Biomedical subjects

Z Kapoula

Publications and source records attributed to Z Kapoula.

At least 19 recordsLinked to original sources

PubMed2005

Prolongation of latency of horizontal saccades in elderly is distance and task specific.

The present study examined horizontal saccades in healthy subjects: 9 adults (20-32 years) and 10 aged subjects (63-83 years), under gap (fixation target extinguishes prior to target onset) and overlap (fixation stays on after target onset). The gap paradigm is known to promote fast initiation of saccades while the overlap paradigm promotes voluntary saccades with longer latency. In real life we perform saccades at various distances. In this study each paradigm was run at three viewing distances-20, 40 and 150 cm, corresponding to a convergence angle of 17.1 degrees, 8.6 degrees and 2.3 degrees, respectively. Eye movements were recorded with the Chronos video eye tracker or with the photoelectric IRIS. The main findings are: (i) increase in latency of saccades with age, with distance and with the overlap condition; (ii) evidence for interaction between these factors, indicating the following anomaly: in the gap condition and at near, aged subjects show short latencies similar to those of young adults; (iii) express type of latencies (between 80 and 120 ms) occur most frequently at near in the gap condition and at similar rates in young (25%) and aged subjects (20%). The specificity of close distance combined with the gap for triggering short latency saccades could be related to both attention and oculomotor fixation disengagement. The strength of coupling between fixation-eye movement control and visual attention control varies for different locations in space, and its decline with aging can be also different.

Adult↗

PubMed2005

The effect of transcranial magnetic stimulation on the latencies of vertical saccades.

In this study, we investigated the effect of transcranial magnetic stimulation (TMS) over the right posterior parietal cortex (PPC) on the latency of two different types of visually-guided vertical saccades: reflexive saccades triggered by the sudden onset of a target, and saccades towards target locations known in advance. For this reason, we used two oculomotor tasks: a gap and a delay task, respectively. Nine normal subjects performed vertical saccades at +/-7.5 and +/-15 degrees . TMS was applied at 80 and 100 ms after target onset in the gap task, and after fixation offset in the delay task. Without TMS, we confirmed a latency asymmetry in the gap task favouring upward saccades at the lower eccentricity (7.5 degrees ), and a latency symmetry in the delay task. TMS increased the latencies of all saccades in the delay task, when delivered at 100 ms. This effect was mostly pronounced for downward saccades at 7.5 degrees . As a result, saccade latencies showed an asymmetry in this condition, similar to the one observed in the gap task without TMS. The gap task with TMS resulted in a variable latency distribution and no significant overall effect on saccade latency. Our results indicate that the right PPC is involved in the initiation of vertical saccades in the delay task, and that this involvement appears to be enhanced for downward saccades. A conclusion for the involvement of this area in the gap task could not be drawn from this study.

Adult↗

PubMed2002

Vertical memory-based disconjugate learning for downward saccades at a viewing distance of 70 cm: relation to horizontal vergence and to vertical phoria.

The goal of this study was to examine vertical disconjugate learning based on memorization of vertical disparity at a viewing distance of 70 cm; such a distance is common in many visual ergonomic conditions, including car driving. For a period of 15 min, eight subjects made downward saccades to a memorized target. The target was rendered disparate with the use of a magnifier placed in front of one eye. Learning occurred in only four of the subjects: saccades became disconjugate and the disconjugacy was retained when the target was not disparate. These observations extend a prior study dealing with a longer distance (1 m) and indicate that such learning is subject dependent for both viewing distances. Importantly, the present study shows, for the first time, that vertical disconjugate learning is related to two other phenomena: (a) the horizontal vergence present during vertical saccades and (b) the degree of vertical phoria normal subjects exhibit.

Adolescent↗

PubMed2001

Effects of transcranial magnetic stimulation of the posterior parietal cortex on saccades and vergence.

Previous studies showed that transcranial magnetic stimulation (TMS) of the posterior parietal cortex (PPC) prolongs the latency of intentional saccades. We examined whether a similar effect exists for reflexive saccades and vergence. To elicit reflexive movements, a gap paradigm was used; lateral saccades and vergence along the median plane were interleaved. TMS was applied on the right PPC 80 ms after target onset. Blocks without TMS were performed and a control experiment with TMS over the primary motor cortex. The latter had no effect on the latency of any type of movements. In contrast, stimulation of the PPC increased the latency of both saccades and vergence, suggesting that the PPC is involved in the triggering of reflexive movements both in direction and in depth.

Adult↗

PubMed2001

Disconjugate oculomotor learning caused by feeble image-size inequality: differences between secondary and tertiary positions.

In order to examine the minimum value of image-size inequality capable of inducing lasting disconjugacy of the amplitude of saccades, six normal emmetropic subjects were exposed for 16 min to 2% image size inequality. Subjects were seated at 1 m in front of a screen where a random-dot pattern was projected and made saccades of 7.5 and 15 deg along the horizontal and vertical principal meridians and to tertiary positions in the upper and lower field. During the training period, compensatory disconjugacy of the amplitude of the saccades occurred for the principal horizontal and vertical meridians; such increased disconjugacy persisted after training, suggesting learning. In contrast, for horizontal saccades to or from tertiary positions made in the upper and lower field, no consistent changes in the disconjugacy occurred, either during training or after the training condition. In an additional experiment, three subjects read sequences of words with the 2% magnifier in front of their dominant eye: in such a task, horizontal saccades to or from tertiary positions at the upper or lower field showed appropriate and lasting disconjugacy for two of the three subjects. We conclude that even a 2% image size inequality stimulates oculomotor learning, leading to persistent disconjugacy of saccades. The small disparity created by the image-size inequality is thus compensated by the oculomotor system rather than tolerated by the sensory system (e.g. by enlarging the Panum's area).

Aniseikonia↗

PubMed2000

Motor execution is necessary to memorize disparity.

Binocular saccades in response to briefly flashed, memorized disparate targets (different for the two eyes) become disconjugate following repeated trials. After 15 min of such training, the disconjugacy persists, even when the target to memorize is no longer disparate. This study examines the hypothesis that disparity memorization has a motor basis. We report here three experiments in which subjects were trained for 15-min periods. In experiment 1, subjects made no saccade after target presentation (static training); in experiment 2 subjects intended to make a saccade, but they actually made a saccade in only 10% or 20% of the trials; in experiment 3 subjects made anti-saccades. For all three experiments, the flashed target was disparate and the memory delay for each trial was 1 s. To examine the effects of learning for all three experiments, before and after training, we recorded memory-guided saccades to non-disparate targets (monocular viewing). Experiments 1 and 2 produced inconsistent (before/after training) changes in the disconjugacy of saccades. Thus, the disparity of potential saccade targets had no lasting effect on the disconjugacy of saccades if a saccade was not made. In contrast, the anti-saccades in experiment 3 developed a disconjugacy opposite to the disparity of the remembered target. These findings indicate that the execution of the saccade is necessary to memorize disparity of the target.

Conditioning, Psychological↗

PubMed2000

Disconjugate memory-guided saccades to disparate targets: temporal aspects.

Memory-guided saccades to disparate targets (i.e., more eccentric for one eye) flashed 1 s earlier become disconjugate (i.e., of different amplitude for the two eyes) after only about 30 trials. After about 225 trials the disconjugacy persists even when the target to remember is no longer disparate. This suggests fast learning based on short-term memorization of disparity. Learning, however, fails to occur if during the training the memory delay for each trial is increased to 2 s. The purpose of the present study was to test the importance of the frequency of stimulus presentation and thereby the rate of saccades. The same memory-guided saccade paradigm was used as in the prior study and a short training period of 225 trials was applied. For each training trial, the memory delay was again 2 s, but the time allocated for fixation of the central dot and the time allocated for fixation of the remembered target in the dark was reduced to increase the frequency of saccades made. Saccades became rapidly disconjugate and their disconjugacy was retained in a subsequent neutral condition using non-disparate targets. These findings indicate that stimulus frequency and thereby saccade frequency is important for disconjugate oculomotor learning based on disparity memorization. Nevertheless, additional experiments using longer memory delays of 3 s or 4 s show a definite failure of memorization and disconjugate learning.

Conditioning, Psychological↗

PubMed2000

Disconjugate vertical memory-guided saccades to disparate targets.

We studied the binocular coordination of normal memory-guided saccades and the possibility of inducing memory-based disconjugate learning. First, we report that normal vertical memory-guided saccades to non-disparate targets are yoked vertically in the two eyes as well as visually guided vertical saccades. To induce disconjugate vertical learning, at each trial we flashed a target that was disparate (i.e. 8% more elevated or more depressed for one eye); the memory delay was 1 s. Memory-guided vertical saccades developed a vertical disconjugacy that was appropriate for the disparity of the remembered target. After 15 min of repetition, this vertical disconjugacy persisted even when the target to be remembered was no longer disparate; this indicates disconjugate vertical learning based on short-term memory. However, this was observed only for a few individual cases and its amplitude was small. This contrasts with prior findings on horizontal saccades associated with horizontal disparities. We conclude that vertical memory-based disconjugate learning is possible but very limited. Together with other studies, this study suggests that the natural vertical conjugacy of vertical saccades relies little on rapid learning mechanisms. Rather it seems to be built-in, and this is consistent with earlier electrophysiological findings.

Convergence, Ocular↗

PubMed1999

Listing's plane rotation with convergence: role of disparity, accommodation, and depth perception.

Earlier studies have reported temporal rotation of Listing's plane with convergence of the eyes causing torsion, which is dependent on eye elevation. The amount by which the planes rotate differs from study to study. To gain insight into the functional significance of the temporal tilt of Listing's plane for vision, we examined whether the rotation of the plane depends on the visual conditions, namely on the stimuli driving vergence. In different conditions, accommodative vergence, disparity-vergence, combinations of disparity with accommodation or depth perception were used and the resulting rotation of Listing's plane was measured. Our findings show, for the first time, that the relationship between convergence and Listing's-plane temporal rotation depends on the stimuli driving vergence. When the stimulus contains only disparity cues, vergence and Listing's plane rotate immediately and consistently among subjects. Accommodative vergence, the mutual couplings between vergence and accommodation, can influence the orientation of Listing's plane, but they do so in a idiosyncratic way. The largest rotation was elicited by stereograms combining disparity-vergence with depth perception. These findings support the idea of a functional role of Listing's plane rotation for binocular vision, perhaps for depth perception.

Accommodation, Ocular↗

PubMed1999

Saccade amplitude disconjugacy induced by aniseikonia: role of monocular depth cues.

The conjugacy of saccades is rapidly modified if the images are made unequal for the two eyes. Disconjugacy persists even in the absence of disparity which indicates learning. Binocular visual disparity is a major cue to depth and is believed to drive the disconjugacy of saccades to aniseikonic images. The goal of the present study was to test whether monocular depth cues can also influence the disconjugacy of saccades. Three experiments were performed in which subjects were exposed for 15-20 min to a 10% image size inequality. Three different images were used: a grid that contained a single monocular depth cue strongly indicating a frontoparallel plane; a random-dot pattern that contained a less prominent monocular depth cue (absence of texture gradient) which also indicates the frontoparallel plane; and a complex image with several overlapping geometric forms that contained a variety of monocular depth cues. Saccades became disconjugate in all three experiments. The disconjugacy was larger and more persistent for the experiment using the random-dot pattern that had the least prominent monocular depth cues. The complex image which had a large variety of monocular depth cues produced the most variable and less persistent disconjugacy. We conclude that the monocular depth cues modulate the disconjugacy of saccades stimulated by the disparity of aniseikonic images.

Aniseikonia↗

PubMed1998

Disconjugate memory-guided saccades to disparate targets: evidence for 3D sensitivity.

The saccadic system has been traditionally regarded as two-dimensional (horizontal, vertical) and basically conjugate in the two eyes. However, saccades to disparate targets (e.g., targets in real three-dimensional space that are located in different directions and at different distances) are naturally disconjugate. We report here that memory-guided saccades to a disparate target flashed 1 s earlier become disconjugate following repeated trials. After 15 min of repetition, the disconjugacy persists even when the target to be remembered is no longer disparate. This suggests fast memory-based learning. Learning, however, fails to occur if, during the repetition trials, the memory delay is 2 s. These findings suggest that the saccadic system has access to a 3D representation of targets and is gifted with 3D short-term memory and learning capacity.

Adaptation, Ocular↗

PubMed1997

Impairment of the binocular coordination of saccades in strabismus.

To examine the link between binocular vision and binocular coordination of saccades we studied subjects with convergent strabismus since childhood with mild or no amblyopia: three subjects had small squint (< 10 prism D) and preserved peripheral binocular visual function with gross stereopsis; four subjects had larger squint (18-35 prism D) and no detectable stereopsis. A standard paradigm was used to elicit horizontal saccades; binocular recordings were made with the IRIS device. For subjects with small strabismus, saccades were disconjugate (unequal between the two eyes) typically by 1 deg. Subjects with larger strabismus exhibited even larger and more variable disconjugacy (typically 1.8 deg). Post-saccadic eye drift was consistently divergent in subjects with small strabismus and tended to reduce the convergent squint angle. In contrast, in subjects with large strabismus drift was convergent. The impairment of the binocular control of saccades is attributed to the deficiency of disconjugate oculomotor adaptive capabilities necessary to compensate for the natural asymmetries or changes in the two oculomotor plants; such deficiency would be more severe in subjects with large strabismus who have neither central nor peripheral binocular vision.

Adult↗

PubMed1997

Deficiency of adaptive control of the binocular coordination of saccades in strabismus.

Disconjugate (different in the two eyes) oculomotor adaptation is driven by the need to maintain binocular vision. Since binocular vision is deficient in strabismus, we wondered whether oculomotor disconjugate adaptive capabilities are deficient in such subjects. We studied eight adult subjects with constant, long-standing convergent strabismus of variable angles (4-30 prism D). No subject had severe amblyopia. Binocular vision was evaluated with stereoacuity tests. Two subjects had peripheral binocular vision and gross stereopsis; two other subjects had abnormal retinal correspondence and abnormal or pseudo gross stereopsis. In the other subjects binocular vision and stereopsis were absent. To stimulate disconjugate changes of saccades, subjects viewed for 20 min an image that was magnified in one eye (aniseikonia). Subjects with residual peripheral binocular vision and even subjects with pseudo or abnormal binocular vision showed disconjugate changes of the binocular coordination of their saccades; these changes reduced the disparity resulting from the aniseikonia. In contrast, for subjects without binocular vision the changes were not correlated with the disparity induced by the aniseikonia. Rather, these changes served to improve fixation of one or the other eye individually.

Adaptation, Physiological↗

PubMed1996

Fast disconjugate adaptations of saccades in microstrabismic subjects.

In normal subjects, saccade amplitude inequality can be induced almost immediately when the image is made larger for one eye. This disconjugacy allows binocular fusion at the point of regard despite the image size inequality. It persists under subsequent monocular viewing which suggests a fast adaptive mechanism. This study tests whether such disconjugacy can be induced in subjects with microstrabismus who do not have foveal fusion. Three microstrabismic subjects viewed a random dot pattern the size of which was 10% larger in one eye. Within 40 sec horizontal saccades became larger in the eye viewing the larger image by 4-10%; the induced disconjugacy persisted under subsequent monocular viewing. Thus, fast disconjugate adaptation is possible in microstrabismus demonstrating that foveal fusion is not necessary to achieve it.

Adaptation, Ocular↗

PubMed1996

Disconjugate adaptation of the vertical oculomotor system.

Conjugate post-saccadic eye drift can be induced in normal humans if a visual pattern is made to drift after every saccade. This study examines the ability of normal humans to create disconjugate vertical post-saccadic drift. Identical fuseable patterns were presented dichoptically, one to each eye. At the end of each vertical saccade one pattern drifted up and the other down, by 5% of the saccade amplitude. Five subjects were trained for 2-3 hr. Eye movements were recorded with eye coils. Normal vertical saccades along the midline were remarkably conjugate and post-saccadic drift was minimal. Training produced only small disconjugate post-saccadic drift (0.14 deg) but substantial saccade amplitude disconjugacy (0.70 deg). For several subjects, the induced disconjugacies persisted even for saccades in the dark indicating that adaptive changes occurred in the binocular coordination of vertical saccades. Apparently vertical disparate post-saccadic retinal slip is not sufficient to stimulate significantly the saccade pulse-step matching mechanism which is believed to control post-saccadic eye drift. The changes we observed aimed to reduce position disparity and not retinal slip in each eye.

Adaptation, Ocular↗

PubMed1995

Position dependency of rapidly induced saccade disconjugacy.

We tested the ability of normal subjects to alter the conjugacy of their saccades in a position-specific manner. Five subjects dichoptically viewed a stereogram produced by two random-dot patterns. They immediately perceived a three-dimensional wedge with its apex closer to them. They were asked to saccade for 15 min back and forth between the apex and two lateral dots of the wedge. For fixation sequences between centre-right-centre, saccades immediately became larger in the right eye. For sequences between centre-left-centre, saccades immediately became larger in the left eye. For two subjects this non-monotonic position-specific disconjugacy compensated for the disparity of the stereogram almost perfectly. The disconjugacy persisted even under monocular viewing of one of the random-dot patterns. It diminished or disappeared immediately, however, when the random-dot pattern was shifted on the screen. We suggest the existence of a fast learning mechanism capable of producing position-specific disconjugacy by associating saccades with disparity. Such a mechanism would use a visual reference rather than the position of the eyes in the orbit.

Adult↗

PubMed1995

Immediate saccade amplitude disconjugacy induced by unequal images.

We tested the ability of normal subjects to make changes in the conjugacy of their saccades. Subjects dichoptically viewed a grid the size of which was 10% larger in one eye. The grids were centred onto a flat screen at 57 cm or 1 m from the subject. Horizontal saccades immediately became larger in the eye viewing the larger grid. For some subjects this disconjugacy persisted even under subsequent monocular viewing. Such persistent changes occurred mainly in the field where the required disconjugacy was divergent for centrifugal saccades, convergent for centripetal saccades. Vertical saccades also developed compensatory disconjugacy; its amplitude was smaller but less variable. To explain these results we propose a fast associative learning mechanism that pairs peripheral disparity with saccades and is capable of producing saccade disconjugacy even in the absence of disparity. For horizontal saccades a secondary conditioning of monocular depth cues by the disparity would also be involved.

Adult↗

PubMed1993

Visually induced cross-axis postsaccadic eye drift.

1. It has been previously shown that, if a visual pattern is transiently moved just after every saccade, it is possible to induce horizontal, postsaccadic, ocular drift after horizontal saccades that persists in the dark. In this study we show that horizontal ocular drift can also be created after vertical saccades. Five human subjects viewed binocularly the interior of a full-field hemisphere filled with a random-dot pattern. They were encouraged to make frequent vertical saccades. During training, eye movements were recorded by the electrooculogram. A computer detected the end of every saccade and immediately moved the pattern to the left after up saccades and right after down saccades. The motion was exponential, its amplitude was 25% of the vertical component of the antecedent saccade, its time constant was 50 ms. Before and after 2-3 h of training, movements of both eyes were measured by the eye-coil/magnetic-field method while subjects were instructed to make vertical saccades in the dark, in the presence of the movable adapting pattern, and between stationary targets for calibration. 2. After training (approximately 20,000 saccades) all subjects developed a zero-latency, exponential ocular drift to the left after up saccades and to the right after down saccades. The amplitude of the horizontal drift, expressed as a percentage of the vertical component of the preceding saccade, was 2.7% in the dark. This rose to 10.2% in the presence of the movable adapting stimulus. The latter rise is not due to visual following systems but to a zero-latency increase in initial drift velocity. 3. The horizontal drifts were usually unequal between the two eyes, indicating the presence of disconjugate movements. We measured intrasaccadic disconjugate horizontal movements of all subjects. In agreement with studies by others of saccades in the light, we measured a divergence during up saccades (1.3 degrees) and a convergence for down (0.4 degrees), but in this case for spontaneous saccades in the dark. After training, these values increased for saccades in the dark but decreased in the light in the presence of the adapting stimulus. These changes were largely idiosyncratic and statistically significant in only a few subjects. 4. The cross-axis postsaccadic drifts were separated into their conjugate and disconjugate components. The disconjugate components were small and idiosyncratic, and the means were small for saccades in the dark. The only consistent trend was in the presence of the adapting stimulus where up saccades were often followed by convergence.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Physiological↗