Biomedical subjects

Maurice Ptito

Publications and source records attributed to Maurice Ptito.

13 recordsLinked to original sources

PubMed2006

Transcranial magnetic stimulation of the visual cortex induces somatotopically organized qualia in blind subjects.

After loss of a particular sensory channel, the deprived cortex can be activated by inputs from other sensory modalities. It is not known whether activation of the rewired cortex evokes subjective experiences characteristic of that cortex or consistent with the rerouted sensory information. In a previous study, blind subjects were trained to perform visual tasks with a tongue display unit, a sensory substitution device that translates visual displays into electrotactile tongue stimulation. This cross-modal sensory stimulation activated their visual cortices. We now extend this finding by using transcranial magnetic stimulation to examine the perceptual correlates of training-induced plastic responses. We find that blind subjects proficient with the use of the tongue display unit report somatopically organized tactile sensations that are referred to the tongue when transcranial magnetic stimulation is applied over the occipital cortex. No such sensations were evoked in trained, blindfolded, seeing control subjects who performed the sensory substitution task equally well. These data show that the perceptual correlate of activity in a given cortical area reflects the characteristics of its novel sensory input source.

Adult↗

PubMed2006

Development of the commissure of the superior colliculus in the hamster.

The development of the corpus callosum (CC) and the anterior commissure (CA) is well known in a wide variety of species. No study, however, has described the development of the commissure of the superior colliculus (CSC) from embryonic state to adulthood in mammals. In this study, by using the lipophylic tracer DiI, we investigated the ontogeny of this mesencephalic commissure in the hamster at various ages. The development of axonal terminals, growth cone morphologies, and axons branching were described for the superior colliculus (SC) contralateral to the tracer injection. The first CSC axons cross the midline at embryonic day 11 (E-11) and grow further into the intermediate layers of the contralateral SC between E-12 and E-14. There is little axon growth therein between E-14 and the day of birth (P-0). Growth cones at the tip of these axons adopt complex morphologies at E-12 and progressively simplify until P-0. Pioneer axons are clearly visible between E-14 and P-1. These are followed by other axons progressively more numerous between P-0 and P-5. Axons do not show any branching until P-2. Between P-3 and P-9, the axons progressively arborize in the intermediate layers. Some axons reach the superficial layers at P-5, and they become more numerous around P-11, and only a few axons remain therein by P-21. Myelinated axons appear at P11 and are very dense at P-21. Our results indicate that the CSC follows developmental schemes similar to those of the CC and the AC but that initial axon midline crossing occurs earlier.

Animals↗

PubMed2005

Pattern-motion selectivity in the human pulvinar.

On the basis of anatomical and physiological data obtained on animal models, we recently proposed that neurons in the main visual extrageniculate nuclei complex, the pulvinar, are actively involved in higher-order visual processing. Pulvinar neurons have been shown to integrate the component signals of a plaid pattern into a coherent global percept (pattern-motion selectivity). Using positron emission tomography (PET), we have investigated the possibility that the human pulvinar is also involved in plaid-defined higher-order motion integration. Plaid patterns were presented to normal observers in two conditions (coherent vs. transparent) created by varying the relative spatial frequency of the two gratings comprising the plaid. Regions of interest analysis revealed a significant activation of the pulvinar in the coherent condition supporting the notion that the human pulvinar nucleus is involved in higher-order motion processing. Plaid pattern activation was also observed in the medial temporal gyrus (area MT/V5), a motion area with strong anatomical connections to the pulvinar. These data provide the first direct evidence that the human pulvinar is involved in complex motion integration, as previously shown in animal models, and further support the existence of cortico-thalamo-cortical computational networks involved in higher-order visual processing.

Adult↗

PubMed2005

Distribution of calcium binding proteins in visual and auditory cortices of hamsters.

The morphology and distribution of neurons immunoreactive (ir) to parvalbumin (PV), calretinin (CR) and calbindin (CB) were studied in the primary visual (V1) and auditory (A1) cortices of hamsters. Cortical cell populations were labelled immunohistochemically using a glucose oxidase-diaminobenzidine-nickel combined revelation method. Quantitative analysis revealed significant differences between V1 and A1 in the density and distribution of their neuronal population. CBir cells exhibited several typologies in both cortical regions. Most cells were multipolar even though many of them had bitufted or bipolar morphologies. These cells were distributed in layers II/III and in layer V of both A1 and V1, but were more numerous in layer V of V1. CRir cells were of the fusiform type with long bipolar dendritic arbours. These were similarly distributed in both cortices with a peak in superficial layers II/III. PVir cells were also found in both cortices and had round or oval-shaped somata with multipolar processes. They were mostly located in layer V for V1 and in layers III/IV for A1. Visual and auditory primary cortices can thus be differentiated on the basis of their immunoreactivity to specific calcium binding proteins.

Animals↗

PubMed2005

Cross-modal plasticity revealed by electrotactile stimulation of the tongue in the congenitally blind.

In sensory substitution, information acquired with one sensory modality is used to accomplish a task which is normally subserved primarily by another sensory modality. We used PET to study cross-modal plasticity in the congenitally blind, using electrotactile stimulation of the tongue. Blind (n = 6) and sighted blindfolded controls (n = 5) were scanned before and after they were trained to use their tongue in a Snellen orientation detection task. Results showed that both groups of subjects learned the discrimination orientation task after seven 1 h training sessions. Before training, no significant changes in regional cerebral blood flow (rCBF) were observed in the occipital cortex in either group. In sharp contrast, activity in the occipital cortex increased after practice for the blind, but not for the sighted, providing evidence for training-induced plasticity in the blind. An inter-regional correlation analysis showed that task-related rCBF changes in left posterior parietal cortex were positively correlated with rCBF changes in the occipital area of the trained blind. These data reveal that cross-modal plasticity in the blind develops rapidly and that the occipital cortex is part of a functional neural network for tactile discrimination in conjunction with the posterior parietal cortex. Our data further show that the tongue can act as a portal to convey somatosensory information to visual cortex.

Adult↗

PubMed2005

Regional analysis of neurofilament protein immunoreactivity in the hamster's cortex.

The laminar distribution of several distinct populations of neurofilament protein containing neurons has been used as a criterion for the delineation of cortical areas in hamsters. SMI-32 is a monoclonal antibody that recognizes a non-phosphorylated epitope on the medium- and high-molecular weight subunits of neurofilament proteins. As in carnivores and primates, SMI-32 immunoreactivity in the hamster neocortex was present in cell bodies, proximal dendrites and axons of some medium and large pyramidal neurons located in cortical layers III, V and VI. A small population of labeled multipolar cells was also found in layer IV. Neurofilament protein immunoreactive neurons were found throughout isocortical areas. Very few labeled cells were encountered in supplemental motor area, insular cortex, medial portion of associative visual cortex and in parietal association cortex. Our data indicate that SMI-32 immunoreactive cells can be efficiently used to trace boundaries between neocortical areas in the hamster's brain. The regional distribution SMI-32 immunoreactivity in the hamster cortex corresponds quite closely with cortical areas as defined by their cytoarchitecture and myeloarchitecture. The primary sensory cortical areas contain the most intense of SMI-32 immunoreactivity and are also those with the highest density of myelinated axons. Very low SMI-32 immunoreactivity was found in orbital, insular, perirhinal, cingulate and infralimbic cortices, which are also poor in myelinated axons. This supports the association between SMI-32 immunoreactivity and myelin contents.

Animals↗

PubMed2005

Cross-modal plasticity in early blindness.

The brain shows a remarkable capacity to reorganize itself following early sensory deprivation or neonatal brain damage. Using two models of deprivation, we will show that the brain does indeed adjust to the loss of either the visual cortex (which receives most of the retinal inputs through the lateral geniculate bodies of the thalamus) or the eyes (which provide the major input to the visual cortex) through cross-modal plastic processes. Hamsters, deprived of their visual system at birth, develop novel and permanent retinal projections to the auditory thalamus. These projections form functional synapses and project to the auditory cortex. When trained on a visual discrimination task, the "rewired" hamsters perform as well as normal hamsters. Lesions of the auditory cortex produce cortical blindness. Congenitally blind human subjects, trained to discriminate the orientation of a stimulus applied to the tongue via an electrotactile device, show activation of their visual cortex, whereas trained blindfolded controls show only activation of the somatosensory cortex representing the tongue. We propose that in blind subjects, there is an unmasking of existing cortico-cortical (parieto-occipital) connections, enabling transfer of somatosensory information to visual cortex.

Blindness↗

PubMed2004

Retinal projections to the lateral posterior-pulvinar complex in intact and early visual cortex lesioned cats.

In intact cats, it is generally considered that the lateral posterior-pulvinar complex (LP-pulvinar) does not receive direct retinal terminals, with the exception of the retino-recipient zone known as the geniculate wing. There is, however, some evidence that early lesions of the visual cortex can occasionally induce the formation of novel retinal projections to the LP nucleus. Given the importance of knowing the connectivity pattern of the LP-pulvinar complex in intact and lesioned animals, we used the B fragment of cholera toxin, a sensitive anterograde tracer, to reinvestigate the retinal projections to the LP-pulvinar in normal cats and in cats with early unilateral lesions of the visual cortex (areas 17 and 18). Immunohistochemical localization of the toxin was performed to show the distribution and morphology of retinofugal terminals. A direct bilateral but predominantly contralateral retinal projection reached the caudal portion of LPl and LPm in the form of patches located mainly along its dorsomedial surface and many scattered terminals. The distribution of retinal projections to LP-pulvinar in intact and operated cats did not differ. Contrary to what had been previously reported, we found no evidence for lesion-induced sprouting of retinal axons in these higher-order thalamic nuclei. Retinal input to the LP-pulvinar might modulate visual responses driven by primary visual cortex or superior colliculus.

Animals↗

PubMed2004

Recovery of anterograde amnesia in a case of craniopharyngioma.

BACKGROUND: Studies of the amnesic syndrome have indicated that telencephalic and diencephalic structures are critical components of the memory system. The exact role of the mammillary bodies (MBs) in human memory remains elusive, since few cases of selective MB damage have been reported. OBJECTIVE: To study a case of severe anterograde amnesia due to a third-ventricle craniopharyngioma with severe MB compression. DESIGN: Case report. SETTING: Neurosurgery clinic of an academic hospital. PATIENT: A 53-year old woman who developed severe anterograde amnesia due to a third-ventricle craniopharyngioma strongly compressing the MBs and, to a lesser extent, the right hippocampus. INTERVENTIONS: Surgical excision of the tumor and neuropsychological testing and positron emission tomography during an associative memory test before and 2 months after tumor removal. A postsurgical magnetic resonance image did not show evidence of damage to the hypothalamus, thalamus, hippocampus, or MBs. MAIN OUTCOME MEASURES: Changes in brain imaging data and results of neuropsychological testing. RESULTS: After tumor removal, the patient showed a complete recovery of memory functions. Performance on the associative memory test was at chance level before surgery and dramatically improved postoperatively. Results of the preoperative positron emission tomographic study showed no activity in memory-related structures. In contrast, a significant blood flow increase occurred in the anterior thalamic nuclei postoperatively. CONCLUSIONS: These behavioral and brain imaging data stress the importance of the MBs in this patient's amnesia. Our data further suggest that the clinical prognosis of decompressing the mammillothalamic tract is excellent, even in cases of massive compression.

Amnesia, Anterograde↗

PubMed2003

Separate neural pathways for contour and biological-motion cues in motion-defined animal shapes.

To determine whether contour and biological motion (BM) cues for motion-defined shapes are subserved by two separate mechanisms, we used PET to measure regional cerebral blood flow in nine human subjects. Subjects were scanned in the following four conditions: (1) contour-defined animals with natural movements (running), (2) motion-defined animals in which the contours were removed and dots were placed on the limbs and moving parts (BM; running); (3) drifting static animal shapes (contours); and (4) drifting dots. The results demonstrate that the perception of BM involves the superior frontal gyrus, the precuneus, the fusiform, the lingual and the medial temporal gyri, the inferior parietal lobe, the hippocampal and parahippocampal areas, and the cerebellum. In addition, the anterior cingulate cortex and the amygdala were significantly activated. The perception of contour-defined shapes produced significant elevation of rCBF in some areas similar to the BM condition, namely the fusiform, medial occipital, medial temporal, and lingual gyri. Only the occipital pole and the inferior temporal cortex were specifically activated by contour-defined shapes. These results are congruent with previous findings that the dorsal visual pathway is important for the perception of BM. They further support psychophysical results showing that contour and BM cues for motion-defined animal shapes are processed by independent channels.

Adult↗

PubMed2002

Cortical areas mediating stereopsis in the human brain: a PET study.

Using PET, we investigated the neural substrates of stereodepth perception in humans. The presentation of Julesz-type random-dot stereograms (RDS) produced significant rCBF elevations in Brodmann areas (BA) 18, 19 and 7, all in the right hemisphere. Activation foci were also found in both middle temporal areas (MT). These results demonstrate that, as in primates, cortical area MT and extrastriate areas are central to stereovision and that a network of predominant right hemispheric regions is recruited to meet visuo-spatial processing demands associated with horizontal binocular disparity inputs.

Adult↗

PubMed2001

Stereological evaluation of neurons and glia in the monkey dorsal lateral geniculate nucleus following an early cerebral hemispherectomy.

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.

Animals↗

PubMedDate not supplied

Retinal projections in the cat: a cholera toxin B subunit study.

The B fragment of cholera toxin (CTb) is a highly sensitive anterograde tracer for the labelling of retinal axons. It can reveal dense retinofugal projections to well-known retinorecipient nuclei along with sparse but distinct input to target areas that are not commonly recognized. Following a unilateral injection of CTb into the vitreous chamber of seven adult cats, we localized the toxin immunohistochemically in order to identify direct retinal projections in these animals. Consistent with previous findings, the strongest projections were observed in the superficial layers of the superior colliculus, the dorsal and ventral lateral geniculate nuclei, the pretectal nuclei, the accessory optic nuclei, and the suprachiasmatic nucleus of the hypothalamus. However, we also found labelled terminals in several other brain areas, including the zona incerta, the medial geniculate nucleus, the lateral posterior-pulvinar complex, the lateral habenular nucleus, and the anterior and lateral hypothalamic regions. The morphological characteristics of the retinal axon terminals in most of the identified novel target sites are described.

Animals↗