Biomedical subjects

J P Guillemot

Publications and source records attributed to J P Guillemot.

At least 19 recordsLinked to original sources

PubMed2001

Spatial disparity sensitivity in area PMLS of the Siamese cat.

Previous studies of the visual system of Siamese cats have shown that binocular cells are scarce in areas 17, 18 and 19, yet significantly more abundant in suprasylvian areas such as the postero-medial lateral suprasylvian area (PMLS). The present study aims at evaluating the sensitivity to spatial disparity of PMLS binocular cells in paralyzed and anesthetized Siamese cats. Centrally located receptive fields were mapped, separated using prisms and then stimulated simultaneously using two luminous bars optimally adjusted to the size of the excitatory receptive fields. Delays were introduced in the arrival of the luminous bars in the receptive fields so as to create the desired spatial disparities. Results indicate that approximately a third of PMLS units are binocular and that these binocular cells can detect spatial disparity cues. Indeed, although the sample was relatively small, cells of the tuned excitatory (14/34), tuned inhibitory (2/34), near (6/34) and far (1/34) types were identified. The spatial selectivity, as measured by the width at half height of the tuning curves of the excitatory and inhibitory cells and the slopes of the near and far cells, was similar to that obtained in PMLS of normal cats but not as precise as that found for primary visual areas in these animals. This suggests that these cells might serve as a substrate for coarse stereopsis.

Action Potentials↗

PubMed2001

Comparative computations of spike synchronization in visual cortex of cats.

In recent years it has been proposed that synchronous activity between neurons is a putative mechanism to bind together various trigger features of an image. Thus the measure of synchronization becomes an important issue since it may be an electrophysiological sign of visual perception. This paper describes and compares six techniques of computing synchronization strength, that is, the central peak of a cross-correlogram. Data were obtained in anesthetized cats prepared for electrophysiological recordings in a conventional fashion. Results indicate that: (1) eye fits are misleading. Visual inspection of cross-correlograms, may be interesting if one needs to estimate approximately synchronization strength and the presence of oscillations in the cross-correlograms, however it may be misleading if one wants to compare different cross-correlograms; (2) regression analysis to compare one method against the others yields a relatively poor correlation suggesting that methods are not directly comparable; (3) the sensitivity of each computational method is unequal. The results may indicate that some functional connections are either under- or over-evaluated depending upon the strategy employed to measure synchronization.

Animals↗

PubMed2001

Contrast dependency of VEPs as a function of spatial frequency: the parvocellular and magnocellular contributions to human VEPs.

The present study investigated the contrast dependency of visual evoked potentials (VEPs) elicited by phase reversing sine wave gratings of varying spatial frequency. Sixty-five trials were recorded for each of 54 conditions: 6 spatial frequencies (0.8, 1.7, 2.8, 4.0, 8.0 and 16.0 c deg(-1)) each presented at 9 contrast levels (2, 4, 8, 11, 16, 23, 32, 64 and 90%). At the lowest spatial frequency, the waveform contained mainly one peak (P1). For spatial frequencies up to 8 c deg(-1), P1 had a characteristic magnocellular contrast response: it appeared at low contrasts, increased rapidly in amplitude with increasing contrast, and saturated at medium contrasts. With increasing spatial frequency, an additional peak (N1) gradually became the more dominant component of the waveform. N1 had a characteristic parvocellular contrast response: it appeared at medium to high contrasts, increased linearly in amplitude with increasing contrast, and did not appear to saturate. The data suggest the contribution of both magnocellular and parvocellular responses at intermediate spatial frequencies. Only at the lowest and highest spatial frequencies tested did magnocellular and parvocellular responses, respectively, appear to dominate.

Adult↗

PubMed2000

Neurons in the posteromedial lateral suprasylvian area of the cat are sensitive to binocular positional depth cues.

Single units in the posteromedial lateral suprasylvian area of the cat are known to be very sensitive to movement. A proportion of these cells can encode movement in depth, but it is unclear whether posteromedial lateral suprasylvian cells only rely upon motion cues to evaluate stimulus depth or whether they can also code for spatial cues. The present study aims at assessing the sensitivity to spatial disparity of binocular cells, in the postero-medial lateral suprasylvian area, in order to determine whether these units are tuned to positional depth cues. A total of 126 single cells located in the posteromedial lateral suprasylvian area of anesthetized, paralyzed cats were examined. As recordings were performed in the central visual field representation, receptive fields were small. A third of the receptive fields were surrounded by an inhibitory region and almost three-quarters of the cells were direction-selective. Most cells (110/114) were binocular, and a large proportion of single neurons responded to stimuli appearing on the fixation plane by increasing (tuned excitatory cells, 43%) or decreasing (tuned inhibitory cells, 14%) their response rate. A smaller proportion of cells increased their firing rate in response to crossed (near cells, 10%) or uncrossed (far cells, 6%) spatial disparities, hence demonstrating respective preference for stimuli presumably appearing in front of or behind the fixation plane. As compared to primary visual cortex, the proportion of disparity-sensitive cells in posteromedial lateral suprasylvian area is similar, but selectivity is significantly coarser. As the posteromedial lateral suprasylvian area can code for both spatial and temporal aspects of stimuli, this area might be involved in the spatiotemporal integration of depth cues, a process that may also participate in the control of accommodation and vergence.

Animals↗

PubMed2000

Spatial properties and direction selectivity of single neurons in area 21b of the cat.

The receptive field properties of single units were assessed in area 21b of the cat visual cortex. Visual cells in this area were binocular and showed relatively large receptive fields. Most cells were strongly sensitive to the direction of drifting gratings. The mean value of the half-widths of the direction tuning curves (32 degrees ) suggests broader direction tunings than are typically found in other visual areas. The spatial frequency tuning functions were either band-pass or low-pass. Cells responded optimally to low spatial frequencies (mean =0.08c/deg) and also showed low spatial resolution (mean =0.29c/deg.). The estimated values of spatial bandwidths (mean=2.2 octaves) suggest that area 21b cells act as relatively good spatial filters. Although some cells exhibited a low contrast threshold, most cells began to respond at intermediate or high contrast values (mean threshold =15.5%). Temporal frequency tuning functions were mostly band-pass and usually broad (mean temporal bandwidth=3.3 octaves). Cells were found that responded optimally to various temporal frequencies (mean optimal temporal frequency=3.2Hz), although the majority preferred a temporal frequency below 4Hz.These results suggest that visual properties (receptive fields sizes, spatial resolution and orientation/direction selectivity) of cells in area 21b differ from those of cells previously observed in the adjoining area 21a. These differences provide evidence in support of functional distinction between these two visual areas.

Animals↗

PubMed2000

Responses of cells to stationary and moving sound stimuli in the anterior ectosylvian cortex of cats.

The azimuthal, directional and angular speed sound selectivities of single units were examined in the posterior part of the anterior ectosylvian cortex. Broadband noise bursts and simulated moving sounds were delivered from 16 loudspeakers fixed on the horizontal plane in a quasi-anechoic sound-isolation chamber. The activity of 78 neurons was recorded and quantitatively analyzed. Most cells responded to at least the static sound. The relative strengths of their responses suggested that the cells could be classed as omnidirectional (37.2%), contralateral hemifield (29.5%), ipsilateral hemifield (2.5%) and azimuth (7.7%) selective. The remaining 23.1% could not be classified. All cells responded to a simulated moving sound displaced at five different speeds. A majority (88%) of them showed some directional preference in that they discharged at least twice as strongly for one direction as for the other for at least one speed. 14.7% displayed angular speed selectivity. Different patterns of neuronal discharges were evoked. For static sounds, most of the cells gave ON-type responses. A large proportion (60%) of the cells responded in a sustained manner to maintained stimulation. Among these, 68% also gave sustained discharges to moving sounds. The spatial tuning and the directional and angular speed selectivity of neurons in the posterior part of the AEC suggest that this area is involved in the processing of static and moving sounds.

Acoustic Stimulation↗

PubMed2000

Cellular response to texture and form defined by motion in area 19 of the cat.

The present study examined the neuronal sensitivity in area 19 of the cat to a motion-defined bar and to texture. Sensitivity was tested in normal, lesioned (areas 17-18) and split-chiasm cats using a kinematogram, as well as a textured bar drifting on a uniform light background and a light bar drifting on a stationary textured background. Texture density was varied. The results indicate that almost all cells of area 19 recorded in the three groups of cats responded to a motion-defined bar or to its edges. Texture density influenced the responses in that the discharge rate increased as density decreased. However, the majority of cells were sensitive to the highest texture density kinematogram. Moreover, the neural responses of all cats were either independent of the density of the textured bar or background, or were modulated by it. These results show that cells in area 19 can signal the presence of a kinetic bar and that the density of either the textured bar, the background or both can influence figure-ground detection. The results are interpreted with respect to how various inputs influence the function of area 19.

Animals↗

PubMed1999

Binocular interactions and spatial disparity sensitivity in the superior colliculus of the Siamese cat.

In Siamese cats, a genetically determined massive misrouting of retinal ganglion cells toward the contralateral hemisphere, as well as an accompanying strabismus, is believed to underlie the extreme paucity of binocular cells in the primary visual cortex. However, binocular cells have been shown to be present in more important numbers at the collicular level. The present study aims at investigating binocular interactions and sensitivity to spatial disparity in the superior colliculus of the Siamese cat. The activity of single units was recorded in the superficial layers of paralyzed and anesthetized Siamese cats. Although most collicular cells were monocularly driven, a significant proportion could be driven through both eyes (34/216 or 16%). Upon isolation of a binocular cell, the receptive fields were separated, then simultaneously stimulated with two light bars. A temporal delay was introduced between the arrival of the bars in the receptive fields to generate spatial disparities (-3 degrees to +3 degrees, in 0.5 degrees or 1 degree steps). Results showed that some binocular cells presented disparity tuning profiles similar to the tuned excitatory (12/34), tuned inhibitory (2/34), near (2/34) and far (3/34) cells found at various cortical levels in the normal cat. These interactions might allow for coarse binocular fusion as well as play a role in the initiation of vergence and the fixation of the eyes upon the appropriate plane of vision.

Animals↗

PubMed1998

Spatial disparity coding in the superior colliculus of the cat.

Cells in the superficial layers of the superior colliculus of the cat have mainly binocular receptive fields. The aim of the present experiment was to investigate the sensitivity of these cells to horizontal spatial disparity. Unit recordings were carried out in the superficial layers of the superior colliculus of paralyzed and anesthetized cats. Centrally located receptive fields were mapped, separated using prisms, and then stimulated simultaneously using two luminous bars optimally adjusted to the size of the excitatory region of the receptive fields. Only binocular cells were tested, and 65% of these units were found to be sensitive to spatial disparities. Some cells (20%) were clearly insensitive to spatial disparity and the remaining 15% showed complex, unclassifiable interactions. The sensitive cells could be divided into four classes based on their disparity-sensitivity profiles: 38% showed excitatory interactions, whereas 9% showed inhibitory interactions. Moreover, 11% and 7% of the cells responded, respectively, to crossed or uncrossed disparities, and were classified as near cells and far cells. Whereas the general shapes of the sensitivity profiles were similar to those of cells in areas 17-18, selectivity in the superior colliculus was significantly coarser. The superficial layers of the superior colliculus project topographically to the deep layers of the superior colliculus, which are known to contain circuits involved in the control of ocular movements. The results thus suggest that disparity-sensitive cells of the superior colliculus could feed information to these oculomotor neurons, allowing for the localization and fixation of objects on the appropriate plane of vision.

Animals↗

PubMed1998

Spatial and temporal matching of receptive field properties of binocular cells in area 19 of the cat.

The spatial and temporal properties of single neurons were investigated in area 19 of the cat. We evaluated the matching of binocular receptive field properties with regard to the respective strength of the ipsilateral and contralateral inputs. Results indicate that most cells in area 19 are well tuned to spatial and temporal frequencies and exhibit relatively low contrast threshold (mean=6.8%) when assessed using optimal parameters and tested through the dominant eye. Spatial resolution (mean=0.75 c/degree), optimal spatial frequencies (mean=0.16 c/degree) were relatively low and spatial bandwidths (mean=2.1 octaves) were broader as compared to those of cells in area 17 but comparable to those of cells in other extrastriate areas. On the other hand temporal resolution (mean=10.7 Hz), optimal temporal frequency (mean=4.5 Hz) and temporal bandwidths (mean=2.9 octaves) were higher and broader than in primary visual cortex. A significant relationship exists between most of the cell's properties assessed through either eye. For some parameters, such as spatial and temporal resolution, ocular dominance was shown to be significantly related to the extent of matching between the two eyes. For these parameters, binocular cells that exhibited a balanced ocular dominance were generally well matched with regard to the receptive field properties of each eye whereas the largest mismatches were found in cells that were more strongly dominated by one eye. These results suggest that visual input contributes to the activation of cells in area 19 in a redundant manner, possibly attesting to the multiplicity of parallel pathways to this area in the cat.

Animals↗

PubMed1998

Striate, extrastriate and collicular processing of spatial disparity cues.

The spatial disparity sensitivity of single units in the primary visual cortex (17-18 border), in extrastriate area 19 and in the superficial layers of the superior colliculus of the cat brain were compared in the present study. Unit recordings were performed in paralyzed and anesthetized animals. Centrally located receptive fields were mapped, separated using prisms and then stimulated simultaneously using two luminous bars optimally adjusted to the size of the excitatory receptive fields. In the three regions studied, cells selective to spatial disparity were found and four classes of disparity sensitivity profiles emerged. Although the disparity sensitivity profiles of the cells in the three regions appeared to have the same general shape, selectivity was clearly different. Cells at the 17-18 border were sharply tuned, those of area 19 were not only less numerous but also less well tuned and collicular cells exhibited coarse selectivity. These differences in selectivity appear to be linked to the projection pattern of the X, Y and W systems to these regions and the roles that these cells might play in vision.

Animals↗

PubMed1997

Unilateral and bilateral temperature comparisons in acallosal and split-brain subjects.

Therapeutic section of the corpus callosum in adult epileptic patients typically results in their incapacity to carry out interhemispheric comparisons of lateralized information. The fact that acallosal and early split-brain subjects display few of these symptoms when tested in the tactile modality has led to the suggestion that these patients may use ipsilateral projections of the somatosensory system more effectively. Compensation, however, is limited by the fact that the lemniscal pathway is strongly lateralized, especially for the distal parts of the body, where few ipsilaterally projecting fibres have been demonstrated. The pathway carrying temperature information has a larger ipsilateral component. Bilateral comparisons within the same hemisphere in subjects who are lacking the corpus callosum should be more common and the development of compensatory mechanisms in early-sectioned or acallosal subjects should be more likely. The objective of the present experiment was to evaluate differential thresholds for thermal stimuli applied on a number of regions either on the same side or on corresponding sites on opposite sides of the body. One subject callosotomized as an adult and one split-brain subject who underwent callosotomy in childhood, as well as three acallosal subjects, were compared to IQ-matched and normal-IQ control subjects. The fingers, forearm and trunk were tested. The comparison temperature was 30 degrees C and the other was varied in an ascending or descending fashion using a modified method of limits. Differential thresholds were similar for within- and between-side comparisons, and comparable to those of the IQ-matched subjects. The results indicate that comparisons involving temperature discrimination for stimuli applied to the two sides of the body do not require the integrity of the corpus callosum.

Adolescent↗

PubMed1997

Positional, directional and speed selectivities in the primary auditory cortex of the cat.

Responses of high-frequency primary auditory cortex (A1) neurons of the cat to noise stimulation were obtained in a quasianechoic chamber using a static and an apparently moving stimulus presented at similar azimuths. Simulated motion toward right or left as well as different simulated velocities were used. Under static stimulation, most units were contralateral-preferring followed by ipsilateral- and midline-preferring. Some were omnidirectional and a few were unclassifiable. Width of tuning was similar for contralateral-, ipsilateral- and midline-preferring units. Overall, about 25% were finely tuned (< 20 degrees) and the remaining were broadly tuned (> or =20 degrees). All cells sampled with static stimulation responded to apparent motion. About one quarter of the units were sensitive to the direction of the simulated moving noise in that they responded at least twice as much to one direction as to the other. Almost all directional contralateral-preferring units responded more when the apparent motion was directed toward ipsilateral azimuths, whereas all directional ipsilateral-preferring units responded preferentially to contralaterally oriented motion. In some units, up to five apparent speeds were tested. About half the units were not speed-selective (46%). The other cells were tuned to a preferential speed (40%), decreased their response as the apparent speed increased (10%) or displayed direction-dependent speed selectivity (4%). These results indicate that moving-sound sources are processed by some A1 single units.

Acoustic Stimulation↗

PubMed1997

Spatial resolution and contrast sensitivity of single neurons in area 19 of split-chiasm cats: a comparison with primary visual cortex.

Electrophysiological recordings were carried out in the callosal recipient zone of area 19 in normal and split-chiasm cats and, for comparison purposes, at the border of areas 17 and 18 of split-chiasm cats. The influences of retinothalamic and callosal inputs on a single cortical neurons were thereby evaluated. Extracellular recordings of single cells were made in anaesthetized and paralysed cats in the zone representing the central visual field. Receptive field properties were assessed using sine wave gratings drifting in optimal directions. Results showed that in area 19 and areas 17/18 one-third of the cells were binocularly driven after section of the optic chiasm. In area 19, the spatial resolution and contrast sensitivity of cells driven via the dominant eye were similar in the normal and split-chiasm groups. In areas 17/18 and area 19 of split-chiasm cats, binocular cells showed significant interocular matching of their receptive field properties (spatial resolution and contrast threshold), although small differences were observed. These small interocular differences were related to the cell's ocular dominance rather than to the signal transmission route (thalamic or callosal).

Animals↗

PubMed1996

Spatial and temporal frequency tuning and contrast sensitivity of single neurons in area 21a of the cat.

The spatial and temporal selectivities of single neurons in area 21a of the adult cat were investigated using sinusoidal gratings. Optimal spatial frequencies and visual acuity (high cut-off frequency) were fairly low and spatial bandwidth was mainly narrow. Contrast threshold was generally low but a substantial number of cells were only excited by high contrast stimuli. The temporal selectivity suggests that cells responded to a wide range of temporal frequencies.

Animals↗

PubMed1996

Distribution of acetylcholinesterase in the developing visual cortex of neonatally hemidecorticate rats.

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.

Acetylcholinesterase↗

PubMed1995

Binaural noise stimulation of auditory callosal fibers of the cat: responses to interaural time delays.

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.

Acoustic Stimulation↗

PubMed1995

Binocular interactions and visual acuity loss in esotropic cats.

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.

Animals↗