cortical representation of sensory inputs arising from bone

In the present study, we show that sensory information from bone reaches the discriminative areas of the somatosensory cortices by electrically stimulating the nerve to the cat humerus and recording evoked potentials on the surface of the primary (SI) and secondary (SII) somatosensory cortex. The SI focus was located over the rostral part of the postcruciate cortex, caudal to the lateral aspect of the cruciate sulcus. The SII focus was identified on the anterior ectosylvian gyrus, lateral to the suprasylvian sulcus. These foci were located adjacent to, or within areas that responded to stimulation of the median, ulnar and/or musculocutaneous nerves. The latency (6-11 ms) to onset of cortical responses in SI and SII were indistinguishable (unpaired t-test; P>0.05), and were consistent with activation of A delta fibers in the peripheral nerve. The amplitudes of the cortical responses were graded as a function of stimulus intensity, and may reflect a mechanism for intensity coding. We did not observe long latency cortical responses (50-300 ms) that would be consistent with C fiber activation in the peripheral nerve, and provide evidence that this may be attributable to inhibition of cortical responsiveness following the initial A delta response. Our finding of discrete, short latency evoked potentials (presumably of A delta origin) in the primary and secondary somatosensory cortices, following stimulation of a nerve innervating bone, may reflect a mechanism for the discriminative component of bone pain.

The Somatotopic Organization Within the Cat's Thalamic Reticular Nucleus

The organization of the somatosensory representation within the cat's thalamic reticular nucleus (TRN) was studied. Focal injections of horseradish peroxidase (HRP), wheatgerm agglutinin conjugated to HRP, and/or [3H]proline were made into somatosensory cortical areas 1 (S1) and 2 (S2). The resultant labelling in the thalamus was analysed. Single injections into S1 result in single zones of terminal labelling in TRN that are restricted to the centroventral part of the sheet-like nucleus. In reconstructions from horizontal sections these zones of labelling resemble thin 'slabs', which lie in the plane of the nucleus parallel to its borders, occupy only a fraction of the thickness of the reticular sheet, and are broadly elongated in the dorsoventral and oblique rostrocaudal dimensions. Thus, the slabs of S1 terminals, which represent large loci of the body surface, and the main distribution of the reticular dendrites have a similar orientation. In comparisons of the zones of labelling following single or double injections at different cortical sites in S1, an inner (medial) to outer (lateral) shift in labelling in the ventrobasal complex (VB) is accompanied by an inner (medial) to outer (lateral) shift in labelling along the thickness of the reticular sheet. Thus, like VB the reticular nucleus receives a topographically accurate projection from S1. Further, the somatotopic map conveyed from S1 to TRN is orientated perpendicular to the plane of the nucleus and repeats the spatial organization of the map in VB. S2 injections result in zones of terminal labelling in that part of TRN that receives S1 inputs. On the basis of these findings, together with those in other mammalian species, two conclusions can be reached about corticoreticular relations. First, although there can be continuity in individual maps of cortical inputs to TRN, there are discontinuities in cortical representations at the inner and outer borders of the reticular sheet. Second, TRN can receive a significant convergence of inputs from different cortical areas.

Interaction between afferent input from fingers in human somatosensory cortex

We recorded somatosensory evoked magnetic fields from eight healthy subjects with a 122-channel whole-scalp SQUID magnetometer. The stimulus sequence consisted of 'standard' stimuli (85%) delivered to palmar side of the left thumb with an interstimulus interval of 0.6 s and of 'deviants' (15%), randomly interspersed among the standards, to little finger, and vice versa. Both stimuli activated four source areas: the contralateral primary somatosensory cortex (SI), the contra-and ipsilateral secondary somatosensory cortices (SII), and the contralateral posterior parietal cortex (PPC). The short-latency (20-40 ms) responses originated in the SI cortex, whereas long-latency responses arose from all 4 areas. At SII and PPC, the deviant stimuli elicited larger responses when presented alone, without intervening standards, than among standards. This implies interaction between afferent impulses from the two fingers and/or partly intermingled cortical representations. Our findings show, in agreement with animal data, different excitatory/inhibitory balance in the various somatosensory areas.

Polysensory evoked potentials in rat parietotemporal cortex: combined auditory and somatosensory responses

A 64 channel microelectrode array was used to map auditory evoked potentials (AEP), somatosensory evoked potentials (SEP) as well as combined auditory and somatosensory evoked potentials (ASEP) from a 7 x 7 mm2 area in rat parietotemporal neocortex. Cytochrome oxidase (CO) stained sections of layer IV were obtained in the same animals to provide anatomical information underlying epicortical field potentials. Epicortical responses evoked by click or vibrissa stimuli replicated earlier findings from our laboratory, and appeared as a family of waveforms centered on primary auditory (AI) or somatosensory (SI) cortical areas as determined from CO histology. Selective microinjections of HRP to AI and SI further confirmed their specific sensory relay nuclei in the thalamus. A small polysensory area between AI and SI, responded uniquely with an enhanced negative sharp wave to combined auditory and somatosensory stimuli. HRP retrograde labeling revealed that the thalamocortical projections to this area were from the posterior nuclear group (Po) and medial division of the medial geniculate (MGm). These data establish close relationships between epicortical AEP, SEP, and especially ASEP and corresponding cortical structures and thalamocortical projections. The neurogenesis of unimodal and polysensory evoked potentials is discussed in terms of specific and non-specific systems.

Connections of somatosensory cortex in megachiropteran bats: the evolution of cortical fields in mammals

The cortical connections of the primary somatosensory area (SI or 3b), a caudal somatosensory field (area 1/2), the second somatosensory area (SII), the parietal ventral area (PV), the ventral somatosensory area (VS), and the lateral parietal area (LP) were investigated in grey headed flying foxes by injecting anatomical tracers into electrophysiologically identified locations in these fields. The receptive fields for clusters of neurons were mapped with sufficient density for injection sites to be related to the boundaries of fields, and to representations of specific body parts within the fields. In all cases, cortex was flattened and sectioned parallel to the cortical surface. Sections were stained for myelin and architectonic features of cortex were related to physiological mapping and connection patterns. We found patterns of topographic and nontopographic connections between 3b and adjacent anterior parietal fields 3a and 1/2, and fields caudolateral to 3b (SII and PV). Area 1/2 had both topographic and nontopographic connections with 3b, PP, and SII. Connections of SII and PV with areas 3b, 3a, and 1/2 were roughly topographic, although there was clear evidence for nontopographic connections between these fields. SII was most densely connected with area 1/2, while PV was most densely connected with 3b. SII had additional connections with fields in lateral parietal cortex and with subdivisions of motor cortex. Other connections of PV were with subdivisions of motor cortex and pyriform cortex. Laminar differences in connection patterns of SII and PV with surrounding cortex were also observed. Injections in the ventral somatosensory area revealed connections with SII, PV, area 1/2, auditory cortex, entorhinal cortex, and pyriform cortex. Finally, the lateral parietal field had very dense connections with posterior parietal cortex, caudal temporal cortex, and with subdivisions of motor cortex. Our results indicate that the 3b region is not homogeneous, but is composed of myelin dense and light regions, associated with 3b proper and invaginations of area 1/2, respectively. Connections of myelin dense 3b were different from invaginating portions of myelin light area 1/2. Our findings that 3b is densely interconnected with PV and moderately to lightly interconnected with SII supports the notion that SII and PV have been confused across mammals and across studies. Our connectional evidence provides further support for our hypothesis that area 1/2 is partially incorporated in 3b and has led to theories of the evolution of cortical fields in mammals.

Electrophysiological examination of the representation of the face in the suprasylvian gyrus of the ferret: a correlative study with cytoarchitecture

Using high-resolution microelectrode mapping methods, we explored the organization of the face representation within the primary somatosensory cortex of ferrets, finding evidence for at least two and probably four representations of the face distributed consecutively from anterior to posterior along the long axis of the suprasylvian gyrus. Examination of the cytoarchitecture (Rice et al., this issue) revealed that these four areas corresponded to four different cytoarchitectonic fields within the crown of the suprasylvian gyrus. The two central, most completely defined representations were oriented so that the dorsal cutaneous surfaces of the face were represented on the lateral side of the gyrus, while the perioral and ventral surfaces were represented on the medial side. The rostral-to-caudal organization within these two representations was reversed; the glabrous rhinaria were represented at the opposite ends of the maps, and penetrations progressively further away from the cortex serving the rhinaria encountered neurons activated by sites progressively more caudal on the face. Receptive fields obtained more rostrally on the gyrus suggested another reversal, implying a third representation. A small area with large receptive fields near the caudal and medial border of the two central maps suggested the presence of a fourth representation. Since the projections of adjacent skin surfaces overlapped considerably, cortical sites serving a particular cutaneous surface were illustrated as enclosed areas that overlapped the territories of other, adjacent representations. The results of this study and of others suggest a need for a re-evaluation of the hypothesis establishing a homology between the representation found in area 3b of primates and that of the primary somatosensory area in nonprimates.

Cytoarchitecture of the ferret suprasylvian gyrus correlated with areas containing multiunit responses elicited by stimulation of the face

The cytoarchitecture was studied in a segment of the ferret suprasylvian gyrus containing at least two and possibly four somesthetic representations of the face that were observed in the primary somatosensory cortex. These representations were restricted to the crown of the gyrus and were surrounded by somesthetically unresponsive cortex that extended down both sides to the base of adjacent sulci. Numerous cytoarchitectonic subdivisions were found on a qualitative basis, and were confirmed quantitatively by cluster analyses and relevant statistical tests of 10 prominent features from layers III, IV, and V. Four distinct cytoarchitectonic subdivisions, each with a well-developed and homogeneous granular layer IV, were found distributed from anterior to posterior along the crown of the gyrus at sites corresponding to the locations of the four facial representations. The surrounding unresponsive cortex had a fragmented cytoarchitecture, especially along the medial bank and base of the coronal sulcus. This unresponsive cortex separated the facial representations from the body representations, which were located on the adjacent posterior cruciate gyrus. Most of the unresponsive subdivisions had a heterogeneous or agranular layer IV and fairly well-developed sublamination in layer III, which may be indicative of extensive corticocortical connections. One set of unresponsive subdivisions had comparable cytoarchitectures that directly bordered the facial representations. Another set of unresponsive subdivisions with comparable architectures occupied most of the lateral bank of the gyrus. The implications of multiple representations and cytoarchitectonic fragmentation of the ferret primary somatosensory cortex are discussed in relation to the organization of the primary somatosensory cortex in other species.

Five topographically organized fields in the somatosensory cortex of the flying fox: microelectrode maps, myeloarchitecture, and cortical modules

Five somatosensory fields were defined in the grey-headed flying fox by using microelectrode mapping procedures. These fields are: the primary somatosensory area, SI or area 3b; a field caudal to area 3b, area 1/2; the second somatosensory area, SII; the parietal ventral area, PV; and the ventral somatosensory area, VS. A large number of closely spaced electrode penetrations recording multiunit activity revealed that each of these fields had a complete somatotopic representation. Microelectrode maps of somatosensory fields were related to architecture in cortex that had been flattened, cut parallel to the cortical surface, and stained for myelin. Receptive field size and some neural properties of individual fields were directly compared. Area 3b was the largest field identified and its topography was similar to that described in many other mammals. Neurons in 3b were highly responsive to cutaneous stimulation of peripheral body parts and had relatively small receptive fields. The myeloarchitecture revealed patches of dense myelination surrounded by thin zones of lightly myelinated cortex. Microelectrode recordings showed that myelin-dense and sparse zones in 3b were related to neurons that responded consistently or habituated to repetitive stimulation respectively. In cortex caudal to 3b, and protruding into 3b, a complete representation of the body surface adjacent to much of the caudal boundary of 3b was defined. Neurons in this area habituated rapidly to repetitive stimulation. We termed this caudal field area 1/2 because it had properties of both area 1 and area 2 of primates. In cortex caudolateral to 3b and lateral to area 1/2 (cortex traditionally defined as SII) we describe three separate representations of the body surface coextensive with distinct myeloarchitectonic appearances. The second somatosensory area, SII, shared a congruent border with 3b at the representation of the nose. In SII, the overall orientation of the body representation was erect. The lips were represented rostrolaterally, the digits were represented laterally, and the toes were caudolateral to the digits. The trunk was represented caudally and the head was represented medially. A second complete representation, PV, had an inverted body representation with respect to SII and bordered SII at the representation of the distal limbs. The proximal body parts were represented rostrolaterally in PV. Finally, caudal to both SII and PV, an additional representation, VS, shared a congruent border with the distal hindlimb representation of both SII and PV. VS had a crude topography, and receptive fields of neurons in VS were relatively large. Many neurons in VS responded to both somatosensory and auditory stimulation.

Thalamic connections of the second somatic sensory area in cats studied with anterograde and retrograde tract-tracing techniques

The thalamic connections of the second somatosensory area in the anterior ectosylvian gyrus of cats have been investigated using the retrograde tracer horseradish peroxidase and the anterograde tracer Phaseolus vulgaris leucoagglutinin. Horseradish peroxidase was injected iontophoretically in several somatotopic zones of the second somatosensory area map of six cats. Sites of horseradish peroxidase delivery were identified preliminarily by recording with microelectrodes the responses of neurons to skin stimulation. Phaseolus vulgaris leucoagglutinin was iontophoretically injected within the ventrobasal complex (one cat) or in the posterior complex (one cat). Horseradish peroxidase injections into cytoarchitectonic area SII retrogradely labeled neurons in the ipsilateral ventrobasal complex and in the posterior complex. Counts of labeled neurons from the ipsilateral thalamus showed that the overwhelming majority of horseradish peroxidase-labeled neurons were in the ventrobasal complex (96.3-96.9%) and few were in the posterior complex (3.1-3.7%). Neurons labeled in the ventrobasal complex were observed throughout the anteroposterior extent of the nucleus, while their mediolateral distribution varied with the site of horseradish peroxidase delivery in the body map of the second somatosensory area, which indicates that the projections from the ventrobasal complex to the second somatosensory area are somatotopically organized. In the cat in which the horseradish peroxidase injection involved both the second somatosensory area proper and the second somatosensory area medial, which lies in the lower bank of suprasylvian sulcus, labeled neurons were almost as numerous in the ventrobasal complex as in the posterior complex. Phaseolus vulgaris leucoagglutinin injected in the ventrobasal complex anterogradely labeled thalamocortical fibers in the ipsilateral anterior ectosylvian gyrus. In this case, patches of labeled fibers and terminals were distributed exclusively within the cytoarchitectonic borders of the second somatosensory area proper. Labeled terminals were numerous in layer IV and lower layer III, but terminal boutons and fibers with axonal swellings, probably forming synapses en passant, were frequently observed also in layers VI and I. Injection of Phaseolus vulgaris leucoagglutinin in the posterior complex labeled thalamocortical fibers in two distinct regions in the ipsilateral anterior ectosylvian gyrus, one lying laterally and the other medially, which correspond, respectively, to the fourth somatosensory area and the second somatosensory area medial. In both areas the densest plexus of labeled fibers and axon terminals was in layer IV and lower layer III, but numerous labeled fibers and terminals were also observed in layer I. In this case, only rare fragments of labeled fibers were present in second somatosensory area proper, but no labeled terminals could be observed.

Somatosensory cortex of the neonatal pig: II. Topographic organization of the secondary somatosensory cortex (SII)

Multiunit microelectrode recording techniques were used to delineate the somatotopic organization of the secondary somatosensory cortex (SII) of the neonatal pig. Barbiturate anesthetized piglets ranging in age from 7 days preterm to 2 months postpartum were used. The SII area, located lateral to the rostral and middle suprasylvian sulci, was found to contain a complete somatotopic representation of the contralateral body surface with a significant proportion of bilateral input for all body regions except the forehoof and forelimb. The SII forelimb and hindlimb representations were found to possess a "striplike" orientation in a rostral to caudal sequence, and the trunk representation was located posterolateral to the hindlimb representation, giving SII an inverted appearance. Two apparently separate face representations were delineated; one posterolateral to the projection from the trunk and the other anterior to the forehoof region. Unlike SI, which possesses a disproportionately large representation of the rostrum, SII has no specialized representation of the rostrum. The overall organization of SII supports the contention that this cortical region provides a more generalized representation of the entire body surface than does SI.

Bilateral receptive fields in cortical area SII: contribution of the corpus callosum and other interhemispheric commissures

The corpus callosum contributes to the interhemispheric transfer of somatosensory information. Since the somatosensory pathways are essentially crossed, a number of studies have postulated that the corpus callosum may be responsible for the presence of bilateral receptive fields (RFs) in cortical area SII. Moreover, subcortical structures, as well as some of the other commissures, may also contribute to the bilateral nature of these cells. In order to assess the relative importance of the corpus callosum, this study compared the RF properties of cells in area SII of callosum-sectioned cats to normal cats, using single-cell recordings. Results showed that the corpus callosum makes an important contribution to the bilateral activation of cells in SII, since the proportion of cells with bilateral RFs found in callosum-sectioned cats was less than half that obtained in normal cats. The decrease in the proportion of bilateral RFs was found for all body regions with the exception of the face. However, the substantial number of bilateral RFs remaining in callosotomized cats indicates that this structure is not the sole contributor to the bilateral activation of cells in SII. In order to determine whether this residual bilateral activation might be mediated by the other interhemispheric commissures, a group of cats was subjected, besides the callosotomy, to the additional transection of their subcortical commissures, including the anterior, posterior, habenular, and intertectal commissures, as well as the massa intermedia. When this group of deep-split cats was compared to the callosotomized group, the results indicated that the contribution of the other commissures to bilateral activation is negligible, since approximately the same proportion of bilateral RFs was encountered in the two groups. The relative importance of the callosal contribution to bilateral RFs of different body regions is discussed with respect to the roles commonly attributed to this structure.

Callosal connections of the somatic sensory areas II and IV in the cat

The homotopic and heterotopic callosal connections in the forelimb representations of the second (SII) and fourth (SIV) somatic sensory areas of cats were investigated by means of the axonal transport of horseradish peroxidase (HRP) in conjunction with microelectrode recording. The tracer was injected in the electrophysiologically identified hand and/or digit zone of SII (six cats) or SIV (four cats). The homotopic area in the contralateral hemisphere was explored with microelectrodes in five animals (three injected in SII and two in SIV) to map neuronal receptive fields. The aim was to correlate in the same experimental case the topography of labelled callosal neurons with the physiological map of the forelimb. Labelled cells and recording sites were plotted on planar maps reconstructed with the aid of a computer from serial coronal sections from the anterior ectosylvian gyrus. After SII injections, labelled callosal neurons were observed throughout the forelimb representation in the contralateral area, but in the tangential plane their distribution was uneven. Each somatotopic zone composing the forelimb map, that is, the arm, hand, and digit zones, contained several subzones in which callosal neurons were either dense or rare. Microelectrode explorations showed that receptive fields mapped from callosal and relatively acallosal subzones representing the same body part were similar in extent and location. After SIV injections, labelled callosal neurons were observed throughout the forelimb and proximal body representation of the contralateral area. Although slight regional variations in the density of labelled cells were apparent, no subzones bare of callosal labelling were observed in SIV. In both SII and SIV, callosal neurons were concentrated mainly in layer III, but a significant number was also evident in the infragranular layers. After HRP injections in the digit zone of SII or SIV, labelled cell bodies were also observed in heterotopic areas of the contralateral hemisphere. Most of these neurons were clustered in the medial bank of the coronal sulcus and in two other heterotopic cortical regions lying, respectively, in the anterior suprasylvian sulcus and in the lateral branch of the ansate sulcus. Some callosal cells interconnecting SII and SIV were also labelled. The results show that the distal forelimb zones in SII and SIV are callosally connected with the respective homotopic zones and with several somatosensory fields located heterotopically in the contralateral hemisphere.

Somatotopic organization of the lateral sulcus of owl monkeys: area 3b, S-II, and a ventral somatosensory area

Multiunit microelectrode recordings and injections of horseradish peroxidase (HRP) were used to reveal neuron response properties, somatotopic organization, and interconnections of somatosensory cortex in the lateral sulcus (sylvian fissure) of New World owl monkeys. There were a number of main findings. 1) Representations of the face and head in areas 3b, 1, and S-II are found on the upper bank of the lateral sulcus. Most of the mouth and lip representations of area 3b were found in a rostral extension along the lip of the lateral sulcus. Adjacent cortex deeper in the lateral sulcus represented the nose, eye, ear, and scalp. 2) S-II was located on the upper bank of the lateral sulcus and extended past the fundus onto the deepest part of the lower bank. The face was represented most superficially in the sulcus, with the hand, foot, and trunk located in a rostrocaudal sequence deeper in the sulcus. The orientation of S-II is "erect," with the limbs pointing away from area 3b. 3) Neurons in S-II were activated by light tactile stimulation of the contralateral body surface. Receptive fields were several times larger than for area 3b neurons. 4) A 1-2-mm strip of cortex separating the face and hand representations in S-II was consistently responsive to the stimulation of deep receptors but was unresponsive to light cutaneous stimulation. 5) Injections of horseradish peroxidase in the electrophysiologically identified hand or foot representations of area 3b revealed somatotopically matched interconnections with mapped hand and foot representations in S-II. 6) A systematic representation of the body, termed the "ventral somatic" area, VS, was found extending laterally from S-II on the lower bank of the lateral sulcus. Within VS, the hand and foot were represented deep in the sulcus along the hand and foot regions of S-II, and the face was lateral near the ventral lip of the sulcus. 7) Neurons at most recording sites in the VS region were activated by contralateral cutaneous stimuli. However, a few sites had neurons with bilateral receptive fields. Receptive field sizes were comparable to those in S-II. In addition, neurons in islands of cortex in the VS region had properties that suggested that they were activated by pacinian receptors, while other regions were difficult to activate by light tactile stimuli but responded to stimuli that would activate deep receptors. 8) A few recording sites caudal to S-II on the upper bank of the lateral sulcus were responsive to somatic stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Thalamic connections of three representations of the body surface in somatosensory cortex of gray squirrels

The anatomical tracer, wheat germ agglutinin, was used to determine the connections of electrophysiologically identified locations in three architectonically distinct representations of the body surface in the somatosensory cortex of gray squirrels. Injections in the first somatosensory area, S-I, revealed reciprocal connections with the ventroposterior nucleus (VP), a portion of the thalamus just dorsomedial to VP, the posterior medial nucleus, Pom, and sometimes the ventroposterior inferior nucleus (VPI). As expected, injections in the representation of the face in S-I resulted in label in ventroposterior medial (VPM), the medial subnucleus of VP, whereas injections in the representation of the body labeled ventroposterior lateral (VPL), the lateral subnucleus of VP. Furthermore, there was evidence from connections that the caudal face and head are represented dorsolaterally in VPM, and the forelimb is represented centrally and medially in VPL. The results also support the conclusion that a representation paralleling that in VP exists in Pom, so that the ventrolateral part of Pom represents the face and the dorsomedial part of Pom is devoted to the body. Because connections with VPI were not consistently revealed, the possibility exists that only some parts or functional modules of S-I are interconnected with VPI. Two separate small representations of the body surface adjoin the caudoventral border of S-I. Both resemble the second somatosensory area, S-II, enough to be identified as S-II in the absence of evidence for the other. We term the more dorsal of the two fields S-II because it was previously defined as S-II in squirrels (Nelson et al., '79), and because it more closely resembles the S-II identified in most other mammals. We refer to the other field as the parietal ventral area, PV (Krubitzer et al, '86). Injections in S-II revealed reciprocal connections with VP, Pom, and a thalamic region lateral and caudal to Pom and dorsal to VP, the posterior lateral nucleus, Pol. Whereas major interconnections between S-II and VPI have been reported for cats, raccoons, and monkeys, no such interconnections were found for S-II in squirrels. The parietal ventral area, PV, was found to have prominent reciprocal interconnections with VP, VPI, and the internal (magnocellular) division of the medial geniculate complex (MGi). The pattern of connections conforms to the established somatotopic organization of VP and suggests a crude parallel somatotopic organization in VPI. Less prominent interconnections were with Pol.(ABSTRACT TRUNCATED AT 400 WORDS)

D-[3H]aspartate retrograde labelling of callosal and association neurones of somatosensory areas I and II of cats

Experiments were carried out on cats to ascertain whether corticocortical neurones of somatosensory areas I (SI) and II (SII) could be labelled by retrograde axonal transport of D-[3H]aspartate (D-[3H]Asp). This tritiated enantiomer of the amino acid aspartate is (1) taken up selectively by axon terminals of neurones releasing aspartate and/or glutamate as excitatory neurotransmitter, (2) retrogradely transported and accumulated in perikarya, (3) not metabolized, and (4) visualized by autoradiography. A solution of D-[3H]Asp was injected in eight cats in the trunk and forelimb zones of SI (two cats) or in the forelimb zone of SII (six cats). In order to compare the labelling patterns obtained with D-[3H]Asp with those resulting after injection of a nonselective neuronal tracer, horseradish peroxidase (HRP) was delivered mixed with the radioactive tracer in seven of the eight cats. Furthermore, six additional animals received HRP injections in SI (three cats; trunk and forelimb zones) or SII (three cats; forelimb zone). D-[3H]Asp retrograde labelling of perikarya was absent from the ipsilateral thalamus of all cats injected with the radioactive tracer but a dense terminal plexus of anterogradely labelled corticothalamic fibres from SI and SII was observed, overlapping the distribution area of thalamocortical neurones retrogradely labelled with HRP from the same areas. D-[3H]Asp-labelled neurones were present in ipsilateral SII (SII-SI association neurones) in cats injected in SI. In these animals a bundle of radioactive fibres was observed in the rostral portion of the corpus callosum entering the contralateral hemisphere. There, neurones retrogradely labelled with silver grains were present in SI (SI-SI callosal neurones). Association and callosal neurones labelled from SI showed a topographical distribution similar to that of neurones retrogradely labelled with HRP. The laminar patterns of corticocortical neurones labelled with D-[3H]Asp or with HRP were also similar, with one exception. In the inner half of layer II, SII-SI association neurones and SI-SI callosal neurones labelled with the radioactive marker were much less numerous than those labelled with HRP. In cats injected in SII, D-[3H]Asp retrogradely labelled cells were present in ipsilateral SI (SI-SII association neurones). Their topographical and laminar distribution overlapped that of neurones labelled with HRP but, as in cats injected in SI, association neurones labelled with silver grains were unusually rare in the inner layer III.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

The representation of the oral structures in the first somatosensory cortex of the cat

Applying light mechanical stimulations to the oral structures, multi-unit recordings (MUR) were performed, and receptive fields of neurons were defined in the somatosensory cortex S I of the cat. Oral representation was confirmed in the circumscribed area of the anterior coronal gyrus rostral to the facial area in the following order: the contralateral lips, the contralateral periodontal tissues such as the gingivae and periodontal membranes, the middle and ipsilateral part of the periodontal tissues, and the palate and tongue in a caudorostral direction. The representation of the lip and perioral tissues of the lower jaw was found on the medial side of the coronal gyrus, while the upper one was found on the lateral side. The same oral representation was confirmed by single-unit recording (SUR) analysis in the anterior coronal gyrus. The representation area of the contralateral, middle and ipsilateral site of the non-hairy lips and periodontal tissues was found in cytoarchitectural field, areas 3b and 2; somatic koniocortex.

Topography and cortical generators of somatosensory potentials evoked by median nerve stimulation in the cat

Epidural and cortical mapping of somatosensory evoked activity after median nerve stimulation was performed under barbiturate anaesthesia in 6 cats. Depth profiles were made to confirm the site of the cortical generators. The area studied revealed two cortical generators in SI and one in SII. In all cases polarity changes in intracortical tracks were demonstrated. The peak latency of all these generators was 12 msec. In SI a P8 was also a consistent finding in epidural, epicortical and intracortical measurements. No evidence, however, could be obtained for a cortical origin of the P8. The most rostral generator of the P12 was localized in the posterior sigmoid gyrus (area 3a). The N12 originates from the dorso-medial bank of the coronal sulcus in SI (area 2). Histological evidence for these projections was obtained with use of electrode markation. Within the second somatosensory area one source was active; the P12 originated just lateral to the anterior aspect of the suprasylvian sulcus in the anterior ectosylvian gyrus.

Somatotopic organization of the third somatosensory area (SIII) in cats

Multiunit microelectrode recording techniques were used to study the location and organization of the third somatosensory area (SIII) in cats. Representations of all major contralateral body parts were found in a small region of cortex along the lateral wing of the ansate sulcus and between the lateral sulcus and the suprasylvian sulcus. The systematic map of the body surface included forepaw and face regions previously identified as parts of SIII. The forepaw representation was generally buried on the rostral bank of the lateral wing of the ansate sulcus. The representations of the face and mystacial vibrissae were largely exposed on the rostral suprasylvian gyrus, but part of the representation of the face was also buried in the lateral wing of the ansate sulcus. Representations of the trunk and hindlimb extended from the suprasylvian gyrus onto the medial bank of the suprasylvian sulcus. We had expected to find these latter body parts in more medial cortex just caudal to the representation of these parts in the first somatosensory area (SI). Instead, neurons in penetrations in cortex caudal to the SI trunk and hindlimb representations were unresponsive to tactile stimulation. The unexpected location of the hindlimb in SIII led us to determine whether the proposed parts of SIII had similar cortical and thalamic connections. Injected anatomical tracers revealed that the representations of both the forelimb and hindlimb were interconnected with SI and a region of the thalamus just dorsal to the ventroposterior nucleus. Similarities in patterns of connections of forelimb and hindlimb portions of SIII supported the conclusion that SIII as presented here is a functional unit of cortex. We conclude that SIII has a somatotopic organization that does not parallel that in SI, and that SIII is not entirely coextensive with either area 5 or area 5a of Hassler and Muhs-Clement (1964).

Microelectrode maps, myeloarchitecture, and cortical connections of three somatotopically organized representations of the body surface in the parietal cortex of squirrels

Microelectrode mapping methods and anatomical procedures were combined in the same animals to reveal the cortical connections of three architectonically distinct representations of the body surface in the somatosensory cortex of grey squirrels. In individual experiments, microelectrode multiunit recordings were used to determine the somatotopic organization of regions of the cortex and to identify sites for injections of the anatomical tracer, wheat germ agglutinin conjugated to horseradish peroxidase. After the brains were perfused, the cortex was separated from the brainstem, flattened, and cut parallel to the flattened surface to facilitate comparisons of areal connection patterns, physiological data, and architectonic subdivisions. Recordings in the primary (S-I) and secondary (S-II) somatosensory fields confirmed earlier descriptions of the somatotopic organization of these fields (Sur et al.: J. Comp. Neurol. 179:425-450, '78; Nelson et al.: J. Comp. Neurol. 184:473-490, '79). In addition, recordings in the cortex caudal to S-I and ventral to S-II revealed a third representation of the body surface in parietal cortex, the parietal ventral area (PV). Neurons in PV were responsive to light tactile stimulation of skin and hairs. Multiple unit receptive fields of neurons in PV were larger than those for neurons in S-I but similar in size to those for neurons in S-II. PV represented the contralateral body surface in a somatotopic manner that can be roughly characterized as an inverted "homunculus" with the limbs directed medially, the trunk located ventrally, and the face congruent with the representations of the upper lip and nose in S-I. Neurons in some electrode penetrations in PV were also responsive to auditory clicks. Microlesions placed at physiologically determined borders allowed all three somatic representations to be related to myeloarchitectonically defined fields. S-I was architectonically distinct as a densely myelinated region. Within S-I, a lightly myelinated oval of the cortex between the representation of the hand and face, the "unresponsive zone" (Sur et al.: J. Comp. Neurol. 179:425-450, '78), was an easily recognized landmark. S-II and PV corresponded to less densely myelinated fields. Other subdivisions such as motor cortex, primary auditory cortex, and visual areas 17 and 18 were distinguished. Connections were revealed by placing injections within S-I, S-II, or PV.(ABSTRACT TRUNCATED AT 400 WORDS)

Second somatic sensory cortical area (SII) in a prosimian primate, Galago crassicaudatus

The cortex adjacent to and along the upper bank of the lateral sulcus (UB-LS) of a prosimian primate, Galago crassicaudatus, was explored to determine the topographical representation of low-threshold cutaneous inputs to this region. The somatic sensory projections to this cortex were considered homologous to those defined in other species as the second somatosensory cortical area (SII). Multiple and single neuron recordings were obtained with tungsten microelectrodes in animals anesthetized with sodium pentobarbital or ketamine hydrochloride; receptive fields were determined by means of manually applied tactile stimuli. The area of SII was located approximately 1-1.5 mm rostral to the posterior limit of LS, extended rostrally approximately 4 mm, and occupied nearly all of the upper bank of the sulcus throughout this region. Receptive fields (RFs) in SII were primarily contralateral except for some bilateral input in the cortex representing portions of the trunk, head, and face. The boundaries of RFs were well defined, especially where recordings were located in the middle layers of the cortex. The distribution of RFs across SII was somatotopically organized into a single, relatively erect representation of the body that involved inputs from the face rostral and medial (superficially along the UB-LS) surrounding an enlarged forelimb area; the latter, in turn, lies rostral and medial to the hindlimb zone. Projections from the tail and sacrum are located furthest caudal and lateral (deeper along UB-LS). Separate regions that were devoted to the glabrous skin surfaces of the distal limbs formed the rostral and lateral boundaries of the distal fore- and hindlimb representations, respectively. In the zone for the glabrous surfaces of the forelimb digits, individual digits dominated discrete components of the SII map, especially medially where digit 1 was represented. The glabrous tip of digit 5 was represented caudal and lateral to the tip of digit 1. A similar radial to ulnar medial to lateral sequence was noted in the area representing the palm. Except for a possible medially located toe 1 zone in the hindlimb representation, separated representations for the glabrous skin of individual toes were not noted. The dorsal hairy surfaces of the digits and toes were, respectively, amalgamated within the representations for the dorsal surfaces of the hand and foot. In these regions, which were found superficial and slightly caudal to their respective glabrous zones, some RFs were found that were devoted only to the distal extremities, but most RFs included more proximal portions of the hand or foot dorsum.(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.

Body surface maps in the somatosensory cortex of rabbit

The organization of somatosensory maps was examined in rabbits with the aid of microelectrode multi-unit recording techniques. Two complete maps of the contralateral body surface are identified in the parietal cortex. The first map, S I, is found entirely on the lateral convexity of the hemisphere and closely resembles S I described in the rat (Welker, '71, '76). It is organized in a complex, though systematic, fashion with the representations of the hindlimb and tail located caudomedially. These representations are followed laterally in sequence by those of the trunk and forelimb and then the representation of the head. Within the head representation the lips are found rostrally, the vibrissae caudomedially, and the displaced representation of the pinna of the ear is located caudolaterally. Unlike the disposition in most other mammals, the dorsal midline of the trunk is represented along the caudal border of S I. Within S I, the representations of the circumoral surfaces, including the lips, philtrum, nose, and vibrissae, are emphasized, occupying approximately 86.4% of the map. It is suggested that S I is contained within a single major koniocortical region, here called the medial parietal area, or Pm. The several previously described parietal regions (Rose, '31; Fleischhauer et al., '80) are interpreted as subregions that are related to particular representations of portions of the body surface. The second map, S II, is located lateral to S I in a region here called the lateral parietal area or Pl. S II shares a common border with S I along the representations of the philtrum, bridge of the nose, and top of the head. The body is oriented in an erect conformation with the head located rostrally and medially and the hindlimb and tail located caudally and laterally.

Somatotopic organization of the second somatosensory area (SII) in the cerebral cortex of the mouse

The somatotopic organization of the parietal cortex of barbiturate-anesthetized, adult mice was studied using tungsten microelectrodes. A complete representation of the contralateral face and body occupying approximately 4.0-4.5 mm2 was found immediately posterior and lateral to the representation of the face in the first somatosensory area (SI). Within this second somatosensory area (SII), the following findings were made: A relatively large region is devoted to representations of the paws and face, especially the sinus hairs associated with the anterior upper lip and mystacial vibrissae. Receptive fields on these body regions are among the smallest found in SII, though larger than corresponding receptive fields in SI. In particular, vibrissae receptive fields always include at least several adjacent whiskers, and paw receptive fields always include at least two adjacent digits. In regions representing proximal body parts, receptive fields are considerably larger, may include both contralateral and ipsilateral limb or trunk surfaces, and sometimes include the entire body and face. Responses to both somatosensory and auditory stimulation were consistently found in the body (i.e., trunk and limb) representation, but rarely found in the face region. The face is represented most anteriorly, and the hindlimb and tail most posteriorly. Forepaw and hindpaw digits and anterior aspects of the face (e.g., perioral sinus hairs and the incisors) are represented laterally, while the back, caudal head, and mystacial vibrissae are represented medially. Within SII, therefore, a "musculus" can be viewed as having an upright body orientation with the face area bordering the face representation within SI. By comparison with SI, SII is characterized by a less pronounced layer IV, which is of irregular thickness and packing density, and by less uniformity in the layering of pyramidal cells in lamina V. In addition, SII is generally thicker from pia to white matter than SI. These results are in general accord with earlier findings from evoked potential studies in mice, but are at variance with recent reports in mice and rats that the mystacial vibrissae have only a minimal, or no, representation within SII. Indeed, the present findings suggest that the representation of the whiskers in SII may have a specialized function paralleling that in SI.

Homotypical ipsilateral cortical projections between somatosensory areas I and II in the cat

In 11 cats, small quantities of horseradish peroxidase conjugated to wheat germ agglutinin were placed into cortical zones of somatosensory area I representing the distal digits (n = 3), distal toes (n = 2), toes and digits (n = 1), proximal forelimb (n = 1), proximal hindlimb (n = 1), trunk (n = 2), and the face and nose (n = 1). Reconstruction of the pattern of retrograde labeling in somatosensory area II revealed dense, heavily labeled patches of cells in regions that were precisely homotypical to the injection site as determined by electrophysiological recordings. This dense, homotypical patch of labeled cells was usually surrounded by a less densely populated fringe of labeled cells that bordered, but did not appear to enter, heterotypical zones. In two animals, however, some retrogradely labeled cells were found in the cortex representing somatotopic zones adjacent to the sites injected with horseradish peroxidase. These results indicated that somatosensory area II primarily sends homotypical projections to somatosensory area I. In a few cases, however, some retrogradely labeled cells may represent either homo-or heterotypical projections depending on how receptive field sizes and the areal extent of labeling in somatosensory areas I and II are interpreted.

Distribution of the neurons of origin of the great cerebral commissures in the cat

Large injections of horseradish peroxidase throughout major portions of the right cerebral hemispheres of four cats revealed extensive distributions of the neurons of origin of the corpus callosum, the anterior commissure and the hippocampal commissures in the uninjected left hemispheres. The distributions of labelled neurons were mapped by semiautomatic computer microscope. The radial and tangential neuron distributions presented here are of a higher density and greater extent than those in previously published studies based on injections of transportable label to more circumscribed areas of the cerebral cortex of the cat. Generally, commissural neurons in the cat were distributed in a bilaminar fashion with supragranular cells more numerous than infragranular cells.

Ipsilateral cortical connections from the second and fourth somatic sensory areas in the cat

The ipsilateral corticocortical connections of the second and fourth somatic sensory areas (SII and SIV) were traced with the aid of anterograde or retrograde axonal transport techniques involving horseradish peroxidase conjugated to wheat germ agglutinin (HRP-WGA) or tritiated amino acids. The injections were placed into physiological defined components of the body representation in SII or SIV. The results from cases with localized injections into SII showed precise topographically organized, reciprocal connections with SI and motor cortex area 4. The distribution of connections in SI included areas 3a, 3b, and 1-2. A uniform pattern of cell and fiber labeling was seen across area 3b and 1 within the zones that were homotypical to the injection site in SII as though only a single representation of the cutaneous surface of the body existed in SI. Intrinsic connections within SII were also topographically arranged. Additional areas found to be interconnected with SII included, in decreasing order of density: area 5, insula, perirhinal cortex (area 36), and ventrolateral orbital cortex. SII connections with area 6 were seen only in the region of the lateral bank of the presylvian sulcus. There may be interconnections between SII and SIV but these were from possible local intrinsic connections in the AEG. The results from injections involving SIV showed reciprocal connections with area 5, the suprasylvian fringe, insula, dorsolateral orbital area, and area 6. The densest connections for SIV were with area 5. No topography was noted in the connections for SIV.

Specialized subregions in the cat motor cortex: anatomical demonstration of differential projections to rostral and caudal sectors

Ipsilateral cortico-cortical and thalamo-cortical projections to the cat motor cortex were determined from the locations of retrogradely labeled neurons following single small intracortical injections of HRP in area 4 gamma. These projections were also examined by studying the distribution of anterogradely transported axonal label following multiple injections of HRP or of tritiated amino acids in areas 1-2 of SI and in area 2pri (SII). The number of retrogradely labeled cells in areas 1-2 and in area 2pri differed markedly between HRP injection sites located in the precruciate (anterior sigmoid gyrus) and postcruciate (posterior sigmoid gyrus) subregions of area 4 gamma. These associational projections from primary and secondary somatosensory cortices were dense to postcruciate subregions but weak to the precruciate subregions. The associational projections from areas 1-2 and from area 2pri to the postcruciate subregion of area 4 gamma were topographically organized, but no clear topographic organization could be demonstrated for the precruciate projection. Anterograde terminal labeling following injection of either HRP or tritiated amino acids into areas 1-2 and area 2pri confirmed the preferential projection of somatosensory cortex to the postcruciate subregion of motor cortex. The projection from somatosensory areas 1-2 was uniform over its terminal field, but that from area 2pri was more patchy and complex. HRP injections in area 4 gamma gave rise to lamellae of labeled neurons in the ventrolateral nucleus of thalamus (VL). A topographic relationship was found between the site of injection and the location of the lamella of labeled neurons. The percentage of retrogradely labeled neurons in the shell zone surrounding the border of the ventrolateral nucleus and the ventrobasal complex (VB) was greater following postcruciate than precruciate injections, whereas fewer retrogradely labeled neurons were found in central lateral nucleus (CL) after postcruciate injections than after precruciate injections. These observations support the hypothesis that differential cortical and thalamic projections to different subregions of area 4 gamma may give rise to the different physiological properties of neurons observed in these subregions (Vicario et al. 1983; Martin et al. 1981).

The connections of cortical somatosensory areas I and II with separate nuclei in the ventroposterior thalamus in the raccoon

The thalamocortical afferents to cortical somatosensory areas I (SI) and II (SII) were investigated in the raccoon using the horseradish peroxidase technique. The purpose of this study was to determine if the cell bodies of origin for thalamocortical afferents to these cortical regions were localized in the same or different nuclei in the ventroposterior region of the thalamus. Horseradish peroxidase was injected into subdivisions of SI or SII and after post-injection survival periods of 12-72 hours the horseradish peroxidase in the tissue was reacted with the chromogens dihydrochlorobenzidine or tetramethylbenzidine in the presence of hydrogen peroxide. The results show that SI and SII receive projections from neurons in separate and distinct nuclei in the ventroposterior thalamus. Following injections into subdivisions of area I, a topographical distribution of retrogradely-labelled cell bodies was observed in the ventrobasal complex. Following injections of horseradish peroxidase into subdivisions of area II, a topographical distribution of labelled cell bodies was observed in the ventroposterior inferior nucleus. No labelled cell bodies were observed in the ventrobasal complex. The thalamocortical connections of somatosensory cortices I and II in raccoon are compared with those in other animals and it is suggested that these two cortical areas may be involved in differential processing of tactile information.

Second somatic sensory area in the cerebral cortex of cats: somatotopic organization and cytoarchitecture

The cortex of the anterior ectosylvian gyrus and adjoining ectosylvian and suprasylvian sulci was explored with tungsten microelectrodes to determine the distribution of responses to light cutaneous stimulation in barbiturate-anesthetized cats. Recordings were spaced between 125 and 250 micrometers and, in several cases, nearly all of the somatic areas in this cortex were explored in the same brain. Four somatic sensory areas were identified on the basis of responses properties, sequences of receptive fields, and cytoarchitecture. The largest area, which occupied the rostral and medial two-thirds to three-fourths of the exposed, relatively flat portion of the anterior ectosylvian gyrus, was called the second somatic sensory area (SII). Receptive fields in SII were primarily from the contralateral side of the body; they were well defined and somatotopically organized into an erect representation of the body. The top of the head was located next to a similar representation of the periphery in a portion of the first somatic sensory area (SI). Individual distal digits and toes occupied discrete components of the SII map. Another representation for the distal forelimb and hindlimb was noted medially along the lateral bank of the anterior suprasylvian sulcus. Receptive fields and response properties in this region were equivalent to those seen in SII proper. However, only a crude anteroposterior, fore- to hindlimb topographical organization was noted, but with more distal parts of the limbs generally located closer to the fundus of the sulcus in this medial representation. As the cytoarchitecture in this medial region was similar to the rest of SII it was considered a medial subdivision of SII. A third, topographically organized zone was located lateral to SII largely within the upper bank of the anterior ectosylvian sulcus and adjoining lateral crest of the anterior ectosylvian gyrus. Large, stockinglike, contralateral receptive fields were common; ipsilateral components to the receptive fields were present. Some individual digit receptive fields were located in the rostral part of the forelimb zone within the anterior ectosylvian sulcus. This lateral somatic area is probably equivalent to a fourth somatic sensory area (SIV) recently identified by Clemo and Stein ('82). Posterior to the hindlimb zones of SII and medial to SIV was another region that responded to cutaneous plus auditory stimulation. There was no detectable topography in this area; nearly all of the receptive fields were large, frequently bilateral, and often involved the whole body or all four extremities. This area's cytoarchitecture was comparable to previous descriptions of the suprasylvian fringe (Rose, '49). The location and physiology of these four areas were discussed in reference to previous controversies regarding the topography of the body representation in SII and the location of an acallosal zone in this region of cortex.

Somatosensory cortex: a 'new' somatotopic representation

A 'new' and orderly representation of the body surface was found in the cerebral cortex of the cat. This somatotopic map was located in the anterior ectosylvian sulcus (AES), an area known to represent several modalities, but believed to be distinguished from primary sensory cortex by its lack of sensory topographies. Auditory and visual cells were also found in the AES, but were not randomly intermixed with the somatic representation. These data, coupled with those from previous studies strongly suggest the necessity for a revision of the traditional distinctions between cortical regions.

The organization of somatosensory area II in tree shrews

Microelectrode multiunit recording methods were used to determine the somatotopic organization of the second somatosensory area, S-II, in tree shrews. Neurons were activated by light tactile stimuli, and receptive fields were located on the contralateral body surface only. The orientation of S-II was such that the top of the head adjoined S-I and the distal limbs pointed away from S-I so that the representation could be characterized as "erect". In general, the distortions of the body surface in S-II were similar to those found in S-I of the tree shrew (Sur et al., '80), with the exception that proportionately less cortex was devoted to the glabrous nose. The representation in S-II was more continuous than that in S-I. Finally, cortex bordering S-II caudally was found to be responsive to generally more intense somatosensory stimuli such as taps to the body surface.

Somatotopographic organization in the second somatosensory area of M. fascicularis

The second somatosensory area (SII) of awake, untrained cynomolgus monkeys was surveyed with recordings from nearly 1,000 single neurons. A detailed somatotopographic organization could be demonstrated in SII because the majority of these neurons had contralateral, moderate to well-defined receptive fields of < 10 cm2, and because neighboring neurons possessed receptive fields that were only slightly displaced from one another. Different body regions were represented in successive anterior to posterior strips that were oriented across the parietal operculum with an anterolateral to posteromedial slant. Neurons with trigeminal receptive fields were found in the anterior portion of SII; these neurons were the only ones in SII with predominantly bilateral receptive fields (r.f.'s.). Neurons with digit or hand r.f'.s form the largest component of the map, and were located posterior to those with face r.f.s. Most of these neurons had only contralateral activation. The hand and digit region was followed in turn by the arm, the upper and lower trunk, and the hindlimb regions. Although the overall SII orientation was along an anterior-posterior gradient, recordings at individual coronal planes often demonstrated isolated sequences of receptive fields that exhibited a medial-lateral progression. The principle example of this latter gradient was seen in the forelimb region where digits one through five were represented in an overlapping sequence across the parietal operculum. Except for portions of the digit representation, neighboring sequences of neurons in SII do not form a precise topologic map of the body that is comparable to the somatotopic maps observed in areas 3b and 1. The present findings contrast with previous physiological studies of SII in the primate. These discrepancies are discussed in relation to methodological differences and in terms of distinctions used to define the boundaries of SII.

Representation pattern in the second somatic sensory area of the monkey cerebral cortex

The body representation in the second somatic sensory area of macaques has been studied by tracing with anatomical techniques the projections from defined parts of the body representation in the first somatic sensory area (SI) to their terminal regions within the lateral sulcus. The second somatic sensory area (SH), as identified in terms of cytoarchitecture and its connection with the thalamic ventrobasal complex, is the only region of the lateral sulcus to receive a projection from SI. The nearby retroinsular area and area 7b receive a projection from area 5. Within SII the face and head representations lie anteriorly, occupying the dorsalmost part of the insula and portions of the front-parietal operculum. The digits, hand, and arm are represented posterior to the face and may take up the mediolateral extent of the parietal operculum in the region immediately in front of the posterior pole of the insula. The trunk representation is lateral to the arm representation, i.e., deep within the superior circular sulcus and on the dorsal insula. The hindlimb appears behind the trunk also occupying the superior circular sulcus in addition to the deepest 2--3 mm of the upper bank of lateral sulcus immediately posterior to the insula. Areas 3b, 1, and 2 each project to SII, and their projections appear to converge within the representation of a given body part. Injections of anterogradely transported tracers in SI label vertically oriented columnar arrays of a terminal ramifications in SII, resembling those previously described for other cortico-cortical projections within the sensory-motor region. In experiments which combined anterograde and retrograde labeling, cells projecting from SII to SI formed columns exactly coinciding with the columns of anterogradely labeled axons terminating in SH. The cells of origin of cortico-cortical projections emanating from SII formed two distinct laminar populations, one in the supragranular layers and the second mainly in layer VI. There is evidence for fiber terminations within the layer VI mainly underlying the column formed by the terminal ramifications in layers I and through IV.

Somatic submodality distribution within the second somatosensory (SII), 7b, retroinsular, postauditory, and granular insular cortical areas of M. fascicularis

Somatic response properties were determined for over 1,300 neurons isolated within and near the lateral sulci of unanesthetized and unparalyzed cynomolgus monkeys. Somatic stimuli unequivocally activated the majority of units studied in SII (93%) and in cortical fields surrounding SII: area 7b (65%), the retroinsular field (74%), and the granular insula (76%). No activation other than somatic was seen for SII neurons, and noxious somatic stimulation was rarely required. The SII units almost always responded in a rapidly adapting manner to hair or skin stimulation, but not both; however, the submodality distribution seen in SII varied as a function of peripheral receptor locations. Two small zones within SII contained neurons that responded only if the animal actively interacted with the stimulus. In contrast, one-half of the sample of neurons from area 7b unequivocally responded only to somatic stimulation. Although many neurons in the lateral parts of area 7b were vigorously activated by innocuous tactile stimulation, others demonstrated little association with an identifiable somatic submodality, had sluggish responses, required complex, noxious, visual or other non-somatic stimuli for activation, and had labile response properties and receptive fields. Indeed, the responses of some area 7b neurons suggested a possible relationship with the animal's attention towards or anticipation of a noxious or a novel somatic stimulus. Neurons within the retroinsular cortex (Ri), which receives projections from the posterior nucleus (PO), primarily responded to light tactile stimulation of rapidly adapting skin receptors; less than 3% responded to moderate or high threshold mechanical stimulation. The sensitivity to tactile stimulation in Ri closely resembled the responses of SII neurons. Neurons in the granular insula (Ig) often responded to gentle hair deflection within receptive fields covering large areas of the body. Ig and area 7b were the principle loci within the lateral sulcus that contained neurons responding to noxious stimulation. Owing to the great similarity in the somatic response properties within these areas in the awake and unparalyzed animal, the designation of cortical areas could only be made after correlating the recording sites with connectional and cytoarchitectonic analyses in the same animal. Consequently, previous physiological studies may have attributed to SII some of the response characteristics of neurons in neighboring areas.

Contextual organization of unitary information processes in the cortex by the thalamus and basal ganglia and the central control of attention

Short-term memory experiments suggest the presence within the neocortex of a basic information unit (a data structure plus an algorithm) able to store and return a relatively fixed number of items. Units of a similar format may be involved in all cortical motor and sensory activities of an integrated nature, and may interact to yield higher-order-information units dealing with interrelationships among groups of units. If individual units can reference other similar units, a hierarchical organization of computing is possible that could form the basis for contextual information processing and the hierarchical structuring characteristic of many specifically human forms of behavior. Vertical columns of neurons in the cortex may provide the physical basis for information units of the type suggested. The selection and organizing of activity in groups of columns may be regulated by a neural loop involving the basal ganglia and the thalamus, which would thus implement the central adjustment of attention.

The organization of the second somatosensory area (SmII) of the grey squirrel

Microelectrode mapping methods were used to determine the organization of the second somatosensory representation, SmII, in grey squirrels. A systematic representation of the contralateral body surface was found in lateral parietal cortex adjoining the first somatosensory representation, SmI (Sur et al., '78a). The representation of the body in SmII was found to be much less distorted than in SmI. Under our recording conditions, almost all recording sites were activated from strictly contralateral body locations. The most important finding was that the basic orientation of the body representation in SmII is "erect" rather than "inverted." This orientation allows SmII and SmI to be adjoined along a common border representing the top of the head and face. This type of border has been called congruent (Allman and Kaas, '75; Kaas, '77), and it may have significance in the development of sensory representations.

Somatotopic organization of mechanosensory projections to SII cerebral neocortex in the raccoon (Procyon lotor)

The pattern of projections of peripheral receptors to the neocortex in the second somesthetic receiving area (SII) was mapped in raccoons. The purpose was to determine if the projection area of peripheral receptors to the forepaw area in the SII region is disproportionally enlarged as it is in SI. Tungsten microelectrode recording procedures were used to map thoroughly the inferior wall of the suprasylvian sulcus for regions responsive to mechanical stimulation of peripheral receptors. The results show that: 1. The forepaw area in SII shows an enlargement commensurate with that found in the SI. This suggests that those factors that are selective for tactile acuity of the raccoon forepaw were operating in the evolution of SII as they were in SI. 2. The somatotopic organization of mechanoreceptive projections to SII is reversed mediolaterally compared to previous descriptions of this arrangement in other mammals: projections form axial structures lie medially and those from apical structures lie laterally along the inferior bank of the suprasylvian sulcus in the raccoon.

The second somatic sensory area (SmII) of opossum neocortex

Organization of the neocortical second somatic sensory area (SmII) of anesthetized Virginia opossums has been examined utilizing micro-electrode recording techniques. SmII is situated between the first somatic sensory area (SmI) medially, and the rhinal fissure laterally. The head representation is located anteromedially within SmII, and the hindlimb representation posterolaterally, with the forelimb representation in between. Approximately 49% of SmII is devoted to representation of the head, 36% to forelimb representation, and 15% to trunk and hindlimb representation. All peripheral receptive fields (RF's) were either contralateral or bilateral. Approximately 63% of head RF's, 25% of forelimb RF's and 100% of hindlimb RF's were bilateral. For a given body locus, SmII RF's are larger than those for SmI. SmII is contained entirely within an area yielding evoked potentials responses to auditory click stimuli.