André Roucoux

Stimulation of the superior colliculus in the alert cat

SummaryElectrical stimulation of the cat superior colliculus (SC), in conjunction with the accurate measurement of elicited eye movements and histologically verified electrode positions, has revealed a striking antero-posterior variation in collicular organization. Three zones could be defined in the SC on the basis of eye movement patterns and associated neck muscle EMG activity evoked from the deeper layers. The Anterior zone was coextensive with the central 25 ° of the visual retinotopically coded map contained in the superficial layers. Saccades evoked from this zone were also retinotopically coded, and the latency of EMG activity depended on the position of the eye in the orbit. A similar observation applies to the entire monkey SC. The Intermediate zone was coextensive with the 25 °–70 ° of visual projections. Saccades evoked from this region were “goal-directed” and were associated with invariant, short latency EMG responses. The Posterior zone was found in the extreme caudo-lateral portion of the SC. Eye movements evoked from this zone were centering saccades associated with constant latency EMG activity. The present results in conjunction with previously demonstrated antero-posterior variations in projections to the SC, suggest that the motor strategies controlling gaze shifts toward visual targets vary depending on the location of the target in the visual field.

Neuronal Mechanisms of Perceptual Learning: Changes in Human Brain Activity with Training in Orientation Discrimination

Using 15O-water 3D positron emission tomography, regional cerebral blood flow was measured twice in six human subjects: before and after extensive training in orientation discrimination. In each session subjects performed two orientation discrimination tasks, during which they discriminated the orientation of a grating at either the trained or untrained reference orientation, and a control task, during which they detected a randomly textured pattern. By comparing the discrimination to the detection tasks, we observed a main effect of task bilaterally in the posterior occipital cortex, extending into the left posterior fusiform gyrus and the right inferior occipital gyrus, bilaterally in the intraparietal sulcus, as well as in the cerebellum, thalamus, and brainstem. When we compared the activation pattern before and after the training period, all the changes observed were activity decreases. The nonspecific changes, which were not related to the orientation used during the training, were situated in the cerebellum and bilaterally in the extrastriate visual cortex. The orientation-specific changes, on the other hand, were restricted to the striate and extrastriate visual cortex, more precisely the right calcarine sulcus, the left lingual gyrus, the left middle occipital, and the right inferior occipital gyrus. These findings confirm our hypothesis concerning the existence of learning related changes at early levels of visual processing in human adults and suggest that mechanisms resulting in neuronal activity decreases might be involved in the present kind of learning.

Horizontal Eye Position-related Activity in Neck Muscles of the Alert Cat

SummaryThe activity of neck muscles was recorded in the alert, head-fixed cat together with the horizontal and vertical components of eye movements. Electromyographic activity of obliquus capitis cranialis and caudalis, and longissimus capitis, is closely related to horizontal eye position in the orbit both during spontaneous eye movements and vestibular nystagmus. The activity of splenius also shows this relationship but the coupling is less tight, probably because of the postural function of this muscle.

Development of Fixation and Pursuit Eye-movements in Human Infants

Visual fixation and pursuit abilities of human infants were tested during their first year of life. Eye as well as head position was measured. Results show that the fixation of visual targets is accomplished by a head rotation accompanied by a series of small eye saccades. The number of these saccades increases with target eccentricity but progressively decreases with age. Pursuit of a moving visual target is performed by a smooth eye and head movement only if the target velocity is low. The maximum speed of pursuit progressively increases with age. The results are compatible with the relatively late development of the fovea.

PET STUDY OF HUMAN VOLUNTARY SACCADIC EYE MOVEMENTS IN DARKNESS : EFFECT OF TASK REPETITION ON THE ACTIVATION PATTERN

Using H215O 3D Positron Emission Tomography (PET), regional cerebral blood flow (rCBF) was measured in six human subjects under two different conditions: at rest and while performing self‐paced horizontal saccadic eye movements in darkness. These two conditions were repeated four times each.

Current Source Density Analysis of CNV During Temporal Gap Paradigm

The present report studied the contingent negative variation during Gap and Non-Gap conditions using visual stimulation and manual responses. The reaction times during the Gap condition were facilitated compared with those of the Non-Gap condition. The contingent negative variation component was obtained during the preparatory period from electrodes placed at 58 scalp sites for both Gap and Non-Gap conditions. The comparison between both conditions: Gap and non-gap did not show statistically significant differences during the preparatory period. The topography of the voltage and current source density maps showed three different foci: (i) an early negativity centred in electrodes overlying the supplementary motor area and cingulate motor areas, (ii) an activation over the primary motor cortex contralateral to the finger movement, and (iii) a bilateral activation on posterior sites. All these results suggest that the facilitation induced by the warning stimuli occurs in neural circuits that would be recruited for the subsequent processing of the imperative stimulus. The facilitation of the reaction times during the gap condition with respect to non-gap condition must be justified by neural events occurring during the gap period.

Smooth eye movements evoked by electrical stimulation of the cat's superior colliculus.

Head-fixed gaze shifts were evoked by electrical stimulation of the deeper layers of the cat superior colliculus (SC). After a short latency, saccades were triggered with kinematics similar to those of visually guided saccades. When electrical stimulation was maintained for more than 150–200 ms, postsaccadic smooth eye movements (SEMs) were observed. These movements were characterized by a period of approximately constant velocity following the evoked saccade. Depending on electrode position, a single saccade followed by a slow displacement or a “staircase” of saccades interspersed by SEMs were evoked. Mean velocity decreased with increasing deviation of the eye in the orbit in the direction of the movement. In the situation where a single evoked saccade was followed by a smooth movement, the duration of the latter depended on the duration of the stimulation train. In the situation where evoked saccades converged towards a restricted region of the visual field (“goal”-directed or craniocentric saccades), the SEMs were directed towards the centre of this region and their mean velocity decreased as the eye approached the goal. The direction of induced SEMs depended on the site of stimulation, as is the case for saccadic eye movements, and was not modified by stimulation parameters (“place” code). On the other hand, mean velocity of the movements depended on the site of stimulation and on the frequency and intensity of the current (“rate” code), as reported for saccades in the cat. The kinematics of these postsaccadic SEMs are similar to the kinematics of slow, postsaccadic correction observed during visually triggered gaze shifts of the alert cat. These results support the hypothesis that the SC is not exclusively implicated in the control of fast refixation of gaze but also in controlling postsaccadic conjugate slow eye movements in the cat.

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

SummaryAn experimental study of head-free and headfixed 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 (≈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.

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.

Amplitude transition function of human express saccades.

The gap paradigm often promotes the occurrence of express saccades, which are supposed to be short latency, visually guided saccades, often forming a separate peak in saccadic latency distribution. We designed six experiments in which we compared the amplitudes of anticipatory, express and regular saccades, for various conditions of target eccentricities, target direction, and predictability. Then, saccadic amplitude was expressed as a continuous function of latency, for the various target eccentricities. From the obtained results, it is proposed that a saccade of a given amplitude is prepared during the gap period, on the basis of internal cues. The latency range of express saccades is a transition zone when the target begins to influence the already prepared saccade. The resulting amplitude will be a weighted average of the value determined during the gap and of the value defined by the target, the weighting being determined by the latency of the saccade. If the preprogrammed saccade is wrongly directed, the target will not be able to correct the saccadic amplitude and the express saccade will have the same amplitude as anticipatory saccades. Regular saccades are delayed sufficiently so that a wrongly directed preprogrammed saccade can be canceled or the amplitude of a rightly directed saccade can be adjusted according to the exact position of the visual target.