Zoï Kapoula

Effects of distance and gaze position on postural stability in young and old subjects.

Visual stabilization of posture is known to improve when the distance to target fixation decreases; this is attributed to increased angular size of retinal slip induced by body sway. At near distance, however, the eyes converge and efferent or afferent oculomotor signals could also be involved in posture stabilization. The goal of this study is to test whether the distance effect exists for both young and elderly and to test the role of vergence itself and of gaze position. Eighteen young (25.3xa0years) and 17 elderly (61.6 years) subjects were asked to fixate a target in quiet stance presented either at close (40xa0cm) or at far distance (200xa0cm); the vergence angle was 9° and 2°, respectively. For each distance, three gaze positions were studied straight-ahead (0°), 15° up or down. We found a decrease in the surface of center of pressure (CoP), of standard deviation of antero-posterior and lateral body sway and of speed variance at near distance that occurs for both young and elderly. At far distance, the surface of CoP is smaller for 15° up or down gaze in comparison with straight-ahead position, but at near distance there is no such gaze position effect. In an additional experiment, subjects fixated a target at far distance (200xa0cm) but prisms were used to cause the eyes to converge by an amount similar to that required for 40xa0cm viewing distance. The use of prisms decreased surface of CoP to values similar to those for natural near viewing distance. The effect of gaze position and of convergence (experiment with prisms) leads us to suggest that in addition to retinal slip, the ocular motor signals and perhaps related neck muscle activity are involved in postural stabilization. Finally, the elderly presented higher speed variance of CoP than the young subjects even though the surface itself was similar to adult values. We suggest that increment of speed variance is the first sign of senescence in postural control.

Binocular coordination of saccades at far and at near in children and in adults

The goal of the study was to test the quality of binocular coordination of saccades in children and adults, and its dependency upon the viewing distance. Fourteen normal children (4.5-12 years old) and 10 normal adults (22-44 years old) participated. Two distances were studied: far (150 cm) and close (20 cm). Horizontal saccades from both eyes were recorded simultaneously by a photoelectric device. The results show (i) poor binocular coordination of saccades in children is distance dependent: coordination is particularly poor at near and could compromise single binocular vision; and (ii) post-saccadic drift analysis indicates that stability of the eyes after the saccade as well as the quality of binocular alignment during fixation are poor in children, regardless of the viewing distance. The results provide a reference for the normal development of binocular motor control and have important implications for reading in young children.

Eye movements in reading isolated words: evidence for strong biases towards the center of the screen.

Three experiments were conducted that compared the eye movement pattern to a peripheral word or letter string as a function of the position of an initial fixation stimulus relative to the center of the screen and the straight-ahead position. Results revealed a strong bias of the eye behavior towards the center of the screen, but not towards the straight-ahead position. Saccades were greater in length, and landed closer to the center of words/strings when launched from a position left of center than when launched from either center or right part of the screen. In addition, the initial saccade launch site was deviated to the right, or to the left of the initial fixation stimulus depending on where relative to the center of the screen the fixation stimulus was displayed. Data were interpreted with the assumption that saccades are programmed in a visual reference framework, with saccade amplitude being computed in relative coordinates. Further research will determine whether the observed bias generalizes to text reading.

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

Abstractu2002Earlier 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.

Role of the Posterior Parietal Cortex in the Initiation of Saccades and Vergence: Right/Left Functional Asymmetry

Abstract: 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.

Abnormality of vergence latency in children with vertigo.

Abstract.It is well known that vergence movements are important forndistance appreciation, depth vision and stereopsis. Moreover,nvergence movements are very probably used by the CNS during headnand body motion to adjust the gain of the vestibulo-ocularnreflex (VOR) according to the viewing distance. A recentnclinical study of Anoh-Tanon et al. suggested that vertigo innchildren with normal vestibular function could be associatednwith abnormal vergence clinically assessed. The purpose of thisnstudy was to test this hypothesis with objective vergence eyenmovement recordings. We examined the latency of vergence,nsaccades and combined movements in twelve children with thencomplaint of vertigo but without vestibular abnormality.nConvergence and saccades combined with convergence or withndivergence had abnormally long latencies (relative to normalnchildren of matched age). In contrast, divergence and isolatednsaccades showed only mild latency increase relative to normals.nLengthening of latency could be due to impaired corticalncontrol. Orthoptic vergence training reduced all latencies;nhowever, even the reduced latency of vergence and of combinednmovements was still abnormal. The improvement after orthopticnvergence training could be due to increased visual attention,nalthough such mechanism cannot eliminate completely theninitiation deficit of vergence movements. Objective eye movementnrecordings are thus useful for a diagnosis and treatment ofnchildren with vertigo.

Speed-accuracy of saccades, vergence and combined eye movements in children with vertigo

Vergence abnormalities could lead to inappropriate vestibulo-ocular reflex (VOR), causing vertigo and imbalance (Brandt 1999). Indeed, a recent study by Anoh-Tanon et al. (2000) reported the existence of a population of children with symptoms of vertigo in the absence of vestibular dysfunction but with abnormal vergence findings in orthoptic tests. The purpose of this study was to examine in such children the accuracy, duration and mean velocity of vergence and saccades; additionally, for a few subjects, the effect of orthoptic vergence training on these parameters was also investigated. LEDs were used to stimulate saccades, pure vergence along the median plane and combined saccade-vergence movements. Movements from both eyes were recorded with a photoelectric device (Bouis). The results show that children with vertigo perform saccades as normal subjects of comparable age. In contrast, vergence, particularly convergence, shows abnormalities: poor accuracy, long duration and low speed. During combined movements, the well known reciprocal interaction between the saccade and the vergence is present only for saccades combined with divergence; for saccades combined with convergence such interaction is abnormal: the saccade is slowed down by the convergence but the convergence is not accelerated by the saccade. Orthoptic training improves significantly the accuracy of all eye movements; such improvement was significant for all types of eye movements except for divergence (pure and combined). Furthermore, convergence remains abnormal and the lack of acceleration by the saccade persists. These specific convergence deficits could be of both subcortical and cortical origin. Orthoptic training improves the accuracy presumably via visual attentional mechanisms, but cannot completely override deficits related to subcortical deficiencies.

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.

Distance impairs postural stability only under binocular viewing

Prior studies indicate that postural stability under binocular viewing is not better than under monocular viewing. This was tested at the distances of 145 cm [Fox, C.R. (1990). Some visual influences on human postural equilibrium: binocular versus monocular fixation. Perception and Psychophysics, 47 (5), 409-422] and 90 cm [Isotalo, E., Kapoula, Z., Feret, P.H., Gauchon, K., Zamfirescu, F., & Gagey, P.M. (2004). Monocular versus binocular vision in postural control. Auris Nasus Larynx, 31 (1), 11-17]. On the other hand, postural stability is known to decrease with distance increase. We re-examined the effect of binocular versus monocular viewing on postural stability at near and far distances (40 and 200 cm), and for both young (25.7+/-2.7 years), and old subjects (61.2+/-4.6 years). For both groups of subjects, proximity decreased the area of CoP, the standard deviation of antero-posterior sway (SDy) and the variance of speed. The group of elderly presented increased variance of speed at far distance in comparison with young subjects. The novel finding is the interaction between distance and viewing condition. Under binocular viewing, the area of CoP was significantly higher at far distance than at near; in contrast, monocular viewing produced similar CoP values at both distances. Increased instability at far distance when both eyes are viewing is attributed to decreased sensitivity to binocular disparity cues and to visual motion in depth resulting from body sway. Monocular viewing would provide similar stability at far and at near distance, because sensitivity to lateral visual motion, detected monocularly, decreases less with distance than sensitivity to binocularly detected motion in depth. Alternatively, such monocular viewing could increase subjects attention and lead to tighter postural control regardless of the distance.

EEG activity related to preparation and suppression of eye movements in three-dimensional space

Eight normal subjects made visually guided eye movements to four LED targets placed at two different distances (20 and 70xa0cm) and on either side (±10°) at 70xa0cm. Four types of eye movements were elicited: pure saccades, convergence, divergence, and combined (divergent saccades). EEG activity was recorded from 62 electrodes and was aligned to stimulus onset. A negativity peaked after 140xa0ms and was modulated according to the location of the stimulus in space and the type of movement prepared, mainly in central and posterior cortex. For saccade targets, we confirmed a stimulus-related negativity in the posterior and central cortical area, contralateral to target direction. For convergence and divergence targets, this negativity was bilaterally distributed; convergence targets activated a rather extended cortical network in the central and posterior area, while divergence targets activated a more confined posterior area, spreading ventrally from the occipital cortex. Cortical activity for combined targets was lateralised contralaterally to stimulus direction but its topography resembled more closely that after the divergence stimulus. When observers suppressed the relevant eye movement to the stimulus, EEG activity was enhanced on the right hemisphere, showing the more pronounced effect on the right occipital-temporal and central-parietal electrode sites.