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A Roucoux

Publications and source records attributed to A Roucoux.

15 recordsLinked to original sources
PubMed1992

Experimental study and modeling of vestibulo-ocular reflex modulation during large shifts of gaze in humans.

An experimental study of head-free and head-fixed gaze shifts explores the role of the vestibulo-ocular reflex (VOR) during saccadic and slow phase components of the gaze shifts. A systematic comparison of head-free and head-fixed gaze shifts in humans revealed that while the VOR is switched off as soon as the saccade starts, its function is progressively restored during the terminal phase of the saccade. The duration of this restoration period is fairly constant; therefore, the faster the gaze saccade, the sooner the VOR function starts to be restored. On the basis of these experimental data, a new eye-head coordination model is proposed. This model is an extension of the one proposed by Laurutis and Robinson (1986) where VOR gain is a function of both the dynamic gaze error signal and head velocity. This extension has also been added to another eye-head coordination model (Guitton et al. 1990). Both modified models yield simulation results comparable to experimental data. This study pinpoints the high efficiency of the gaze control system. Indeed, a fixed period of time (approximately 40 ms) is needed to restore the inhibited VOR; the gaze control system thus must have a knowledge of its own dynamics in order to be able to anticipate the end of the saccadic movement.

Adult↗
PubMed1989

Neck muscle activity in eye--head coordinated movements.

The electromyographic (EMG) activity of different neck muscles in relation to gaze orientation has been studied in alert trained cats. When the head is kept fixed, the activity of these muscles is proportional to eye eccentricity in the horizontal as well as in the vertical planes. On basis of this tonic activity, a preferential orientation can be attributed to each muscle: upward and lateral for biventer, rectus and complexus, and downward and lateral for longissimus, splenius and obliquus capitis cranialis. Fluctuations in this modulation of the EMG activity by eye position can be observed. When the head is free to move, the muscles show phasic discharges having similar preferential orientations. For a given muscle, this orientation covers a quite large angle: many muscles contribute to a given movement. The timing of the discharge of the different muscles as a function of the direction of the head movement was examined. It was found that the latency, i.e. the delay between the discharge and movement onset, progressively increases as the movement direction diverges from the preferential orientation of the muscle. It has been noted that the muscles having an upward preferential orientation may show, in relation to downward movements, inhibition occurring prior to the onset of the head movement. The same muscles may also increase their activity around the midcourse of downward movements. Thus, the head motor system controls the direction and amplitude parameters not only by selectively activating the appropriate muscles but also by sequencing their activity in a subtle way to start, control the trajectory and stop the movement, reminiscent of what has been described for limb movements.

Action Potentials↗
PubMed1976

Eye movements evoked by superior colliculus stimulation in the alert cat.

(1) The electrical stimulation of the superior colliculus (SC) in the cat evokes exclusively conjugate and contraversive eye saccades. (2) Their maximum velocity is markedly higher than that of spontaneous saccades. (3) The stimulus parameters (intensity, frequency, pulse width) have but little effect on the characteristics of the saccades. (4) In the anterior half of the SC, corresponding to the projection of the 12-15 central degrees of the retina, amplitude and direction of saccades depend exclusively on the position of the electrode. (5) In the posterior half, corresponding to the projection of the peripheral retina, saccades are 'goal directed' and the position of the goal is determined by the location of the electrode. (6) An increase in stimulus train length produces, in the anterior part, a succession of identical saccades and, in the posterior part, a goal fixation. (7) Taking all these data into consideration, a model of the foveation process in the cat is proposed.

Animals↗