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Biomedical subjects

M Ptito

Publications and source records attributed to M Ptito.

At least 55 records · Page 3Linked to original sources
PubMed1994

Sensory modality distribution in the anterior ectosylvian cortex (AEC) of cats.

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.

Acoustic Stimulation↗
PubMed1994

Development and regulation of alpha adrenoceptors in kitten visual cortex.

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.

Animals↗
PubMed1993

Binocular interaction and disparity coding in area 19 of visual cortex in normal and split-chiasm cats.

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.

Animals↗
PubMed1993

Paradoxical improvement of residual vision following corpus callosotomy in brain damaged cats.

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.

Animals↗
PubMed1992

Somatosensory receptive field properties of corpus callosum fibres in the raccoon.

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.

Animals↗
PubMed1992

Binocular interaction and disparity coding at the 17-18 border: contribution of the corpus callosum.

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.

Animals↗
PubMed1992

Loss of stereopsis following lesions of cortical areas 17-18 in the cat.

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.

Animals↗
PubMed1991

Stereopsis in the cat: behavioral demonstration and underlying mechanisms.

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.

Animals↗
PubMed1991

Target detection and movement discrimination in the blind field of hemispherectomized patients.

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.

Brain↗
PubMed1990

Bilateral interaction in the second somatosensory area (SII) of the cat and contribution of the corpus callosum.

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.

Animals↗
PubMed1990

Maturation of short latency somatosensory evoked potentials by median nerve stimulation: a cross-sectional study in a large group of children.

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.

Adolescent↗
PubMed1990

[Plasticity of the callosal system].

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.

Adolescent↗
PubMed1988

Initial acquisition of visual discriminations following selective cortical lesions in cats.

Split-brain adult cats with various visual cortex lesions were tested for their ability to acquire several pattern discriminations using monocular vision. Different lesions were made in the two hemispheres to control for possible individual differences in learning speed due to factors other than the lesion. Results indicated that removal of either areas 17, 17 + 18, 18 + 19, or 19 did not abolish the ability to learn pattern discriminations. However, when compared to control animals, lesioned animals learned more slowly the various discriminations, especially subjects having lesions in 18 + 19. These results tend to confirm the hypothesis that the integrity of individual visual cortical areas for the perception and discrimination of simple patterns is important but not essential.

Animals↗
PubMed1987

Receptive field properties of somatosensory callosal fibres in the monkey.

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.

Animals↗
PubMed1987

Electrophysiological evidence for interhemispheric connections in the anterior ectosylvian sulcus in the cat.

We report electrophysiological data regarding the contribution of the corpus callosum to visual responses in the cortex around the anterior ectosylvian sulcus (AES). The experiments were performed in cats in which the optic input from each eye was surgically restricted to the ipsilateral hemisphere (split-chiasm cats), and where neuronal responses to stimulation of the contralateral eye were mediated by interhemispheric connections. A very high proportion of cells were driven by stimuli presented to either eye indicating that they were activated not only through an intrahemispheric pathway from the ipsilateral eye, but also through an interhemispheric pathway from the contralateral eye. With few exceptions, both receptive fields (RFs) of each binocular neuron abutted or were in the vicinity of the vertical meridian. All neurons responded well to moving stimuli and most of them showed directional selectivity. A few cells were activated by stimuli moving in depth. Following an additional section of the posterior half of the corpus callosum, cells in AES responded only to stimulation of the ipsilateral eye, demonstrating thus that the input from the contralateral eye was conveyed by this part of the corpus callosum. By contrast following a section of the anterior half of the corpus callosum, all visually responsive AES neurons were binocularly activated. These results suggest that the interhemispheric visual input to this ectosylvian region is conveyed via a polysynaptic loop involving visual cortical areas that are connected through the posterior portion of the corpus callosum.

Animals↗
PubMed1987

Distribution of visual callosal projection neurons in the siamese cat: an HRP study.

In this experiment we studied the distribution of callosal cells in various visual cortical areas of both common and siamese cats using the HRP method. The results showed that the most notable difference between the two strains concerned the primary visual areas. Indeed, labeling in areas 17 and 18 was significantly lower in siamese cats and only a few scattered cells were encountered at the 17/18 border. In general, the number of callosal cells forming interhemispheric connections within areas 17, 18 and the 17/18 border was dramatically reduced in the siamese. For all other visual areas (19, lateral suprasylvian subdivisions, 20 a, b and 21 a, b) the distribution was comparable for both species. A study of the laminar distribution indicated that, for common and siamese cats, medium and large pyramidal cells were mainly concentrated in layers III and IV whereas in deeper layers we found a mixture of small pyramidal and fusiform neurons.

Animals↗