Biomedical subjects

Olivier Coubard

Publications and source records attributed to Olivier Coubard.

5 recordsLinked to original sources

PubMed2005

Role of the posterior parietal cortex in the initiation of saccades and vergence: right/left functional asymmetry.

This study explored in humans the role of the posterior parietal cortex (PPC) in saccades, vergence, and combined saccade-vergence movements by means of transcranial magnetic stimulation (TMS). TMS was applied to the right PPC at 80 ms, 90 ms, or 100 ms after target onset in experiment 1, and to the left PPC in experiment 2. Control experiments were also run in which TMS was applied over the primary motor cortex at 90 ms after target onset. Relative to no-TMS trials, TMS over the right PPC prolonged significantly the latency of almost all eye movements (saccades in either direction, convergence, divergence, and components of combined eye movements). Such latency increase was significant mostly when TMS was delivered 90 ms after target onset. In contrast, TMS of the left PPC increased the latency only for saccades to right, convergence, and convergence combined with rightward saccades; latency increase occurred for all time windows of TMS deliver (80, 90, or 100 ms after target onset). TMS over the vertex had no effect on the latency for any type of eye movement. TMS of either the left or the right PPC or of the motor cortex did not alter the accuracy of any type of eye movement. Thus, the effects of TMS on latency are time-, area-, and eye-movement-specific. We suggest that the right PPC is involved primarily in the processing of fixation disengagement, whereas the left PPC participates in the initiation of eye movements via different spatial selective mechanisms that concern exclusively targets to the right and/or to near.

Adult↗

PubMed2004

Contextual influence of TMS on the latency of saccades and vergence.

This study examines the effects of TMS of the right PPC on the latency of saccades and vergence alone or combined and the role of experimental design. Two designs were used: pure blocks with exclusively no-TMS or TMS trials; mixed blocks in which no-TMS and TMS trials were interleaved; a control study with TMS of the primary motor cortex (pure blocks) was also conducted and showed no effects on latencies. In contrast, in the experiment with TMS of the PPC latencies for TMS trials increased relative to no-TMS trials for almost all eye movements (isolated saccades, convergence, divergence, and for saccade and divergence components of combined eye movements). However, such increase was significant for pure blocks only. In mixed blocks no difference between TMS and no-TMS was found mainly because the latency of no-TMS trials increased relative to corresponding latencies in pure blocks. A second study centered on isolated convergence and divergence confirmed the interaction between block-design and TMS effects, and showed significant TMS/no-TMS differences only for the pure design and for a design in which the rate of TMS trials was high (75%). Again, the absence of difference was due to increase of latency for no-TMS trials in mixed blocks with low rates of TMS trials (50% or 25%), but also to decreased effects for the TMS trials themselves. We conclude that latency of all eye movements, saccades and vergence is highly influenced by the context. Such a contextual factor is the number of TMS versus no-TMS trials within a block; low numbers of TMS trials (50% or less) increases baseline latencies. The design of mixed blocks with 50% or less of TMS trials should not be recommended as it underestimates the direct effects of TMS on cortical processing. In fact, the majority of TMS studies on eye movements do use paradigms with high rates of TMS trials (75% or more). Our study confirms the validity of such paradigms.

Adult↗

PubMed2004

Transcranial magnetic stimulation of the posterior parietal cortex delays the latency of both isolated and combined vergence-saccade movements in humans.

To explore the 3D visual environment most frequently we make combined saccade-vergence eye movements. We studied the effect of transcranial magnetic stimulation (TMS) of the right posterior parietal cortex (rPPC) on such combined eye movements versus isolated saccade and vergence. In the main experiment, TMS was applied on the rPPC 80, 90 or 100 ms after target onset. In a control experiment, TMS was applied over the primary motor cortex at 90 ms after the target presentation. TMS trials were compared with no-TMS trials. TMS of the motor cortex had no effect at all on eye movements. TMS of the rPPC had no effect on the accuracy of eye movements, but it caused a latency increase: the increase was similar for the two components of the combined saccade-vergence movements, and it did not alter the naturally existing tight relationship of latency between the two components. Furthermore, the amount of latency prolongation was similar to that of isolated vergence, and of saccades in either direction (ipsilateral or contralateral relative to the stimulated site). Latency prolongation was time-specific but in a different way for different types of eye movements: for combined and convergence eye movements, the critical time window was -130 ms or more prior to the onset of eye movement, while for saccades and divergence TMS was disruptive later, -110 ms or more prior to the onset of eye movements. The latency increase is attributed to the interference by the TMS with the fixation disengagement process, for which the rPPC is believed to be instrumental. These results suggest that fixation disengagement occurs earlier for convergence and combined eye movements than for saccades and divergence.

Adult↗

PubMed2003

Gap effects on saccade and vergence latency.

To explore visual space, we make saccades, vergence, and, most frequently, combined saccade-vergence eye movements. The initiation of saccades is well studied, while that of vergence is less explored. Saccade latency is influenced by the fixation task: when the target appears simultaneously with the offset of the fixation point, latencies tend to be regular, whereas the introduction of a gap period before target onset causes the emergence of express latencies (80- to 120-ms). This study examines in ten normal adults whether the gap paradigm has a similar effect on the latency of vergence and combined eye movements. The second goal is to identify contextual factors that favor the emergence of short latencies, by comparing a condition in which gap and simultaneous trials were performed in separate blocks (pure blocks) with a condition in which the two types of trials were interleaved randomly (mixed blocks). The results are: (1) the gap paradigm reduced similarly (by approximately -30 ms) the mean latency of saccades, convergence, divergence, and both the saccadic and vergence components of combined eye movements; (2) the gap paradigm was responsible for the emergence of 80- to 120-ms latencies for saccades and divergence (pure or combined), but rarely for convergence; (3) inspection of the latency distributions showed that such short latencies formed a clearly distinct population, different from anticipatory responses or regular latencies, for saccades (pure or combined) but not for pure vergence; importantly, distinct express latencies were found also for the convergence and divergence components of combined eye movements; (4) no difference was found for the group of subjects between pure and mixed blocks, but the latter yielded shorter latencies for some subjects, suggesting an idiosyncratic phenomenon. We suggest that distinct express latencies are specific to saccades and could correspond to a specific mode of saccade initiation. Interestingly, the express mode of triggering can be transferred to the vergence component in the ecological condition in which saccade is combined with vergence.

Adolescent↗

PubMed2003

Effects of TMS over the right prefrontal cortex on latency of saccades and convergence.

PURPOSE: The prefrontal cortex (PFC) is known to inhibit unwanted saccades through its connections to the superior colliculus (SC). Indeed, transcranial magnetic stimulation (TMS) of the PFC decreases saccade latency by increasing the rate of express saccades. This study examined whether a similar phenomenon exists for vergence. METHODS: In a gap paradigm, six healthy subjects were asked to look at LEDs placed in a horizontal plane and to make lateral saccades, pure convergence along the median plane, and combined eye movements. Eye movements were recorded binocularly. TMS was applied over the right (r)PFC synchronously with the onset of the target. In a control condition, TMS was applied over the motor cortex (MC). RESULTS: TMS over the MC had no effect on the latency of any type of eye movements. In contrast, TMS over the rPFC (1) decreased significantly (P = 0.00367) the latency of contralateral pure saccades, (2) had no effect on the latency of pure convergence, (3) and caused a mild decrease in the latency of both the saccadic and the convergence components of combined eye movements, and the effect was bilateral. Decreased latencies were mainly due to an increase of the rate of express movements. CONCLUSIONS: The inhibition exerted by PFC over SC and preventing express movements from occurring is presumably a saccade-specific mechanism. When the saccade is combined with convergence, the express triggering can be transferred to a certain extent to the convergence.

Adult↗