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C Milleret

Publications and source records attributed to C Milleret.

28 records · Page 2Linked to original sources
PubMed1987

[Reorganization of callosal interhemispheric connections in the adult cat: effects of monocular occlusion after chiasmotomy].

Two groups of adult cats were chiasmotomized and their cortical receptive fields (17-18 boundary) were compared after a postoperative period of ca 6 weeks. In one group, binocular vision was maintained during that period, in the other one, one eye was sutured at the time of the chiasmotomy, depriving one hemisphere from patterned vision through the direct pathway. In monocular chiasmotomized animals, the receptive fields to stimulation of the contralateral eye were significantly larger than in the binocular ones.

Animals↗
PubMed1987

[Response of neurons of the visual cortex (area 18) to extraocular proprioceptive stimulation: development in normal or dark-reared cats and interaction with visual activity].

Single unit responses to stimulation of the extraocular muscle receptors or of their afferent fibers were recorded from the visual cortical area 18 in normally or dark-reared cats. These responses were improved by a random visual stimulation delivered during the test. These proprioceptive responses, elicited with or without random visual activation, changed with age and as a function of the postnatal rearing conditions. In the normally-reared group, neurons which were activated by stimulation of the afferents from the Obliquus inferior displayed a preferred orientation for visual stimulus oriented at right angle to action plane of this particular muscle.

Aging↗
PubMed1986

Role of eye movements in developmental processes of orientation selectivity in the kitten visual cortex.

Six-week-old dark-reared kittens were exposed for 6 hr to a normal lighted environment in which they were free to move, but with eyes immobilized. Receptive field properties (orientation selectivity and ocular dominance) of the visual cortical cells (Area 17) were studied. No restoration of orientation selectivity could be observed when only the eyes were immobilized either by oculomotor nerve sections or by extraocular muscles removal, in contrast to what had been observed in intact free moving animals. From these results one can conclude that eye movements must be associated with vision to allow developmental processes of orientation selectivity in the primary cortex. These new results are compared to those obtained previously in paralyzed or restrained animals and those obtained in animals with interruption of orbital afferents.

Animals↗
PubMed1984

Receptive field sizes and responsiveness to light in area 18 of the adult cat after chiasmotomy. Postoperative evolution; role of visual experience.

Receptive field sizes to stimulation of the ipsilateral temporal retina were studied in area 18 of adult cats at different times after complete midsagittal section of the optic chiasm. A specific postoperative evolution could thus be noticed: immediately after section, the average area of the receptive fields was reduced, as compared to control preparations, owing to the disappearance of large fields located at more than 20 degrees of eccentricity. A progressive reappearance of these large fields occurred between 8 and 45 days after chiasmotomy, provided that the animal was placed in normal visual conditions during its postoperative period. No such recovery could be assessed after as long as 55 days, in animals kept in complete darkness after operation. Chiasmotomized cats also displayed a reduction of their percentage of light reactive cells with respect to controls, as expected from the suppression of the contralateral input. This percentage was at first very low and progressively increased, during postoperative recovery but again not when the animal had been kept in the dark. Finally, an increase of cells with "diffuse responses" was observed in the late postoperative recovery stage. This latter evolution also appeared to depend upon postoperative visual experience. On the other hand, no clear indication of an interhemispheric transfer could be obtained in these experiments, even at the 17-18 boundary.

Animals↗
PubMed1984

[Postnatal development of functional properties of the visual cortical cells of area 18 in kittens raised with or without visual experience].

866 units were recorded extracellularly in area 18 of anaesthetized and paralysed kittens from 13 to 66 days of age. The development of their receptive field properties was studied in normally (EN) and dark-reared (EO) kittens. In addition to orientation selective (S) and non-selective (NS) cells, we found a number of non-selective units whose receptive field was surrounded by a peripheral zone (NSp) where stationary stimuli were effective. In EN kittens, the orientation selectivity developed with age and concomitantly, NS and NSp cells disappeared. Ocular dominance distribution was also gradually modified from a contralateral monocular dominance at 13 days of age to an adult-like binocularity at 58 days. In EO kittens, the early orientation selectivity began to decrease at the 5th week. From then on, the process of despecification started and progressed until nearly all cells were NS. Absence of visual experience also delayed the development of mature binocularity. In 6 week old EO kittens, a 6 hrs. visual exposure induced a fast but uncomplete specification with decrease of both NS and NSp cells and a slight modification of the ocular dominance distribution. The comparison of these results with those obtained in area 17 shows that functional properties vary more slowly in area 18 than in area 17.

Animals↗
PubMed1983

[Responsive characteristics of the cells of cortex area 18 of the adult cat after chiastomy. Postoperative development; importance of the visual experience].

Patterns of unit responses in area 18 to stimulation of the ipsilateral temporal retina were studied in adult Cats at different times after complete midsagittal section of the optic chiasm. A specific postoperative evolution could thus be followed. Immediately after section, the percentage of cells reacting to light was reduced, as compared to control preparations, as well as the average areas of the receptive fields, owing to the disappearance of large eccentric fields. A progressive recovery of the normal reactive patterns occurred between 8 and 45 days after chiasmotomy, provided that the animal was placed in normal visual conditions during its postoperative period. No such recovery could be observed after 55 days, in animals kept in complete darkness after operation. Visual experience thus seems essential to this kind of functional recovery in the adult. Another characteristic of chiasmotomised Cats also appeared to depend upon postoperative visual experience, namely the development of cells with "diffuse" responses to light.

Animals↗
PubMedDate not supplied

Visual inter-hemispheric processing: constraints and potentialities set by axonal morphology.

The largest bundle of axonal fibers in the entire mammalian brain, namely the corpus callosum, is the pathway through which almost half a billion neurons scattered over all neocortical areas can exert an influence on their contralateral targets. These fibers are thus crucial participants in the numerous cortical functions requiring collaborative processing of information across the hemispheres. One of such operations is to combine the two partial cortical maps of the visual field into a single, coherent representation. This paper reviews recent anatomical, computational and electrophysiological studies on callosal connectivity in the cat visual system. We analyzed the morphology of individual callosal axons linking primary visual cortices using three-dimensional light-microscopic techniques. While only a minority of callosal axons seem to perform a strict 'point-to-point' mapping between retinotopically corresponding sites in both hemispheres, many others have widespread arbors and terminate into a handful of distant, radially oriented tufts. Therefore, the firing of a single callosal neuron might influence several cortical columns within the opposite hemisphere. Computer simulation was then applied to investigate how the intricate geometry of these axons might shape the spatio-temporal distribution of trans-callosal inputs. Based on the linear relation between diameter and conduction velocity of myelinated fibers, the theoretical delays required for a single action potential to reach all presynaptic boutons of a given arbor were derived from the caliber, g-ratio and length of successive axonal segments. This analysis suggests that the architecture of callosal axons is, in principle, suitable to promote the synchronous activation of multiple targets located across distant columns in the opposite hemisphere. Finally, electrophysiological recordings performed in several laboratories have shown the existence of stimulus-dependent synchronization of visual responses across the two hemispheres. Possible implications of these findings are discussed in the context of temporal tagging of neuronal assemblies.

Animals↗
PubMedDate not supplied

Microglia and astrocytes may participate in the shaping of visual callosal projections during postnatal development.

In the adult cat, axons running through the corpus callosum interconnect the border between the visual cortical areas 17 and 18 (A17 and A18) of both hemispheres. This specific pattern emerges during postnatal development, under normal viewing conditions (NR), from the elimination of initially exuberant callosal projections. In contrast, if the postnatal visual experience is monocular from birth (MD), juvenile callosal projections are stabilised throughout A17 and A18. The present study aimed at using such a model in vivo to find indications of a contribution of glial cells in the shaping of projections in the developing CNS through interactions with neurones, both in normal and pathological conditions. As a first stage, the distribution and the morphology of microglial cells and astrocytes were investigated from 2 weeks to adulthood. Microglial cells, stained with isolectin-B4, were clustered in the white matter below A17 and A18. Until one month, these clustered cells displayed an ameboid morphology in NR group, while they were more ramified in MD animals. Their phenotype thus depends on the postnatal visual experience, which indicates that microglial cells may interact with axons of visual neurones. It also suggests that they may differentially contribute to the elimination and the stabilisation of juvenile exuberant callosal fibres in NR and MD animals respectively. Beyond one month, microglial cells were very ramified in both experimental groups. Astrocytes were labelled with a GFAP-antibody. The distributions of connexins 43 (Cx43) and 30 (Cx30), the main proteic components of gap junction channels in astrocytes, were also investigated using specific antibodies. Both in NR and MD groups, until 1 month, GFAP-positive astrocytes and Cx43 were mainly localised within the subcortical white matter. Then GFAP, Cx43 and Cx30 stainings progressively appeared within the cortex, throughout A17 and A18 but with a differential laminar expression according to the age. Thus, the distributions of both astrocytes and connexins changed with age; however, the monocular occlusion had no visible effect. This suggests that astrocytes may contribute to the postnatal development of neuronal projections to the primary visual cortex, including visual callosal projections.

Animals↗
PubMedDate not supplied

Unilateral paralytic strabismus in the adult cat induces plastic changes in interocular disparity along the visual midline: contribution of the corpus callosum.

Neurones activated through the corpus callosum (CC) in the cat visual cortex are known to be almost entirely located at the 17/18 border. They are orientation selective and display receptive fields (RFs) distributed along the central vertical meridian of the visual field ("visual midline"). Most of these cells are binocular, and many of them are activated both from the contralateral eye through the CC, and from the ipsilateral eye via the direct retino-geniculo-cortical (GC) pathway. These two pathways do not carry exactly the same information, leading to interocular disparity between pairs of RFs along the visual midline. Recently, we have demonstrated that a few weeks of unilateral paralytic strabismus surgically induced at adulthood does not alter the cortical distribution of these units but leads to a loss of their orientation selectivity and an increase of their RF size, mainly toward the ipsilateral hemifield when transcallosally activated (Watroba et al., 2001). To investigate interocular disparity, here we compared these RF changes to those occurring in the same neurones when activated through the ipsilateral direct GC route. The 17/18 transition zone and the bordering medial region within A17 were distinguished, as they display different interhemispheric connectivity. In these strabismics, some changes were noticed, but were basically identical in both recording zones. Ocular dominance was not altered, nor was the spatial distribution of the RFs with respect to the visual midline, nor the amplitude of position disparity between pairs of RFs. On the other hand, strabismus induced a loss of orientation selectivity regardless of whether neurones were activated directly or through the CC. Both types of RFs also widened, but in opposite directions with respect to the visual midline. This led to changes in incidences of the different types of position disparity. The overlap between pairs of RFs also increased. Based on these differences, we suggest that the contribution of the CC to binocular vision along the midline in the adult might be modulated through several intrinsic cortical mechanisms.

Action Potentials↗