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The effects of an early, unilateral cerebral hemispherectomy on the cytoarchitecture of the dorsal lateral geniculate nucleus (dLGN) were quantitatively evaluated in the green monkey. The dLGN ipsilateral to the lesion showed a 73% reduction in size, more than 99% neuronal cell loss, 50% increase in glial cell density, but a 50% reduction in the total number of glial cells. The total number of neural and glial cells estimated for the dLGN contralateral to the ablation did not differ from control values. Despite evidence for substantial degeneration of the ipsilateral dLGN, cytochrome oxidase histochemistry revealed a small population of surviving cells that exhibited features of neuronal cells. More surviving cells were found in the parvocellular than in the magnocellular layers, and surviving parvocellular cells had the same size-frequency distribution as Nissl-stained neurons in an intact animal. These findings suggest that the intrinsic geniculate circuitry may be able to sustain the residual interneurons that can, in turn, contribute to maintaining retinal and brainstem afferents. The remaining neurons in the dLGN following hemispherectomy appear to be insufficient in number to be importantly implicated in the residual visual functions that have been reported in some hemispherectomized patients.

We have used positron emission tomography to investigate the cortical areas of the normal human brain involved in processing inward (Expansion) and outward (Contraction) radial motion simulated with an optic flow stimulus. The optical flow display was made out of dots moving radially away from or toward the center of the display monitor. In the Control condition, the dots' motion was randomized in order to remove any sensation of radial motion. In the Expansion condition, several loci of activation were observed: visual areas V2-V3 and the superior parietal lobule (BA 7), predominantly in the right hemisphere. In the Contraction condition activation sites were found in the same visual areas (V2 and V3) in the right hemisphere but the increase in rCBF in these regions was much lower than in the Expansion condition. BA 7 was activated in both hemispheres. When the motion component of the stimulus was isolated by subtracting the static condition from the incoherent motion condition, we obtained activations of areas V2, V3, and MT (putative V5). These results indicate that the detection of radial motion derived from an optic flow stimulus is mediated by structures forming the dorsal part of the visual cortical system and confirm that area MT is not specifically involved in flow analysis., (Copyright 2001 Academic Press.)

Hemispherectomized patients are able to perceive tactile and painful stimuli on their nonparetic as well as paretic body halves. We have used functional MRI to study the cortical mechanisms underlying this preserved somatosensory capacity. Nonpainful brushing and painful heat were applied to the skin of the legs in four hemispherectomized patients and, for comparison, in four normal subjects. Cortical activation was studied with a 1.5 T scanner using a BOLD (blood oxygen level dependent) protocol. All patients rated both the brushing and the heat pain as almost equally intense on each leg and the ratings were similar to those in normals. Brushing on the nonparetic leg activated primary and secondary somatosensory cortices (S1 and S2) in all patients, similar to findings in normals. Brushing on the paretic leg activated S1 in two patients and S2 in one of these patients. Heat pain activated S2, insular cortex and anterior cingulate cortex to a similar degree for both legs, but the activation was weaker in the patients than in the normals. For the individual patient, there was generally no obvious correlation between cortical activation as studied with the BOLD technique and psychophysical performance. The findings from tactile stimulation of the nonparetic leg, that the activation was similar to the contralateral activation in normals, suggest that tactile information processing in the hemisphere contralateral to the stimulation is independent of the corpus callosum. In contrast, the pain activation for the nonparetic leg was weaker than in normals, suggesting that pain activation in the hemisphere contralateral to the stimulation is dependent on transcallosal information processing. The latter finding was corroborated by a subnormal capacity for pain localization on the nonparetic foot in two of the patients. The findings from stimulation of the paretic leg show that areas typically involved in the processing of tactile and painful stimuli can be activated by ipsilateral pathways directly from the periphery. The tactile-evoked ipsilateral S1 activation may be due to subcortical reorganization, since it was not observed in the normal subjects.

The anatomical consequences of unilateral cerebral hemispherectomy in some animal models are reviewed. We have shown that the retinogenigulate pathway undergoes severe degenerative changes in hemispherectomized monkeys, greater than those shown in cats and we proposed that remaining retinal terminals to the dorsal lateral geniculate nucleus have little potential for conveying visual information any further. All subdivisions of the pulvinar undergo severe degeneration following hemispherectomy showing that the ascending tectofugal pathway is also shut off. On the other hand, the retina subserving the blind field is not depleted of ganglion cells which still send normal appearing terminals to the midbrain pretectum and superior colliculus. Visual information from the blind hemifield can thus gain access to the brain and could potentially reach the contralateral cerebral cortex through the midbrain commissure and possibly through thalamic commissural cells.

We studied visually guided behavior and the visual response properties of single auditory cortex (A1) neurons in neonatally operated hamsters with surgically induced, permanent, ectopic retinal projections to auditory thalamic nuclei and to visual thalamic nuclei which normally receive little direct retinal input. The surgically induced retino-thalamo-cortical pathways can mediate visual guided behaviors whose normal substrate, the pathway from the retina to the primary visual cortex via the primary thalamic visual nucleus, is missing. The visually evoked response properties of A1 neurons resemble in many respects those of neurons in V1 of normal hamsters: many A1 neurons have well-defined visual receptive fields and preferences for orientation or direction of movement. In addition, some visually responsive cells in A1 are bimodal--they also respond to auditory stimuli. The visually responsive neurons in A1 probably account for the capacity of the auditory cortex to mediate visual behavior in 'rewired hamsters'.

Destruction of the striate cortex has traditionally been thought to lead to permanent blindness in the contralateral visual field and to the dogma that this region is indispensable for vision in primates. For over 25 years now, evidence has been accumulating that hemianopic human subjects and monkeys possess wide-ranging residual visual capacities or 'blindsight' in the blind part of their visual field. For some researchers, isolated islands of the striate cortex have been associated with patches of degraded vision and made responsible for blindisight. Artefacts such as light scatter, criterion effects, macular sparing, eccentric fixation and minute eye movements have also been linked with the residual vision. For others, the fact that certain aspects of the visual information can be processed without the geniculostriate pathway suggests mediation by the visual subsystems such as extrastriate visual cortical areas which receive visual information via subcortical pathways, that escaped the cortical damage. Subjects who have had a whole cerebral hemisphere removed or disconnected (for the treatment of uncontrollable epilepsy) and who show residual vision in their blind field offer a remarkable opportunity to help clear the controversy regarding the neural substrate of blindsight. Because it is certain that no functional striate or extrastriate cortex remains on the ablated side, these subjects have contributed significantly to identifying the critical pathways involved in blindsight.

The total number, distribution and peak density of ganglion cells were evaluated in the Nissl-stained retina of the ostrich (Struthio camelus). The mean (n = 4) total number of retinal ganglion cells (RGC) was estimated at 2,274,128 (s.d. = 273, 152). The ostrich retina exhibited a prominent horizontal visual streak along which a central area located nasal to the pecten had a peak density of 9,500 cells/mm2. A high concentration of cells with a peak density of 2,646 cells/mm2 was also observed in the temporal retina, slightly dorsal to the visual streak. The results further showed that the ostrich eye has a 15-mm pupil entrance diameter, its mean axial length is 39.81 mm, the estimated retinal magnification factor is 0.4075 mm/deg and the maximum visual acuity along the well-defined visual streak was estimated to be 19.32 cycles/deg. The latter component of the retina might subserve vision along the horizon while the temporal region mediates binocular processing. The data also showed that the degree of retinal illumination in this bird could be comparable to that noted in some nocturnal species. The findings in this study suggest that the ostrich might not be restricted to diurnal activity., (Copyright 2002 S. Karger AG, Basel)

Combined lesions of retinal targets and ascending auditory pathways can induce, in developing animals, permanent retinal projections to auditory thalamic nuclei and to visual thalamic nuclei that normally receive little direct retinal input. Neurons in the auditory cortex of such animals have visual response properties that resemble those of neurons in the primary visual cortex of normal animals. Therefore, we investigated the behavioral function of the surgically induced retino-thalamo-cortical pathways. We showed that both surgically induced pathways can mediate visually guided behaviors whose normal substrate, the pathway from the retina to the primary visual cortex via the primary thalamic visual nucleus, is missing.

Transneuronal retrograde cell changes in the retina of the primate have been well documented after lesions to striate cortex, but little is known about the effects of hemispherectomy, a surgical procedure used in humans for the treatment of intractable epilepsy. In order to follow the time course of this degenerative process, we examined the retinae of six monkeys who underwent a total hemispherectomy at various postnatal ages with a survival period of 4 years. We demonstrate that transneuronal retrograde degeneration in the retina following hemispherectomy is inversely correlated with age at the time of the lesion. This degeneration is maximal when the lesion is induced within the first 4-6 months of life and less pronounced from 8 months to adulthood.

The notion of blindsight was recently challenged by evidence that patients with occipital damage and contralateral field defects show residual islands of vision which may be associated with spared neural tissue. However, this possibility could not explain why patients who underwent the resection or disconnection of an entire cerebral hemisphere exhibit some forms of blindsight. We present here a model for the detection of intraocular scatter, which can account for human sensitivity values obtained in the blind field of hemidecorticated patients. The model demonstrates that, under controlled experimental conditions i.e. where the extraocular scatter is eliminated, Lambertian intraocular scatter alone can account for the visual sensitivities reported in these patients. The model also shows that it is possible to obtain a sensitivity in the blind field almost equivalent to that in the good field using the appropriate parameters. Finally, we show with in-vivo spectroreflectometry measurements made in the eyes of our hemidecorticated patients, that the relative drop in middle wavelength sensitivity generally obtained in the blind field of these patients can be explained by selective intraocular spectral absorption.

To assess the topographical distribution of sleep spindles and K-complexes, four 15-minute samples of stage 2 sleep in a group of eight healthy young adults were analyzed. Results show that a majority of spindles generated are detected over central regions, and that K-complexes are markedly predominant over prefrontal and frontal regions. These findings are consistent with the single-spindle generator hypothesis and raise questions concerning the Rechtschaffen and Kales rules for scoring K-complexes.

The topographical distribution of density and the soma size of retinal ganglion cells (RGCs) were studied in the St. Kitts green monkey (Cercopithecus aethiops sabeus). The total number of RGCs, estimated from light microscopic analysis of wholemounted and of transversely sectioned retinae, ranged between 1,183,721 and 1,273,715 (mean 1,228,646). These estimates are comparable to the number of optic nerve fibres (1,220,000) estimated from semithin sections. The topographic distribution of RGCs shows a strong centroperipheral gradient. The soma size distribution of RGCs in Nissl-stained flatmounts falls within a range of between 5.7 microm and 22.9 microm and is comparable to other primate species. Somata of RGCs were found to be generally smaller within the fovea than in peripheral regions. Ganglion cells, as reported for other diurnal primates, are nonuniformly distributed with a slight nasotemporal elongation of isodensity contours, and they exhibit nasotemporal asymmetry in the frequency distribution of soma size. The topography of the RGC distribution of this semiarboreal, ground-dwelling monkey is similar to what has been found in other diurnal Old World species.

Investigations of dream mentation in brain damaged patients have shed some light on the controversial issue of cerebral lateralization of dreaming. To examine further the relationships between brain function and dreaming, we studied REM sleep dream recall and content in four patients having undergone right functional or anatomical hemispherectomy and eight matched control subjects. Patients were found to have the capacity to report dreams to much the same extent as control subjects. Further, the patients' dream content was overall similar to that of the control subjects. The results provide strong evidence that dreaming is not a right-hemisphere function, and that the left hemisphere may be more critical for the generation of dreams. In addition, some characteristics of hemispherectomized patients' dream content (characters, smells) are consistent with the possibility that a history of epilepsy may influence REM sleep imagery over the long term.

The present study provides a demonstration of blindsight in two hemispherectomy patients who showed a visual spatial summation effect across the vertical meridian despite their lack of visual awareness in one hemifield. Such an effect cannot be related to light diffusion onto the sighted hemifield because it was not present when one of the stimuli fell into the retinal blind spot of control subjects. We conclude that blindsight phenomena of the simple type described in the present study can be subserved by sub-cortical mechanisms and do not necessarily require cortical processing.

Using a guessing paradigm we measured visual sensitivity in the blind and normal half-fields of four cerebrally hemidecorticated patients. In the blind field, sensitivity was reduced by approximately 3 long units. Stimuli which produced significant detection also evoked conscious sensations of light and colour. Control experiments showed that although sensitivity in the blind field depended in a normal fashion on background luminance it was independent of the luminance of a local platform, and showed no spatial summation. This residual vision can be explained by intraocular light diffusion and reflection.

The present study investigated the postnatal establishment of the laminar pattern of acetylcholinesterase (AChE) activity in the visual cortex (Oc1) of normal and neonatally hemidecorticate rates. Rat pups received a hemidecortication on post-natal day (PND) 3 and sacrificed at three day intervals starting at PND-6 through PND-24. Laminar patterns of AChE activity in Oc1 are described qualitatively and quantitatively using optical densitometry. The postnatal development of the laminar distribution of AChE activity is similar in normal and hemidecorticate rats. In both cases, AChE activity is intense in layer I, in the deep layer III as well as in layer IV. This pattern is first detected at the end of the first postnatal week. AChE activity reaches a peak intensity during the third postnatal week and gradually declines to adult levels during the fourth postnatal week. Hemidecortication has no significant effect on the intensity of AChE activity measured in the visual cortex. Neonatal hemidecortication does not affect AChE activity levels, structure of AChE neurites or the laminar distribution pattern, nor does it affect the time course of the establishment of this pattern in Oc1 of the remaining cortex. These data do not support the hypothesis that massive cortical lesions in rats result in an increase in contralateral cholinesterase activity nor do they suggest terminal sprouting of basal forebrain projections to the visual cortex.

In this series of studies, we have attempted to characterize anatomically the organization of the retinofugal pathways in monkeys that underwent the surgical removal in infancy of the entire left cerebral hemisphere. Hemidecordication in baby monkeys produced a transneuronal retrograde degeneration of the retinal ganglion cells (RGCs) that affected mainly the foveal rim. Although the density of RGCs in this region was drastically diminished, the soma sizes of the surviving cells remained normal. The lateral geniculate nucleus (dLGN) ipsilateral to the removed cortex was dramatically reduced in size although it still showed normal layering. There was a marked reduction in the number of neurons in both the parvocellular and magnocellular layers and a heavy gliosis. By contrast, the superior colliculus ipsilateral to the lesion was remarkably well preserved: although slightly reduced in volume, it showed little gliosis and a metabolic activity, as revealed by cytochrome oxidase histochemistry, similar to the superior colliculus contralateral to the lesion. Behavioral perimetry indicated a partial sparing of vision up to 45 degrees in the 'blind' hemifield. We argue that the preservation of the retino-tectal pathway mediates most of the residual visual functions found in the 'blind field' of hemispherectomized human subjects.

Visual acuity was measured behaviorally in various groups of cats by using a two-choice discrimination procedure. Cats in group 1 were rendered strabismic soon after birth by sectioning the tendon of the lateral rectus muscle (unilateral esotropia); at adulthood, their visual acuity (VA) was evaluated, after which the optic chiasm was sectioned and VA reassessed. Cats in group 2 were not only tenotomized but also chiasmatomized neonatally, while cats in group 3 underwent a neonatal section of the optic chiasm only. VA was measured at adulthood in the two latter groups. Group 4 consisted of adult cats whose VA was evaluated before and after an optic chiasm section. Stimuli consisted of square-wave gratings of various spatial frequencies. Results showed that in normal cats, the average threshold values under monocular viewing were identical for each eye (4.76 cycles/degree); however, following optic chiasm section, monocular VA was reduced to 1.23 cycles/degree. VA in early optic chiasm section cats was lower than that of the normal cats but higher than that of late-lesioned animals (2.33 cycles/degree). In strabismic cats, mean VA was 1.25 cycles/degree for the deviated eye and 2.8 cycles/degree for the normal eye. Following the optic chiasm section at adulthood, VA was lower not only for the deviated eye (< 0.17 cycles/degree) but also for the normal eye (1.14 cycles/degree). Similar results were found when both the deviation and chiasmatomy were performed neonatally. The elimination of interocular interactions through chiasm transection failed to improve VA in the strabismic eye.

In cats raised with surgically induced esotropia and early section of the optic chiasm visually responsive neurons were recorded from areas 17-18 ipsilaterally and contralaterally to the deviated eye. In these animals both cortices were monocularly activated only through the ipsilateral eye. In addition, unlike to that occurring in otherwise intact strabismic animals, the esotropic eye drove a number of neurons equivalent to that driven by the non-deviated eye. We suggest that an inhibitory influence exterted over the ipsilateral afferents of the deviated eye is removed by the early section of the crossed retinal projections.

During normal postnatal development, there is an overproduction and subsequent partial elimination of the callosal projections of cortical areas 17 and 18 in the cat. In the present study, we investigated how neonatal splitting of the optic chiasm affects this process. Our results indicate that neonatal splitting of the optic chiasm exaggerates the normally occurring partial elimination of immature callosal projections: it causes a significant reduction in the total number of neurons in the supragranular layers that send an axon through the corpus callosum. It does not, however, cause a significant change in the number of callosally projecting neurons in the infragranular layers. These data suggest that in addition to other factors previously described, the level or spatial distribution of correlated binocular input to visual cortical neurons may influence the stabilization/elimination of immature callosal connections.

The corpus callosum, the principal neocortical commissure, allows for the interhemispheric transfer of lateralized information between the hemispheres. The aim of the present experiment was to study callosal transfer of auditory information in the cat, with particular reference to its contribution to sound localization. The corpus callosum was approached under direct visual control, and axonic responses were recorded under light anesthesia using glass micro-pipettes. Results showed that auditory information is transmitted in the posterior portion of the callosum. Diotic presentations, in which interaural time delay was manipulated, indicated that, for a large number of fibers, the largest excitatory or inhibitory interactions were obtained at null interaural time delay, a condition which supports the notion of a callosal contribution to auditory midline fusion. However, an important number of callosal fibers was also found to be excited maximally at specific, non-zero interaural time delays, suggesting that they preferred sounds situated at spatial locations other than the midline. The results are discussed in relation to those obtained electrophysiologically for the visual and somesthesic modalities and in terms of results obtained in human and animal behavioral experiments.

Alpha-1 and alpha-2 adrenergic receptors were localized in developing cat visual cortex by using [3H]prazosin and [3H]rauwolscine, respectively as selective ligands. The effects of neuronal input on the development of the two receptor subtypes were also studied in animals with lesions at various sites within the central visual pathways. Binding densities for both ligands increased during the first few postnatal weeks and declined thereafter. For both receptor subtypes, the highest concentration of binding sites was found in the subplate zone of the cortex in neonatal animals. Both ligands showed their highest concentrations in cortical layer IV beginning at postnatal day 30 and in the superficial cortical layers in adulthood. However, the developmental redistribution of alpha-1 receptors began at earlier ages than that of the alpha-2 sites. The alpha-1 sites were still concentrated in the subplate zone up to 60 days postnatal, while the alpha-2 sites in this region disappeared much earlier. Receptor binding densities were also examined in animals with quinolinic acid lesions within cortex, lesions of the lateral geniculate nucleus and lesions of the optic tract. The results indicate that both alpha-adrenoceptor subtypes were mainly located on cortical cells, and that the absence of neuronal activity during development resulted in a reduction of the binding density for both subtypes in the visual cortex. An additional major reduction in alpha-2 but not alpha-1 binding sites was observed following the lateral geniculate nucleus lesion, suggesting that the development of alpha-2 receptors is also dependent on input from the lateral geniculate nucleus. Removal of the lateral geniculate nucleus early in life resulted in a significant increase in alpha-1 receptors in the subplate region, indicating that receptor densities in this zone may be negatively regulated by the lateral geniculate nucleus afferents. These results show that adrenergic receptors reorganize during postnatal cortical development with a strong temporary concentration in the subplate zone. The reorganization process is heavily influenced by cortical inputs.

The behavioural effects of an early period of monocular deprivation can be extremely profound. However, it is possible to achieve a high degree of recovery, even to normal levels of visual acuity, by prompt imposition of certain regimes of part-time reverse occlusion where the initially non-deprived eye is occluded for only part of each day in order to allow a daily period of binocular visual exposure. In this paper we report on the depth perception of five monocularly deprived cats that had recovered normal visual acuity in both eyes following imposition of certain of the above occlusion regimes. Although three of the animals exhibited five- to sevenfold superiority of binocular over monocular depth thresholds, subsequent tests made on two of the animals revealed that they were unable to make stereoscopic discriminations with random-dot stereograms. Despite the recovery of normal visual acuity in both eyes, we conclude that these animals recover at best only local stereopsis.

Modality specificity of neuronal responses to visual, somesthetic and auditory stimuli was investigated in the anterior ectosylvian cortex (AEC) of cats, using single-unit recording techniques. Seven classes of neurons were found, and according to their responsiveness to sensory stimuli regrouped into three categories: unimodal, bimodal and trimodal. Unimodal cells that responded to only one of the three stimulus modalities formed 59% of the units; 30.2% were bimodal, in that they showed a clear increase of neuronal discharges to two of the three stimulus types; 10.8% were defined as trimodal because they responded to all three stimulus modalities. Although the different categories of cells were intermingled within the AEC, indicating a certain degree of overlap between sensory modalities, some clustering of cell types was nonetheless evident. Thus, the somatosensory responsive cells were mainly located in the anterior two-thirds of the dorsal bank of the anterior ectosylvian sulcus. Visually responsive cells were concentrated on the ventral bank of the sulcus, whereas neurons with an auditory response occupied the banks and fundus of the posterior three-quarters of the sulcus. The histological distribution and physiological properties of AEC neurons suggest that this cortical region is a higher-order associative area whose function may be to integrate information from different sensory modalities.

Sensory interactions, namely, the responses of single cells to stimulations originating from the two sides of the body or from the two visual fields, or from more than one sensory modality (namely, visual, auditory and somatosensory), were evaluated within the anterior ectosylvian cortex (AEC) of cats. Results showed that responses of single neurons to a stimulus of one modality can be enhanced or inhibited by the presentation of another stimulus of either the same or another modality. This facilitatory or inhibitory modulation seems to depend upon temporal and/or spatial relationships between the stimuli. These results, taken together with those previously obtained in our laboratory and by others, suggest that neurons in the AEC may be involved in integrating inputs from various modalities and possibly linking sensory input with action.

The nature of residual vision in cats having undergone serial lesions of various visual structures was investigated behaviorally. Following the section of the left optic tract, learning performance on brightness and pattern discrimination was normal; the additional lesion of all the known visual cortical areas of the right hemisphere greatly impaired retention and learning of pattern discriminations. When a corpus callosotomy was further performed, a paradoxical improvement in both the retention and learning of a pattern discrimination was observed. This hemispheric disconnexion, by interrupting the abnormal influences exerted by the lesioned side on the intact deafferented hemisphere, improved visual functions mediated by the latter.

Binocular disparity, resulting from the projection of a three-dimensional object on the two spatially separated retinae, constitutes one of the principal cues for stereoscopic perception. The binocularity of cells in one hemisphere stems from two sources: (1) the ganglion cells in the homonymous temporal and nasal hemiretinae and (2) the contralateral hemisphere via the corpus callosum (CC). The objectives of this study were, on one hand, to determine whether disparity-sensitive cells are present in a "higher order" area, namely area 19 of the visual cortex, of the cat and, on the other hand, to ascertain whether the CC contributes to the formation of these cells. As in areas 17-18, two types of disparity-sensitive neurons were found: one type, showing maximal interactive effects around zero disparity, responded with strong excitation or inhibition when the stimuli presented independently to the two eyes were in register. These neurons are presumed to signal stimuli situated about the fixation plane. The other type, also made up of two subtypes of opposed valencies, gave maximum responses at one set of disparities and inhibitory responses to the other set. These are presumed to signal stimuli situated in front of or behind the fixation plane. Unlike areas 17-18, however, disparity-sensitive cells in area 19 of the normal cat were less finely tuned and their proportion was lower. In the split-chiasm animal, very few cells were sensitive to disparity. These results, when coupled with behavioral data obtained with destriate animals, indicate that (1) area 19 is probably less involved in the analysis of disparity information than area 17, (2) the disparity-sensitive neurons that are sensitive to disparity are not involved in the resolution of very fine three-dimensional spatial detail, and (3) the CC only determines a limited number of these cells in the absence of normal binocular input.

Anatomical studies in a number of species have shown that most areas of the somatosensory cortex are callosally interconnected. This is also true for the raccoon, at least for those parts representing proximal and axial body regions. Electrophysiologically, studies carried out in cats and monkeys have demonstrated that all sensory sub-modalities cross in the callosum. Moreover, cells representing the paws and fingers, though occupying a large portion of areas SI and SII, seem to send proportionately fewer axons through the callosum than axial structures. No comparable study has been carried out in the raccoon. The purpose of the present experiment was therefore to investigate the functional organization of the callosal system in this animal by examining the receptive field properties of the somatosensory fibres crossing in the callosum. Axonal activity was recorded directly through tungsten microelectrodes in the corpus callosum of eight raccoons. Results indicated that somatosensory information is transmitted in its rostral portion. Most receptive fields concerned axial and proximal body regions and the head and face. Some receptive fields represented para-axial regions of the body and a few concerned the hands and fingers. Slowly and rapidly adapting fibres were found, as were all the sensory sub-modalities tested. A substantial proportion of the axons had bilateral receptive fields. These results are discussed in relation to those obtained in other species, with particular reference to: (1) the midline fusion hypothesis of callosal function; (2) the representation within this structure of the distal extremities, and (3) the origin of the bilateral receptive fields.

The effects of bilateral removal of cortical areas 17-18 were investigated in the cat; these areas represent the central portion of the visual field and the effect of their removal was evaluated with reference to the perception of Julesz random-dot stereograms. Animals were trained in a two-choice discrimination box to choose between two stereotargets made out of random dots. When appropriately viewed, one produced a vertical rectangle and the other an horizontal one, which appeared to float out in space (crossed stereopsis). The results indicated that all normal cats could solve the random-dot task. Following the cortical lesions, stereoscopic perception was abolished. We also tested for the possibility that this inability to solve the random-dot problem was due to a more general acuity loss. Vernier-type acuity comparing a continuous to a disjointed line showed this to be within the animals' discriminative ability. Offset acuity of the lines was better than that of the stereodot patterns. On the other hand, the ability to determine the preoperatively acquired brightness and pattern discriminations was preserved, although some retraining was necessary for the more difficult patterns. It is therefore suggested that the primary visual cortex, at least in the cat, is involved in the perception of global stereopsis independently of its implication in the discrimination of bidimensional patterns.

Binocular disparity, resulting from the projection of a three-dimensional object on the two spatially separated retinae, constitutes one of the fundamental cues for stereoscopic perception. The binocularity of cells in one hemisphere stems from two sources: i) from the ipsilateral ganglion cells in the temporal retina which converge with inputs coming from the contralateral nasal retina; the latter axons cross at the chiasma; ii) from inputs originating in the opposite hemisphere which cross in the corpus callosum. The objective of this study was to demonstrate that interactions from both types of inputs can result in the formation of disparity sensitive neurons and presumably that either type could mediate stereoperception based on disparity cues. Two types of disparity sensitive neurons were found in the normal cat: one type, showing maximal interactive effects around zero disparity responded with strong excitation or inhibition when the stimuli were in register. These neurons are presumed to signal stimuli situated about the fixation plane. The other type, also made up of two subtypes of opposed valencies, gave maximum responses at one set of disparities and inhibitory responses to the other set. These were presumed to signal stimuli situated in front of or behind the fixation plane. In the split-chiasm cat, whose cortical binocularity is presumably assured by converging ipsilateral and callosal inputs, three of the four subtypes of disparity sensitive neurons were found, the uncrossed disparity cells being absent in these animals. Moreover, stimulating each eye individually indicated that nearly 80% of the cells in normal and about 40% in split-chiasm cats were binocularly driven. However, both these figures underestimated the amount of binocular interaction in the callosal recipient zone, since stimulating both eyes simultaneously showed that a proportionately larger number of cells were binocularly driven. Disparity sensitive cells also varied as a function of ocular dominance, i.e., cells signaling the fixation plane tended to have balanced dominance whereas units preferring stimuli situated in front of or behind the fixation plane were dominated by the ipsilateral and contralateral eyes, respectively.

Four hemispherectomized patients were tested with a variety of tasks designed to investigate the extent of residual vision in the blind field. The first set of studies was aimed at evaluating the ability of these subjects to detect and localize at various eccentricities three types of targets which differed in their spatiotemporal properties (stationary, flashing and moving). The subjects could detect effectively the presence of any of these stimuli in their blind field and they experienced little difficulty in discerning blank from target trials. When manual pointing was used to measure localization quality, the hemianopes made more errors in their blind field but their accuracy, when detection was correct, was comparable with that observed in their intact visual field and in that of the control subjects. In the second set of experiments, the capacity of 3 of the subjects to detect in their blind field a moving grating, as well as to discriminate between relative grating velocities and directions in their blind field and in both fields simultaneously, was assessed. Two subjects could detect the movement in their blind field, although this was in part affected by stimulus velocity. When discrimination of relative velocities in the blind field and in both fields simultaneously was evaluated, individual differences were observed. However, all subjects showed some capacity to carry out the task. Finally, none of the experimental subjects was able to discriminate the relative directions of the moving gratings. The results are discussed in terms of collicular involvement to account for the residual vision observed in the blind field of hemispherectomized subjects.

Visually responsive neurons have been recorded in the lateral suprasylvian area (LSA) of cats raised with either a convergent or a divergent strabismus. In contrast to areas 17 and 18, where many studies have documented a profound loss of binocularly activated neurons following early strabismus, in the LSA the majority of cells could still be binocularly driven. Acute or chronic section of the splenium of the corpus callosum reduced but did not abolish binocularity in the LSA. We propose that the widespread callosal connections, the large size of the receptive fields and the peculiar internal circuitry of the LSA all concur in permitting the maintenance of binocular coding in spite of early misalignment of the eyes.

The neural substrates subserving stereopsis were investigated behaviorally and electrophysiologically in the cat. In one set of studies, we examined behaviorally the ability of normal cats to perceive depth on the sole basis of spatial disparity using random-dot stereograms. Results showed that the animals were able to carry out this discrimination. We then evaluated the contribution of the optic chiasm, the corpus callosum and the primary visual cortex to this function. Results indicated that: (1) chiasma transection drastically reduced the ability of the animals to solve the random-dot problem; (2) a callosal split had little or no effect on their ability to relearn the same discrimination; (3) a section of both the corpus callosum and optic chiasm abolished this ability; and (4) bilateral lesions of areas 17-18 also abolished it. In another set of studies, we examined electrophysiologically the properties of neurons in the various visual cortical areas where disparity-based depth discrimination processes are presumed to take place. We recorded from areas 17, 18 and 19 of normal and split-chiasm cats. Results showed that: (1) the primary visual cortex of the normal cat contained cells sensitive to stimulus disparity; (2) these disparity sensitive neurons were also present in area 19 although in a much lower proportion and were more widely tuned than those in areas 17-18; and (3) following the section of the optic chiasm, there was a significant decrease in the number of disparity sensitive cells in areas 17-18, whereas in area 19 they were nearly completely absent. The results obtained from the lesion studies and from the single unit recording experiments indicate that stereoscopic depth perception is highly dependent in the cat upon the integrity of the through-the-chiasm geniculo-striate pathway and its target primary visual cortex.

There are indications in the literature that convergent ipsilateral and contralateral input to the second somatosensory area (SII) may interact. Single unit activity of SII bilateral cells was studied to evaluate the impact of simultaneous bilateral stimulation of the receptive fields (RF) on neural discharge. The cellular responses to unilateral ipsilateral and contralateral, as well as to bilateral stimulation were compared. 22% of bilateral cells showed interaction, usually facilitation. Bilaterally evoked responses were found to be as great as 250% of the strongest unilateral response. Only bilateral responses stronger or weaker than the dominant unilateral response by at least 50% were considered as interactive. The great majority of interactive cells had their RF on the forelimb and were responsive to deep stimulation. The corpus callosum appears to be responsible for part of the observed interaction since in callosotomized cats only 5% of bilateral cells were interactive. A non-callosal ipsilateral pathway must be postulated because both bilaterality and bilateral interaction persist to some degree after callosotomy. A putative role for bilateral interaction in sensory-motor integration is discussed.

A series of experiments examined the potential plasticity of the callosal system in both epileptic patients and in kittens submitted to corpus callosotomy at various ages. The patients were tested for unilateral discrimination and interhemispheric transfer of tactile information. The youngest patient was also required to perform additional inter- and intrahemispheric comparisons of visual and tactile stimuli. The animals were tested for interhemispheric transfer of visual discriminations. The results suggest that in both animals and humans there exists a critical period before which callosal section does not disrupt interhemispheric communication. The results also indicate that the compensatory mechanisms used to achieve interhemispheric transfer in the absence of the corpus callosum may vary according to the sensory modality involved. The possible physiological and/or functional mechanisms responsible for callosal plasticity are discussed.

We have conducted two studies to evaluate the maturational changes of SEP in 206 subjects from neonatal period through adulthood. The first study was done with a cephalic reference electrode and included 137 subjects from 41 weeks (conceptional age) to 9 years of age, divided in 5 different age groups. In the second study, we used an extracephalic reference electrode and recorded SEPs in 69 normal subjects aged from 4 to 34 years, divided in 3 age groups. Our results show that there is a highly significant correlation between age and height in all the age groups studied. Peripheral and central components of SEP mature at different rates with adult values being reached for central conduction times (N9-P14, N9-N20, N13-N20 and P14-N20) between 4 and 9 years of age. Male and female subjects were compared in both studies and no significant differences were found in any of the age groups for any of the wave latencies or interpeak latencies analyzed.

A method is described for repeated recording sessions of cortical thalamic units in the unanaesthetized but paralyzed animal. Because the recording chamber is miniaturized, it is well tolerated by the subject between sessions. The system also permits painless and stable immobilization of the head without the use of earbars, avoiding surgical preparation during the actual recording sessions and allowing long periods of recording. Between sessions the animals always display normal behavior.

Effects of electrical stimulation of the basal ganglia (caudate nucleus and putamen) and cortex (gyrus proreus and compositus) on the receptive fields and response properties of units in the visual cortex of cats were assessed using single lines, double lines and multiple lines (gratings). In the single line experiment caudate stimulation, significantly increased the spontaneous activity, optimal firing rate and receptive field size of visual cortex neurons whereas putamen stimulation decreased these parameters. Stimulation of gyrus proreus enhanced, while that of gyrus compositus diminished optimal firing rate without affecting spontaneous activity; in addition, stimulation of ipsilateral proreus and compositus increased the receptive field size whereas their contralateral homologues decreased it. In the double line experiment, proreus and caudate stimulation increased the magnitude of the facilitatory effect of progressive separation of the lines over certain ranges whereas compositus and putamen stimulation increased the inhibitory influences. Orientation selectivity and spatial frequency tuning characteristics were unaffected by the electrical stimulation of any of the four sites. Thus three categories of network properties were delineated: those characterized by remaining invariant to any cerebral stimulation; those characterized by overall activation as by basal ganglia stimulation; and those characterized as interactive which were responsive especially to cortical stimulation.

The sites of origin of afferents to the telencephalon in the pigeon were investigated using the HRP technique (TMB). Following large multiple injections of the enzyme into the cerebral hemisphere, peroxidase-positive neurons were identified at diencephalic levels: ipsilaterally in DLM, DMA, DLP, DIP, DMP, Rt, Ov, SRt and SCE/SCI; bilaterally in DLA and SPC. At mesencephalic and posterior brainstem levels, HRP-labeled neurons were found: ipsilaterally in AVT, ZpNIII, ICo, FRM, FRL, TP and SCv; bilaterally in GCt, ZpFLM, neurons were found: ipsilaterally in AVT, ZpNIII, ICo, FRM, FRL, TP and SCv; bilaterally in GCt, ZpFLM, Annul, LoC, LC and PrV. Smaller injections of HRP into different regions of the telencephalon demonstrated, for some of the latter structures, either single or multiple projections upon topographically distinct subdivisions.

The harmaline-induced tremor of antagonistic muscles is studied in the anaesthetized rat. By recording simultaneously both the rhythmic EMG-activities in flexor and extensor muscles and the concomitant tremorogenic bulbar reticular discharges, we discovered two facts. 1) The extensor burst precedes the flexor muscle discharge by about 10 msec, contrarily to the close synchrony expected from anterior descriptions of harmaline tremor. 2) The previously observed periodical modulation of olivary and Purkinje cell rhythmic activities is transferred, through the reticular formation to the spinal level, in the form of an alteration of periods with tremor and periods with tonic coactivation of antagonistic muscles. The implication of the myotatic loop in the second fact is discussed.

In order to assess the contribution of the lateral suprasylvian area and subcortical pathways other than the corpus callosum in learning and interhemispheric transfer, a series of studies was undertaken in the cat. Groups of animals which received different types of cortical and subcortical lesions were tested for learning and interocular transfer of visual pattern discriminations. Results have shown that : 1. the lateral suprasylvian area is of equal importance in learning and transfer as areas 17, 18 and 19; 2. in the absence of the corpus callosum, other commissures could preserve interocular transfer of visual information. The results obtained on commissurotomized neonate kittens clearly support this assumption.

Intra- and interhemispheric connections between the anterior ectosylvian visual area (AEV) and other visual cortical areas including the lateral suprasylvian (LSS) were examined in the cat using the retrograde double-label fluorescence technique. The areal and laminar distributions of labeled neurons were mapped following injections of different tracers: Evans Blue (EB), Fast Blue (FB) and Nuclear Yellow (NY) made separately into AEV and LSS of the same or opposite hemispheres. The results indicated: (1) reciprocal and bilateral AEV-LSS connections stemming from layers V and VI in addition to a predominant efferent LSS projection upon AEV from both layer III and the posterior lateral (PLLS) subdivision of LSS; (2) homotopic interhemispheric connections to AEV arising from layers III, V and VI and from layers III and V of ipsilateral areas 20 and 21a; (3) differential laminar distributions of the cell populations projecting to the two cortical sites injected including neurons in layer III of LSS which project to contralateral LSS and AEV of either hemisphere via collateral axon branching (double-labeled). The anatomical findings support the functional similarities between AEV and LSS and the possible role of AEV in interhemispheric transfer of visual information is discussed.

The corpus callosum is the principal neocortical commissure which transmits lateralized information between the hemispheres. The aim of the present experiment was to study the receptive field properties of somatosensory callosal fibres in rhesus macaque monkeys. The callosum was approached under direct visual control and axonic responses were recorded using tungsten microelectrodes. All sensory submodalities which could be examined with the available instruments were found (light touch, medium and deep pressure, joint movement and light pinches). Most fibres had receptive fields concerned with the trunk, followed by the head, with only a few responding to stimulation of the extremities. The medial borders of the unilateral receptive fields situated on the trunk and the head extended to the midline. The results are interpreted in terms of the roles of the corpus callosum in midline fusion and interhemispheric transfer.