Imitation Strategies in Callosotomized Patients.

Imitation is a human ability rooted in early life. It allows people to interact with each other by observing and reproducing simple and complex movements alike. Imitation can occur in at least two forms: the rst, de ned as anatomical, seems to be based primarily on the mental construct of the "body schema" because the imitating movement corresponds precisely to the imitated movement in bodily terms, but not in terms of spatial compatibility. For example, a right arm movement of a model is imitated with a right arm movement by a facing imitator in a spatially incompatible fashion. The other form, de ned as specular or mirror-mode, involves a spatially compatible matching between imitated and imitating movements, as when an imitator moves her right arm upon viewing a corresponding left arm movement of a facing model (Chiavarino et al., 2007). In a previous study, healthy subjects showed a slight (61%) preference for the specular mode when freely imitating meaningful and meaningless gestures, whereas they strongly preferred the anatomical mode (93%) when given an intentionally ambiguous instruction such as "use the same (or the opposite) limb as the model" (Pierpaoli et al., 2014). In the present investigation it has been shown that callosotomized patients tended to favour the mirror-mode in both the free (66%) and the instructed condition (61% responses in driven sessions) regardless instructions given by the experimenter. Moreover, present data suggest that the extent of the callosotomy may in uence the patient's performance.

Further evidence for the topography and connectivity of the corpus callosum: an FMRI study of patients with partial callosal resection.

BACKGROUND AND PURPOSE: This functional MRI study was designed to describe activated fiber topography and trajectories in the corpus callosum (CC) of six patients carrying different degree of partial callosal resection. METHODS: Patients receiving gustatory, tactile, and visual stimulation according to a block-design protocol were scanned in a 1.5 Tesla magnet. Diffusion tensor imaging (DTI) data were also acquired to visualize spared interhemispheric fibers. RESULTS: Taste stimuli evoked bilateral activation of the primary gustatory area in all patients and foci in the anterior CC, when spared. Tactile stimuli to the hand evoked bilateral foci in the primary somatosensory area in patients with an intact posterior callosal body and only contralateral in the other patients. Callosal foci occurred in the CC body, if spared. In patients with an intact splenium central visual stimulation induced bilateral activation of the primary visual area as well as foci in the splenium itself. CONCLUSION: Present data show that interhemispheric fibers linking sensory areas crossed through the CC at the sites where the different sensory stimuli evoked activation foci, and that topography of callosal foci evoked by sensory stimulation in spared CC portions is consistent with that previously observed in subjects with intact CC.

Anatomical or mirror mode imitation? A behavioral approach.

Imitation can occur in at least two forms: one, which can be defined as anatomical, is based primarily on the mental construct of the body schema and allows recognition of correspondences between own body anatomy and that of other individuals. The other form, defined as specular or mirror mode, is most probably based on the allocation of some form of attention to the same region of the environmental space both by model and imitator, and to the objects it contains. This study investigated the behavioral strategy of imitation in normal subjects, to assess whether they carried out task instructions using an anatomical or a mirror perspective. Twenty seven adults were asked to imitate intransitive meaningful and meaningless gestures shown by a model in video clips. Instructions about how to perform them were provided before each trial. Trials were free (intended to produce spontaneous imitation) or driven (intended to produce anatomical imitation); further driven trials were administered to verify participants' knowledge of bodily laterality and were used as control. Performances were interpreted as anatomical or mirror imitation, according to the observation of anatomical or spatial reference frames between stimulus and imitator. The results revealed that in spontaneous imitation the mirror mode was more frequent (61% of responses), in line with previous studies. The novel finding was the prevalence (93% of responses) of anatomical imitation in tasks involving detailed driven instructions.

Contribution of the corpus callosum to bilateral representation of the trunk midline in the human brain: an fMRI study of callosotomized patients.

Human brain studies have shown that the cutaneous receptors of trunk regions close to the midline are represented in the first somatosensory cortex (SI) of both hemispheres. The present study aims to establish whether in humans, as in non-human primates, the bilateral representation of the trunk midline in area SI depends on the corpus callosum. Data were obtained from eight callosotomized patients: three with complete callosal resection, one with a partial posterior resection including the splenium and the callosal trunk, and four with partial anterior resections sparing the splenium and in one case also the posterior part of the callosal trunk. The investigation was carried out with functional magnetic resonance imaging. Unilateral tactile stimulation was applied by rubbing ventral trunk regions close to the midline (about 20 x 10 cm in width) with a soft cotton pad (frequency 1 Hz). Cortical activation foci elicited by unilateral stimulation of cutaneous regions adjacent to the midline were detected in the contralateral post-central gyrus (PCG), in a region corresponding to the trunk ventral midline representation zone of area SI, as described in a previous study of intact subjects. In most patients, activation foci were also found in the ipsilateral PCG, again as in subjects with an intact corpus callosum. The data confirm that the skin regions adjacent to the trunk midline are represented bilaterally in SI, and indicate that ipsilateral activation is at least partially independent of the corpus callosum.

Amyotrophic lateral sclerosis and sports: a case-control study.

An increased incidence of amyotrophic lateral sclerosis (ALS) amongst soccer players in Italy has recently been reported. A case-control study (300 cases and 300 matched controls) was conducted to explore the association between ALS and physical/sports activities, with specific reference to trauma-related risk. Neither the practice of competitive sports nor sports-related traumas were found to be associated with an increased risk of ALS. The practice of physical activities or sports is not per se a risk factor for ALS. Our results exclude sports-related microtraumas as etiopathogenic factors in the natural history of ALS.

Glutamic acid decarboxylase immunoreactivity in callosal projecting neurons of cat and rat somatic sensory areas.

The distribution of GABAergic callosally projecting neurons was analysed in the somatic sensory areas of cat and rat cerebral cortex by combining retrograde tracing of nerve cell bodies and glutamic acid decarboxylase (GAD) immunocytochemistry. A retrograde tracer (colloidal gold- labelled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase) was injected in the first or second somatic sensory area. Brain sections were processed for the simultaneous visualisation of the retrograde tracer and GAD immunoreactivity. In all animals, double-labelled neurons were found in the hemisphere contralateral to the injection site (double-labelled callosal neurons). Their proportion was similar in both species (0.8% of all retrogradely-labelled neurons in cat, 0.7% in rat). These results: 1) confirm the existence of a small proportion of GABAergic callosally projecting neurons in rat somatic sensory cortices; 2) indicate the presence of a small but significant proportion of GAD-positive callosally projecting neurons in cat somatic sensory cortices; and 3) show that the proportion of GAD-positive callosal neurons is similar in the two species.

Taste laterality in the split brain.

Two patients with corpus callosum resection, one complete and the other sparing the genu and the rostrum, were tested for discrimination of three basic taste stimuli (sour, bitter, salty) applied to the right or left sides of the tongue. Responses were made by pointing with either hand to written words or images of visual objects corresponding to the stimuli, a language-based discrimination. In both patients, response accuracy was significantly above chance for both hemitongues but there was a significant advantage for the left side. Reaction time was shorter for left stimuli than for right stimuli but the difference was not significant. Eight normal controls matched for age with the patients performed equally well with right and left hemitongue stimuli and so did a third callosotomy patient with sparing of the posterior callosum, including the splenium. Tactile and visual tests showed that the left hemisphere was responsible for language-based responses in the first two patients. The results confirm and extend previous findings in another callosotomy patient, indicating that: (i) taste information from either side of the tongue can reach the left hemisphere in the absence of the corpus callosum; (ii) the ipsilateral input from the tongue to the left hemisphere is more potent functionally than the contralateral input and (iii) in the normal brain, the corpus callosum, specifically its posterior part including the splenium, appears to equalize the effects of the ipsilateral and contralateral gustatory inputs on the left hemisphere. Taken together with evidence about lateralized taste deficits following unilateral cortical lesions, the results also suggest that the gustatory pathways from tongue to cortex are bilaterally-distributed with an ipsilateral predominance that may be subject to individual variations.

Posterior corpus callosum and interhemispheric transfer of somatosensory information: an fMRI and neuropsychological study of a partially callosotomized patient.

Interhemispheric somatosensory transfer was studied by functional magnetic resonance imaging (fMRI) and neuropsychological tests in a patient who underwent resection of the corpus callosum (CC) for drug-resistant epilepsy in two stages. The first resection involved the anterior half of the body of CC and the second, its posterior half and the splenium. For the fMRI study, the hand was stimulated with a rough sponge. The neuropsychological tests included: Tactile Naming Test (TNT), Same-Different Recognition Test (SDRT), and Tactile Finger Localization Test (intra- and intermanual tasks, TFLT). The patient was studied 1 week before and then 6 months and 1 year after the second surgery. Before this operation, unilateral tactile stimulation of either hand activated contralaterally the first (SI) and second (SII) somatosensory areas and the posterior parietal (PP) cortex, and SII and PP cortex ipsilaterally. All three tests were performed without errors. In both postoperative sessions, somatosensory activation was observed in contralateral SI, SII, and PP cortex, but not in ipsilateral SII and PP cortex. Performance was 100% correct in the TNT for the right hand, but below chance for the left; in the other tests, it was below chance except for TFLT in the intramanual task. This case provides the direct demonstration that activation of SII and PP cortex to stimulation of the ipsilateral hand and normal interhemispheric transfer of tactile information require the integrity of the posterior body of the CC.

Role of the corpus callosum in the somatosensory activation of the ipsilateral cerebral cortex: an fMRI study of callosotomized patients.

To verify whether the activation of the posterior parietal and parietal opercular cortices to tactile stimulation of the ipsilateral hand is mediated by the corpus callosum, a functional magnetic resonance imaging (fMRI, 1.0 tesla) study was performed in 12 control and 12 callosotomized subjects (three with total and nine with partial resection). Eleven patients were also submitted to the tactile naming test. In all subjects, unilateral tactile stimulation provoked a signal increase temporally correlated with the stimulus in three cortical regions of the contralateral hemisphere. One corresponded to the first somatosensory area, the second was in the posterior parietal cortex, and the third in the parietal opercular cortex. In controls, activation was also observed in the ipsilateral posterior parietal and parietal opercular cortices, in regions anatomically corresponding to those activated contralaterally. In callosotomized subjects, activation in the ipsilateral hemisphere was observed only in two patients with splenium and posterior body intact. These two patients and another four with the entire splenium and variable portions of the posterior body unsectioned named objects explored with the right and left hand without errors. This ability was impaired in the other patients. The present physiological and anatomical data indicate that in humans activation of the posterior parietal and parietal opercular cortices in the hemisphere ipsilateral to the stimulated hand is mediated by the corpus callosum, and that the commissural fibres involved probably cross the midline in the posterior third of its body.

Localization of the first and second somatosensory areas in the human cerebral cortex with functional MR imaging.

BACKGROUND AND PURPOSE: Our objective was to map by means of a conventional mid-field (1.0 T) MR imaging system the somatosensory areas activated by unilateral tactile stimulation of the hand, with particular attention to the areas of the ipsilateral hemisphere. METHODS: Single-shot echo-planar T2*-weighted imaging sequences were performed in 12 healthy volunteers to acquire 10 contiguous 7-mm-thick sections parallel to the coronal and axial planes during tactile stimulation of the hand. The stimulation paradigm consisted of brushing the subjects' palm and fingers with a rough sponge at a frequency of about 1 Hz. RESULTS: Stimulation provoked a signal increase (about 2% to 5%) that temporally corresponded to the stimulus in several cortical regions of both hemispheres. Contralaterally, activation foci were in the anterior parietal cortex in an area presumably corresponding to the hand representation zone of the first somatosensory cortex, in the posterior parietal cortex, and in the parietal opercular cortex forming the upper bank of the sylvian sulcus and probably corresponding to the second somatosensory cortex. Activation foci were also observed in the frontal cortex. Ipsilaterally, activated areas were in regions of the posterior parietal and opercular cortices roughly symmetrical to those activated in the contralateral hemisphere. The same activation pattern was observed in all subjects. CONCLUSION: The activated areas of the somatosensory cortex described in the present study corresponded to those reported in other studies with magnetoelectroencephalography, positron emission tomography, and higher-field functional MR imaging. An additional area of activation in the ipsilateral parietal operculum, unnoticed in other functional MR imaging studies, was also observed.

The cerebral ventricles, the animal spirits and the dawn of brain localization of function.

This paper reviews the early history of brain localization of function. It analyses the doctrines professed in ancient times by philosophers and physicians, who believed that brain functions were carried out in the cerebral ventricles by the psychic pneuma, or animal spirit, a sort of special and light substance endowed with the power to perform sensory, motor and mental activities. This theory, conceived in the Classic Age and called "ventricular-pneumatic doctrine", evolved in the 4th-5th centuries A.D. into the "three-cell theory", according to which each cerebral ventricle was the seat of a specific function, and contained a unique type of spirit with the power to perform that function. The three-cell theory represents the earliest attempt to localize different mind functions in separate brain sites and was held true by Byzantine, Arabian and Western Latin scholars well beyond the Renaissance. The paper is subdivided into an Introduction and eight sections. The first two sections report a brief history of the philosophical and medical doctrines about the pneuma as mediator of all vital functions, the ventricular-pneumatic doctrine elaborated by Galen of Pergamon, and his theory of nerve physiology based on the assumption that the pneuma, set in motion by active brain movements and flowing in the hollow nerves, could transfer sensations from the sense organs to the anterior ventricles, and motor commands from the posterior ventricle to the muscles. The third and fourth sections trace the ways in which these doctrines were transmitted to the Byzantine physicians and then to the Arabs, until they reached the Latin West. Here, throughout the Middle Ages they not only formed the background of medical and natural philosophy, but also influenced Christian theologians. The fifth section is devoted to the ventricular localization of mind faculties, called internal senses by Arabian and Western Latin scholars. Most authors recognized three basic internal senses: imagination, cognition and memory, and generally localized imagination in the anterior ventricle, cognition in the middle and memory in the posterior one, while other scholars adopted complex lists including up to seven faculties, each carried out by a specific type of animal spirit and localized, or sub-localized, in different ventricular sites according to complex topographical patterns. This section reports more than sixty patterns of ventricular localization from various authors (summarized in a Table), the rationale of complex ventricular localization, and the naive interpretations of Medieval physicians and surgeons of the impairment of the internal senses caused by brain disease and trauma. The sixth section deals with the decline of the three-cell theory, which was first challenged in the early 16th century and then drastically revised by several Renaissance and post-Renaissance experimentalists, anatomists and philosophers, although some remnants of the Galenic pneumatic neurophysiology survived in medicine until the 18th century. The penultimate section analyses bibliographical data on the earliest localizationists and shows that, independently of chronological priority, Nemesius of Emesa was the source of the pattern of ventricular localization of function adopted by later Byzantines and by the Arabs, and then transmitted to Latin Western scholars. The last section discusses the legacy of the three-cell theory to later generations of neuroscientists.

Glutamate decarboxylase immunoreactivity in corticocortical projecting neurons of rat somatic sensory cortex.

Combined retrograde tracing and immunocytochemical experiments were carried out on rats to ascertain whether corticocortical projecting neurons in the somatic sensory areas are immunoreactive to an antiserum against glutamate decarboxylase. Injections of a retrograde tracer (colloidal gold-labelled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase) in the first somatic sensory area labelled neurons in the injected area, in the second somatic sensory area, and in the parietoventral area of the ipsilateral hemisphere. The topographical and laminar distribution of these retrogradely-labelled corticocortical neurons in the first and second somatic sensory areas and in the parietoventral area was in line with a previous description (Fabri M. and Burton H. (1991b) J. comp. Neurol. 311, 405-424). In sections processed for the simultaneous visualization of the retrograde tracer and glutamate decarboxylase immunoreactivity, a number of neurons were double-labelled. Double-labelled neurons were most numerous in the first somatic sensory cortex, where they accounted for 5% of all retrogradely-labelled neurons. Outside this region, double-labelled cells were observed in the second somatic sensory cortex and in the parietoventral cortex, where they amounted respectively to 2.8% and 2.3% of all corticocortical neurons labelled in these two areas. Glutamate decarboxylase-immunopositive corticocortical neurons were mainly concentrated in the infragranular layers (73.8% of all double-labelled neurons in the first somatic sensory area, 81.7% in the second somatic sensory area, and 76.5% in the parietoventral area). The results indicate the presence of a small but significant contingent of GABAergic inhibitory neurons in the associative connections of the somatic sensory areas.

Laminar pattern of termination of the ipsilateral cortical projection from SII to SI in cats.

The present light and electron microscopic experiments were carried out on the first somatic sensory area (SI) of cats to determine the laminar distribution of axon terminals from the ipsilateral second somatic sensory area (SII) and to identify the types of synapses between these terminals and the neuronal elements of SI. Phaseolus vulgaris-leucoagglutinin (PHA-L) was iontophoretically injected into multiple sites and at different cortical depths of the forepaw representation zone of SII. Fixed brain blocks containing the injected SII and ipsilateral SI were cut into slices and processed immunocytochemically to stain PHA-L-filled fibers and terminals. Light microscopic examination of SI revealed patches of anterograde labeling in the forepaw representation zone, concentrated mainly in supragranular layers. In these layers, thin immunolabeled fibers branched extensively and formed a dense plexus that was more prominent in layers II and I. Conversely, the infragranular layers contained fragments of vertically oriented thick fibers that rarely emitted axon collaterals. PHA-L-labeled axons had numerous swellings along their course, interpreted as boutons en passant, and stalked boutons. Of 19,661 labeled terminals (17,833 beads and 1,828 stalked boutons), 84.74% were observed in supragranular layers, with the highest concentration in layer II (33.15%) and lower in layers I (26.27%) and III (25.30%). The proportion of terminals was lower in layers IV (6.49%) and V (5.45%) and lowest in layer VI (3.32%). These counts also showed that boutons en passant were the majority (90.70%) and stalked boutons, the minority (9.30%). The ratio of these two types of presynaptic specializations was similar (9:1) in all six layers. Electron microscopic examination of the labeled regions of SI showed that both axon swellings and stalked boutons formed synapses of the asymmetric type with SI neuronal elements. The majority (85.37%) of a sample of 130 labeled terminals synapsed on SI neurons in layers I-III. The identified postsynaptic profiles were dendritic spines (61.11%) or medium-sized and small dendrites (38.89%). These results are discussed in relation to those of a companion study on the laminar pattern of the projection from SI to SII of cats (P. Barbaresi, A. Minelli, and T. Manzoni, 1994, J. Comp. Neurol. 343:582-596). Based on the anatomical organization of these reciprocal connections, there seems to be no clear hierarchicalal relationship between SI and SII in cats.

Topographical relations between ipsilateral cortical afferents and callosal neurons in the second somatic sensory area of cats.

Experiments were carried out on the second somatic sensory area (SII) of cats to study 1) the laminar distribution of axon terminals from the ipsilateral first somatic sensory cortex (SI); and 2) the topographical relations between their terminal field and the callosal neurons projecting to the contralateral homotopic cortex. To label simultaneously in SII both ipsilateral cortical afferents and callosal cells, cats were given iontophoretic injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) in the forepaw zone of ipsilateral SI, and pressure injections of horseradish peroxidase (HRP) in the same zone of contralateral SII. The possibility that ipsilateral cortical axon terminals synapse callosal neurons was investigated with the electron microscope by combining lesion-induced degeneration with retrograde HRP labelling. Fibers and terminations immunolabelled with PHA-L from ipsilateral SI were distributed in SII in a typical patchy pattern and were mostly concentrated in supragranular layers. Labelled fibers formed a very dense plexus in layer III and ramified densely also in layers I and II. Labelled axon terminals were both en passant and single-stalked boutons. Counts of 8,303 PHA-L-labelled terminals of either type showed that 82.40% were in supragranular layers. The highest concentration was in layer III (43.99%), followed by layers II (30.32%) and I (8.09%). The remaining terminals were distributed among layers IV (6.96%), V (4.93%), and VI (5.68%). The same region of SII containing anterogradely labelled axons and terminals also contained numerous neurons retrogradely labelled with HRP from contralateral SII. Callosal projection neurons were pyramidal, dwelt mainly in layer III, and were distributed tangentially in periodic patches. Patches of anterograde and retrograde labelling either interdigitated or overlapped both areally and laminarly. In the zones of overlap, numerous PHA-L-labelled axon terminals were seen in close apposition to HRP-labelled pyramidal cell dendrites. Combined HRP-electron microscopic degeneration experiments showed that in SII axon terminals from ipsilateral SI form asymmetric synapses with HRP-labelled dendrites and dendritic spines pertaining to callosal projection neurons. These results are discussed in relation to the layering and function of the SI to SII projection, and to the evidence that SII neurons projecting to the homotopic area of the contralateral hemisphere have direct access to the sensory information transmitted from ipsilateral SI.

Calcitonin gene-related peptide (CGRP) in the cat neocortex: evidence for a sparse but widespread network of immunoreactive fibers.

The morphology, and laminar and topographic distribution of fibers containing calcitonin gene-related peptide (CGRP) immunoreactivity were studied by light and electron microscopic methods in the cerebral cortex of adult cats using a rabbit antiserum raised against the C-terminal region of the rat alpha-CGRP. At the light microscopic level, a sparse number of CGRP-positive fibers were observed in the frontal, parietal, and occipital cortices. They showed numerous irregularly spaced varicosities, were mostly oriented vertically, and in rare cases gave rise to boutons terminaux as they ascended toward the pial surface. At the border between layers I and II, they branched into horizontal fibers that could be followed for several hundred microns in layer I and gave rise to terminal clusters of boutons. In some sections, CGRP-positive fibers were seen in close association with blood vessels. At the electron microscopic level, CGRP immunoreactivity was found in axon terminals containing few mitochondria and clear synaptic vesicles. CGRP-positive axon terminals were very sparse, and mainly of small size. The majority formed conventional synapses, all of the asymmetric type. CGRP-positive fibers showed an uneven topographic distribution through the cortical mantle, with the frontal areas exhibiting the highest density and the occipital cortex the lowest. These results show that CGRP-containing axons are more widely distributed than previously thought since they were observed in all the cortical areas examined, and cast some doubts on the hypothesis that the functional role of this peptide is restricted to the processing of visceral sensory information.(ABSTRACT TRUNCATED AT 250 WORDS)

Substance P-containing pyramidal neurons in the cat somatic sensory cortex.

Light and electron microscopic immunocytochemical methods were used to verify the possibility that neocortical pyramidal neurons in the first somatic sensory cortex of cats contain substance P. At the light microscopic level, substance P-positive neurons accounted for about 3% of all cortical neurons, and the vast majority were nonpyramidal cells. However, 10% of substance P-positive neurons had a large conical cell body, a prominent apical dendrite directed toward the pia, and basal dendrites, thus suggesting they are pyramidal neurons. These neurons were in layers III and V. At the electron microscopic level, the majority of immunoreactive axon terminals formed symmetric synapses, but some substance P-positive axon terminals made asymmetric synapses. Labelled dendritic spines were also present. Combined retrograde transport-immunocytochemical experiments were also carried out to study whether substance P-positive neurons are projection neurons. Colloidal gold-labelled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase was injected either in the first somatic sensory cortex or in the dorsal column nuclei. In the somatic sensory cortex contralateral to the injection sites, a few substance P-positive neurons in layers III and V also contained black granules, indicative of retrograde transport. This indicates that some substance P-positive neurons project to cortical and subcortical targets. We have therefore identified a subpopulation of substance P-positive neurons that have most of the features of pyramidal neurons, are the probable source of immunoreactive axon terminals forming asymmetric synapses on dendritic spines, and project to the contralateral somatic sensory cortex and dorsal column nuclei. These characteristics fulfill the criteria required for classifying a cortical neuron as pyramidal.

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.

Matching of receptive fields in the association projections from SI to SII of cats.

Anatomical and electrophysiological experiments were performed on cats to investigate the pattern of divergence and convergence in the association projections from the first (SI) to the second (SII) somatic sensory cortex and to ascertain whether diverging and converging fibre components from SI have receptive fields (RFs) matching those of target neurons in SII. In the first group of six cats, a single deposit of horseradish peroxidase (HRP) was iontophoretically placed (2-4 microA for 20 minutes) into an electrophysiologically identified site of the SII map: the digit (3 cats), forepaw (2 cats), and arm (1 cat) zones. The forelimb representation in ipsilateral SI was subsequently explored with microelectrodes and RFs from small clusters of neurons systematically mapped. Planar maps of this area were reconstructed with the aid of a computer from serial sections, to correlate on the tangential plane the topographical distribution of retrogradely labelled association neurons with the physiological map of the forelimb. Since diverging projections were observed from a zone of SI to multiple zones of SII, double-labelling experiments were carried out in a second group of three cats, in which two retrograde fluorescent dyes (diamidino yellow and fast blue) were injected by pressure into two different sites of the SII map, to ascertain whether SI sends diverging projections by branching axons. HRP injections in SII retrogradely labelled a discrete number of association neurons in SI. Their distribution area was several tens of times wider than that covered by the injection site. This suggests that a remarkable amount of divergence and convergence exists in the association projections from SI to SII. Despite the substantial difference in the extent of the injected and labelled areas, RFs of afferent and target neurons corresponded closely. Injections covering a small region within a single digit zone of SII labelled neurons throughout the entire representation of the same digit in SI, while neurons labelled in somatotopically inappropriate zones were rare. RFs mapped from several sites of the labelled region in SI were individually smaller than the RF mapped from the injection site in SII, but the overall size of afferent RFs encompassed that of target neurons. Divergence and convergence in the SI projections to SII zones representing more proximal portions of the forelimb may be even greater since HRP injections in the forepaw and arm zones of SII labelled a number of neurons also in the digit zone of SI, providing the RFs mapped from the injection sites were sufficiently wide to include the digits.(ABSTRACT TRUNCATED AT 400 WORDS)