Biomedical subjects

Zoï Kapoula

Publications and source records attributed to Zoï Kapoula.

At least 19 recordsLinked to original sources

PubMed2006

Evidence for frequent divergence impairment in French dyslexic children: deficit of convergence relaxation or of divergence per se?

AIM: There is a controversy as to whether dyslexic children present visuo-motor disabilities such as vergence and accommodative problems assessed with orthoptic tests. The purpose of this study is to re-examine this issue in a large population of children. METHODS: Extensive orthoptic evaluation was made in 57 dyslexic and 46 non-dyslexic ("normal") age-matched children. Convergence and divergence capacities were evaluated at two distances (30 cm and 400 cm). RESULTS: Binocular vision measured with stereo-acuity tests was normal in dyslexics. In contrast, the near point of convergence was significantly more remote in dyslexics; most importantly, divergence at both far and near distance was significantly more reduced in dyslexics (median value 4 pD and 10 pD, respectively, at far and near) than in "normals" (median value 6 pD and 12 pD, at far and near). CONCLUSION: The existence of the divergence deficit at far distance indicates the presence of deficit of divergence per se, independently from convergence and accommodation relaxation. This result is novel and corroborated by physiological studies indicating distinct control of convergence and divergence, both at the cortical and subcortical premotor level. We conclude that vergence deficits are frequently present in dyslexics, and that dyslexics should be re-educated; training should address distinctively convergence and divergence subsystems.

Accommodation, Ocular↗

PubMed2006

Aging does not affect the accuracy of vertical saccades nor the quality of their binocular coordination: a study of a special elderly group.

Fine binocular coordination of vertical saccades is a complex process requiring appropriate distribution of innervations to all six extraocular muscles. Loss of such coordination causes vertical binocular disparities that are particularly bothersome. We studied the quality of binocular control of vertical saccades in healthy subjects, 11 young adults (20-28 years) and 11 elderly adults (63-75 years). We used LED targets at 7.5 degrees or 15 degrees from the center (fixation), up or down in four conditions: gap and overlap tasks, each done at two distances--near (40 cm) and far (150 cm). Vertical eye movements were recorded with video-oculography (CHRONOS). The results showed: aged subjects performed vertical saccades as accurately as young subjects. Importantly, the binocular coordination of vertical saccades was well preserved in the elderly; the mean difference of vertical saccades between the two eyes was 0.10 degrees and 0.09 degrees in young and elderly subjects, respectively. Upward saccades were associated with divergence, downward ones with convergence. This secondary phenomenon was also the same and of the similar amplitude for young (1.30 degrees ) and elderly (1.25 degrees) subjects. Thus, despite its complexity, the quality of binocular coordination of vertical saccades remains intact with age. The other observations are mostly dependent of several aspects on the direction (up/down), viewing distances and eccentricities; the horizontal vergence during or after vertical saccades was found to be larger for downward saccades than for upward saccades, for saccades at far distance than at close, and for the more eccentric targets (15 degrees versus 7.5 degrees). All these phenomena are the same for both young and elderly subjects. We conclude that the accuracy and the binocular coordination of vertical saccades, at least for target steps less than 15 degrees , are preserved in elderly subjects <75 years who maintain good physical and intellectual form. The data are consistent with the idea of the existence of non-aging system function in the human CNS.

Adult↗

PubMed2006

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 subject's attention and lead to tighter postural control regardless of the distance.

Adult↗

PubMed2006

Binocular coordination of saccades in children with vertigo: dependency on the vergence state.

The present study examines the quality of binocular coordination of saccades at far and near distance in 15 children with symptoms of vertigo headache and equilibrium disorders; these children show normal vestibular function but abnormal convergence eye movements (e.g., long time preparation, slow execution and poor accuracy, see ). The results show normal binocular saccade coordination at far distance, but large abnormal disconjugacy for saccades at near distance. During combined saccade-vergence movements (studied in six of these children), convergence remains abnormally slow. This supports the interpretation according to which poor binocular yoking of the saccades is linked to the reduced ability to produce fast convergence during the saccade; a learning mechanism based on rapid vergence would help to reduce the abducting-adducting asymmetry of the saccades. An alternative interpretation would be reduced learning ability for monocular adjustment of the saccade signals.

Adolescent↗

PubMed2006

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 (25.3 years) and 17 elderly (61.6 years) subjects were asked to fixate a target in quiet stance presented either at close (40 cm) or at far distance (200 cm); the vergence angle was 9 degrees and 2 degrees , respectively. For each distance, three gaze positions were studied straight-ahead (0 degrees ), 15 degrees 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 degrees 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 (200 cm) but prisms were used to cause the eyes to converge by an amount similar to that required for 40 cm 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.

Adaptation, Physiological↗

PubMed2005

The control of vertical saccades in aged subjects.

In real life we produce vertical saccades at different distances and eccentricities, and while our fixation is more or less actively engaged. The goal of this study is to examine vertical saccades in aged and young subjects, taking into consideration all these parameters. Eleven adults (20-28 years) and 11 aged subjects (63-83 years) were recruited. We used LED targets at 7.5 degrees or 15 degrees, up or down in four conditions: gap and overlap tasks, each done at two distances-at near (40 cm) and at far (150 cm). In the gap task fixation target extinguishes prior to target onset, while in the overlap condition it stays on after target onset; consequently, visual attention and fixation are employed differently in the two tasks. Eye movements were recorded with the Chronos video eye tracker. Results showed that vertical saccades were longer for aged subjects than for young adults under almost all conditions. For both aged and young subjects, latencies were shorter under the gap than under the overlap task. Latencies for eccentric targets at 15 degrees were significantly longer than those at 7.5 degrees but for aged subjects only; this effect was more pronounced for upward saccades under the overlap condition. Express type of latencies (80-120 ms) occurred frequently in the gap task and at similar rates for young adults (16%) and aged subjects (12%); in the overlap task express latencies were scarce in young adults (0.4%) and aged subjects (1.8%). Age deteriorates the ability to trigger regular volitional saccades but not the ability to produce express type of saccades. Latency increase with aging is attributed to the degeneration of central areas, e.g. oculomotor cortical areas involved in the initiation of vertical saccades.

Adult↗

PubMed2005

Binocular coordination of saccades in 7 years old children in single word reading and target fixation.

The purpose of the present study was to examine in 7 years old normal children who just learnt to read, saccade and fixation characteristics during reading single words. Eight children were studied and their results were compared to those of eight normal adults doing the same task. For each group word reading data were also compared with data in a task requiring saccades and fixations to target-LEDs. Horizontal saccades from both eyes were recorded with a photoelectric device (Oculometer, Bouis). Latencies of saccades both to words and to LEDs presented at predictable location were similar, and they were also similar between children and adults. In contrast, disconjugacy of saccades was significantly increased for children and similar in the two tasks (LEDs or words). Disconjugate post-saccadic drift and its velocity were also significantly higher in children and similar in the two tasks. Substantial conjugate leftward drift was present for children only, and for the word task only. Finally, fixation duration on words was significantly longer in children than in adults. We conclude that binocular coordination and fixation stability is poor in children and that it could be partially responsible for the long fixation duration. Binocular coordination does not depend on the task (LEDs or words) neither for adults, nor for children; this contrasts prior reports.

Adult↗

PubMed2005

Dorsolateral prefrontal cortex prevents short-latency saccade and vergence: a TMS study.

This study explores whether vergence eye movements along the median plane can be triggered with short latencies, and the role of the dorsolateral prefrontal cortex (DLPFC) in controlling such movements. We used a gap paradigm and applied transcranial magnetic stimulation (TMS) in 10 humans making saccades or vergence. TMS over the motor cortex had no effect on any eye movement parameter. TMS over DLPFC influenced eye movement initiation but not their metrics. TMS over the right DLPFC accelerated the triggering of saccades bilaterally but did not influence divergence. TMS over the left DLPFC speeded up the triggering of ipsilateral saccades and exacerbated the anticipatory mode of triggering of divergence. For convergence, TMS effects were mild: rightward TMS increased the proportion of short latencies but failed to shorten the group mean latency; leftward TMS influenced triggering in some individuals only. For saccades and convergence under TMS, some subjects showed an emerging population of short latencies in their latency distribution. Horizontal saccadic intrusions (80% of trials) and vertical saccades (recorded in one subject) intruding on vergence were unlikely to assist vergence triggering. We conclude that the prefrontal mechanisms underlying voluntary eye movement control are similar for saccades and vergence although some specificities exist.

Adult↗

PubMed2005

Influence of gap and overlap paradigms on saccade latencies and vergence eye movements in seven-year-old children.

The latency of eye movements is influenced by the fixation task; when the fixation stimulus is switched off before the target presentation (gap paradigm) the latency becomes short and express movements occur. In contrast, when the fixation stimulus remains on when the target appears (overlap paradigm), eye movement latency is longer. Several previous studies have shown increased rates of express saccades in children; however the presence of an express type of latency for vergence and combined movements in children has never been explored. The present study examines the effects of the gap and the overlap paradigms on horizontal saccades at far (150 cm) and at close (20 cm) viewing distances, on vergence along the median plane, and on saccades combined with convergence or divergence in 15 normal seven-year-old children. The results show that the gap paradigm produced shorter latency for all eye movements than the overlap paradigm, but the difference was only significant for saccades at close viewing distances, for divergence (pure and combined), and for saccades combined with vergence. The gap paradigm produced significantly higher rates of express latencies for saccades at close viewing distances, for divergence, and for saccades combined with divergence; in contrast, the frequencies of express latencies for saccades at far viewing distances and for convergence (pure or combined) were similar in the gap and the overlap paradigms. Interestingly, the rate of anticipatory latencies (<80 ms) was particularly high for divergence in the gap paradigm. Our collective findings suggest that the initiation of saccades at close viewing distances and of divergence is more reflexive, particularly in the gap paradigm. The finding of frequent anticipatory divergence that occurs at similar rates for seven-year-old children (this study) and for adults (Coubard et al., 2004, Exp Brain Res 154:368-381) indicates that predictive initiation of divergence is dominant.

Aging↗

PubMed2005

Inhibition of saccade and vergence eye movements in 3D space.

Inhibitory capacity was investigated by measuring the eye movements of normal subjects asked to fixate a central point, and to suppress eye movements toward visual distracters appearing in the periphery or in depth. Eight right-handed young adults performed such a suppression or distracter task. In different conditions, the distracter could appear at 10 degrees left or right at a distance of 20, 40, or 150 cm (calling for horizontal saccades), or in a central position far or close (calling for convergence or divergence), or 7.5 degrees up or down at 40 or 150 cm (calling for vertical saccades). Eye movements were recorded binocularly with an infrared light eye-movement device. Results showed that (1) suppression performance was not perfect, as the subjects still produced eye movements; (2) errors were distributed unequally in three-dimensional space, with more frequent errors toward distracters calling for convergence, or leftward and downward saccades at a close distance; (3) distracters calling for saccade suppression yielded saccades in the direction of the distracter (that we called prosaccades), and saccades directed away from it (that we called spontaneous antisaccades); (4) for vergence, only distracters calling for convergence yielded errors, which were always promovements; (5) in addition, a small convergent drift was found for convergence distracters. Differences in the errors between saccade and vergence suggest that different inhibitory mechanisms may be involved in the two systems. Spatial left/right, up/down, and close/far asymmetries are interpreted in terms of attentional biases.

Adult↗

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↗

PubMed2004

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.

Adolescent↗

PubMed2004

Saccade-vergence dynamics and interaction in children and in adults.

Peak velocity, duration and accuracy of eye movements (saccade, vergence and combined saccade-vergence eye movements) were investigated in fourteen normal children (4.5 to 12 years of age) and ten normal adults (22 to 44 years of age). Horizontal movements from both eyes were recorded simultaneously by the oculometer, a photoelectric device. Peak velocity of all eye movements, saccades, and vergence (convergence and divergence), attains adult levels by the age of 4.5 years and there is no significant change over the age range studied (4.5 to 44 years). Vergence duration is longer only in young children (below 8 years of age). The reciprocal interaction between saccade and vergence during combined movements known in adults, i.e. acceleration of the vergence by the saccade (increase of velocity and decrease of duration) and deceleration of the saccade by the vergence (decrease of velocity and increase of duration) was found to be similar in children. The accuracy of eye movements is good on average for both saccades and vergence by the age of 4.5 years, and does not change with age; an exception is the variability of saccade amplitude, which is higher in children less than 8 years old. Taken together, the results indicate early maturation of brainstem structures controlling spatio-temporal aspects of saccades, vergence and their interaction.

Adult↗

PubMed2004

Abnormality of vergence latency in children with vertigo.

It is well known that vergence movements are important for distance appreciation, depth vision and stereopsis. Moreover, vergence movements are very probably used by the CNS during head and body motion to adjust the gain of the vestibulo-ocular reflex (VOR) according to the viewing distance. A recent clinical study of Anoh-Tanon et al. suggested that vertigo in children with normal vestibular function could be associated with abnormal vergence clinically assessed. The purpose of this study was to test this hypothesis with objective vergence eye movement recordings. We examined the latency of vergence, saccades and combined movements in twelve children with the complaint of vertigo but without vestibular abnormality. Convergence and saccades combined with convergence or with divergence had abnormally long latencies (relative to normal children of matched age). In contrast, divergence and isolated saccades showed only mild latency increase relative to normals. Lengthening of latency could be due to impaired cortical control. Orthoptic vergence training reduced all latencies; however, even the reduced latency of vergence and of combined movements was still abnormal. The improvement after orthoptic vergence training could be due to increased visual attention, although such mechanism cannot eliminate completely the initiation deficit of vergence movements. Objective eye movement recordings are thus useful for a diagnosis and treatment of children with vertigo.

Adolescent↗

PubMed2004

Latency of saccades, vergence and combined saccade vergence movements in a subject with manifest latent nystagmus.

PURPOSE: To study eye movements in 3D space - saccades, vergence and combined eye movements - in a subject with Manifest Latent Nystagmus (MLN). METHODS: A 13-year-old girl diagnosed with MLN participated in this study. Saccades, pure vergence along the median plane and combined saccade-vergence movements were recorded under both binocular and monocular viewing. Horizontal eye movements from both eyes were recorded simultaneously with a photoelectric device (Bouis Oculometer). RESULTS: The recordings of saccades, vergence or both components of combined movements show that such movements have a staircase trajectory. The consequence of this staircase behavior is that the time for approaching the target is prolonged. The latency of all types of eye movements is extended when the subject is viewing binocularly, while latency values are normal when viewing monocularly; the difference between the viewing conditions is significant for vergence. CONCLUSION: The normal latency values under monocular viewing are attributed to facilitation of eye movement initiation by the increased nystagmus.

Adolescent↗

PubMed2004

TMS over the left posterior parietal cortex prolongs latency of contralateral saccades and convergence.

PURPOSE: This study explored the role of the left posterior parietal cortex (PPC) in saccades, vergence, and combined saccade-vergence movements by means of transcranial magnetic stimulation (TMS) in humans. METHODS: TMS was applied over the left PPC for 80, 90, or 100 ms after target onset. In a control experiment, TMS was applied over the primary motor cortex at 90 ms after target onset. RESULTS: Relative to no TMS trials, TMS over the left PPC increased the latency of only certain types of eye movements: saccades to right, convergence, and convergence combined with rightward saccades. It had no such effect on saccades to the left, divergence, saccades combined with divergence, or left saccades combined with convergence. TMS over the vertex had no effect on the latency of any type of eye movement. Thus, the effects of TMS on latency are both area- and eye-movement specific. TMS of the left PPC or the motor cortex did not alter the accuracy of any type of eye movement. The results are in contrast to those of prior studies of the right PPC, which is involved in the initiation of any type of eye movement. CONCLUSIONS: The findings suggest that the right PPC is involved primarily in the processing of fixation disengagement, whereas the left PPC provides the signal that is also necessary to initiate eye movements and could occur in parallel with the process (model of Findlay and Walker). Such a functional role of the left PPC seems to be topographically organized and concerns a saccade to the right and convergence alone or combined with a saccade to the right.

Adult↗