Biomedical subjects

Jean-Paul Guillemot

Publications and source records attributed to Jean-Paul Guillemot.

10 recordsLinked to original sources

PubMed2005

Development of visual-evoked potentials to radially modulated concentric patterns.

The visual processing of radially modulated concentric patterns was studied in human participants, aged 3-22 years, by recording event-related potentials. These stimuli are known to activate the fusiform face area as well as area V4 in normal adults. The electrophysiological data showed a P1 latency that reached a maturation asymptote before 3 years of age, whereas that of N1 and P2 became adultlike by 13 years of age. In addition, the distribution of the P2 component over the scalp was focalized in the primary visual cortex before adolescence and became distributed over the entire brain after adolescence. Radially modulated concentric stimuli thus induce brain activation that is not mature until 13 years of age.

Adolescent↗

PubMed2005

Role of primary visual cortex in the binocular integration of plaid motion perception.

This study assessed the early mechanisms underlying perception of plaid motion. Thus, two superimposed gratings drifting in a rightward direction composed plaid stimuli whose global motion direction was perceived as the vector sum of the two components. The first experiment was aimed at comparing the perception of plaid motion when both components were presented to both eyes (dioptic) or separately to each eye (dichoptic). When components of the patterns had identical spatial frequencies, coherent motion was correctly perceived under dioptic and dichoptic viewing condition. However, the perceived direction deviated from the predicted direction when spatial frequency differences were introduced between components in both conditions. The results suggest that motion integration follows similar rules for dioptic and dichoptic plaids even though performance under dichoptic viewing did not reach dioptic levels. In the second experiment, the role of early cortical areas in the processing of both plaids was examined. As convergence of monocular inputs is needed for dichoptic perception, we tested the hypothesis that primary visual cortex (V1) is required for dichoptic plaid processing by delivering repetitive transcranial magnetic stimulation to this area. Ten minutes of magnetic stimulation disrupted subsequent dichoptic perception for approximately 15 min, whereas no significant changes were observed for dioptic plaid perception. Taken together, these findings suggest that V1 is not crucial for the processing of dioptic plaids but it is necessary for the binocular integration underlying dichoptic plaid motion perception.

Adult↗

PubMed2004

Early- and late-onset blind individuals show supra-normal auditory abilities in far-space.

Blind individuals manifest remarkable abilities in navigating through space despite their lack of vision. They have previously been shown to perform normally or even supra-normally in tasks involving spatial hearing in near space, a region that, however, can be calibrated with sensory-motor feedback. Here we show that blind individuals not only properly map auditory space beyond their peri-personal environment but also demonstrate supra-normal performance when subtle acoustic cues for target location and distance must be used to carry out the task. Moreover, it is generally postulated that such abilities rest in part on cross-modal cortical reorganizations, particularly in the immature brain, where important synaptogenesis is still possible. Nonetheless, we show for the first time that even late-onset blind subjects develop above-normal spatial abilities, suggesting that significant compensation can occur in the adult.

Acoustic Stimulation↗

PubMed2004

Disparity sensitivity in the superior colliculus of the cat.

The present study aims at evaluating the spatial disparity response profiles of binocular cells in the superficial layers of the superior colliculus of the cat using drifting light bars and phase-shifted spatial frequency gratings. Results show that a total of 64% of the cells were sensitive to phase disparities and had large tuning profiles. Similarly, a large proportion (75%) of those tested with position offsets showed one of the four classic disparity profiles, those of the tuned cells being rather coarse. When tested with both position and phase disparities, 54% of the cells showed sensitivity profiles to the two types of stimuli. The overall results suggest that the superior colliculus is involved in the analysis of coarse stereopsis and/or the planning and initiation of saccades during vergence eye movements and/or the control of fine adjustments to maintain fixation as the stimulus moves in depth.

Action Potentials↗

PubMed2004

Effective binocular integration at the midline requires the corpus callosum.

To study the role of the corpus callosum (CC) in midline binocular integration, the effects of late callosotomy and congenital CC agenesis on the ability to perceive dichoptic plaid motion was assessed. Coherent motion was well perceived at all locations in the visual field under dioptic viewing but not along the vertical meridian (VM) when the components were dichoptically presented. This deficit was totally abolished in the agenesis subject and reduced in the callosotomized individual when stimulus size was increased beyond the VM. Electrophysiological correlates were also examined by recording visual evoked potentials and these showed that the P1/N2 components were abnormal for small dichoptic stimuli presented on the midline. These findings attest to the importance of the contribution of CC to midline binocular integration and the effects of cerebral plasticity.

Adult↗

PubMed2003

Magnocellular and parvocellular developmental course in infants during the first year of life.

The visual system undergoes major modifications during the first year of life. We wanted to examine whether the magnocellular (M) and parvocellular (P) pathways mature at the same rate or if they follow a different developmental course. A previous study carried out in our laboratory had shown that the N1 and P1 components of pattern visual evoked potentials (PVEPs) were preferentially related to the activity of P and M pathways, respectively. In the present study, PVEPs were recorded at Oz in 33 infants aged between 0 and 52 weeks, in response to two spatial frequencies (0.5 and 2.5 c deg(-1)) presented at four contrast levels (4, 12, 28 and 95%). Results indicate that the P1 component appeared before the N1 component in the periods tested and was unambiguously present at birth. The P1 component showed a rapid gain in amplitude in the following months, to reach a ceiling around 4-6 months. Conversely, the N1 component always appeared later and then gained in amplitude until the end of the first year without reaching a plateau. Latencies were also computed but no developmental dissociation was revealed. Results obtained on amplitude are interpreted as demonstrating a developmental dissociation between the underlying M and P pathways, suggesting that the former is functional earlier and matures faster than the latter during the first year of life.

Contrast Sensitivity↗

PubMed2003

Comparison of sensitivity to first- and second-order local motion in 5-year-olds and adults.

We compared sensitivity to first- versus second-order motion in 5-year-olds and adults tested with stimuli moving at slower (1.5 degrees s(-1)) and faster (6 degrees s(-1)) velocities. Amplitude modulation thresholds were measured for the discrimination of the direction of motion (up vs. down) for luminance-modulated (first-order) and contrast-modulated (second-order) horizontal sine-wave gratings. At the slower velocity (1.5 degrees s(-1)), the differences in threshold between 5-year-olds and adults were small but significant for both first- and second-order stimuli (0.02 and 0.05 log units worse than adults' thresholds, respectively). However, at the faster velocity (6 degrees s(-1)), the differences in threshold between the children and adults were 8 times greater for second-order motion than for first-order motion. Specifically, children's thresholds were 0.16 log units worse than those of adults for second-order motion compared to only 0.02 log units worse for first-order motion. The different pattern of results for first-order and second-order motion at the faster velocity (6 degrees s(-1)) is consistent with models positing different mechanisms for the two types of motion and suggests that those mechanisms mature at different rates.

Adult↗

PubMed2002

Phase-disparity coding in extrastriate area 19 of the cat.

Binocular interactions were investigated in area 19 of the anaesthetized cat using dichoptically presented phase-shifted static spatial frequency gratings that flickered at a fixed temporal rate. More than two-thirds of the binocular cells showed phase specificity to static phase disparities leading to either summation or facilitation interactions. This proportion of spatial disparity selectivity was higher than that shown for the same area (one-third of the units) when drifting light bars or drifting spatial frequencies were used to create disparities. The range of phase disparities encoded by binocular cells in area 19 is inversely related to the optimal spatial frequency of the dominant eye. Thus, cells in this area are tuned to coarse spatial disparities which, as supported by behavioural studies, could reflect its involvement in the analysis of stereoscopic pattern having gross disparities but devoid of motion cues. Because of the nature of its interconnections with numerous visual cortical areas, area 19 could serve as a way station where stereoscopic information could be first analysed and sent to other higher order areas for a complete representation of three-dimensional objects.

Animals↗

PubMed2002

Influence of NO downregulation on oscillatory evoked responses in developing rat superior colliculus.

Nitric oxide (NO) is involved in neuronal transmission by modulating neurotransmitter release in adults and in stabilizing synaptic connections in developing brains. We investigated the influence of downregulation of NO synthesis on oscillatory components of ON and OFF evoked field potentials in the rat superior colliculus. NO synthesis was decreased by inhibiting nitric oxide synthase (NOS) with an acute microinjection of N(omega)-nitro-L-arginine methyl ester (L-NAME). The study focuses on rhythmic activity by analyzing fast Fourier transform (FFT). Collicular responses were recorded in anesthetized rats, at postnatal days (PND) 13-19 and adults. This time window was chosen because it is centered on eye opening. NO downregulation resulted in a dual effect depending on age and response-type. NO synthesis inhibition decreased the magnitude of oscillations in ON responses in the youngest animals (PND13-14), whereas oscillations of frequencies higher than 20 Hz in OFF responses were increased in all age groups of developing rats. In adults NO downregulation increased oscillations in ON responses and decreased oscillations in OFF responses. L-Arginine was used to increase NOS activity and its injection produced effects opposite to those seen with L-NAME. Slow oscillatory components (7-12 Hz) were unaffected in all experiments. Our data together with results reported in the literature suggest that rhythmic patterns of activity are NO-dependent.

Animals↗

PubMed2002

Phase disparity in area 19 of the cat.

Binocular cells in area 19 are tuned to positional disparities. In effect, up to one-third of the cells respond preferentially to small incongruities between the optimal bar stimuli presented within the receptive fields of each eye. The aim of the present study was to determine whether cells in area 19 are also sensitive to phase disparities. Both types of disparities have been proposed as mechanisms through which stereoperception is achieved. Results indicate that phase disparities produced coherent interactions in 38% of the binocular cells, resulting in facilitation or summation. The remaining cells were phase insensitive. The overall results suggest that cells in area 19 code phase disparities in a proportion comparable to stimulus disparities, confirming that this area is implicated in binocular integration, albeit in a relatively smaller proportion than some of the other visual areas.

Animals↗