Plantar cutaneous afferents influence the perception of Subjective Visual Vertical in quiet stance.

The estimation of Subjective Visual Vertical (SVV) involves the allocentric, gravitational and egocentric references, which are built by visual, vestibular and somatosensory afferents. Our goals were to assess the influence of plantar cutaneous afferents on the perception of SVV, and to see if there is a difference according to the efficiency of plantar cutaneous afferents. We recruited 48 young and healthy subjects and assessed their SVV and postural performances in quiet stance with a force platform, at 40 or 200 cm, in four ground conditions: on firm ground, on foam, with a bilateral, or with a unilateral 3 mm arch support. We also assessed the efficiency of our subjects' plantar afferents with the plantar quotient method and divided them in two groups: subjects with a normal use of plantar afferents and subjects with Plantar Exteroceptive Inefficiency (PEI). The results showed significant decreases in the counter clockwise SVV deviation only with the unilateral arch support, at near distance, and among the typically behaving subjects. We conclude that asymmetric foot cutaneous afferents are able to bias the egocentric vertical reference and hence influence the perception of SVV. This influence disappears among subjects with PEI, probably because of a distortion of the plantar signal.

Controlling Posture and Vergence Eye Movements in Quiet Stance: Effects of Thin Plantar Inserts.

The purpose of this study was to assess properties of vergence and saccade eye movements as well as posture in quiet stance, and the effects of thin plantar inserts upon postural and oculomotor control. The performances of 36 young healthy subjects were recorded by a force platform and an eye tracker in three testing conditions: without plantar stimulation, with a 3 millimetre-thick plantar insert, either a Medial or a Lateral Arch Support (MAS/LAS). The results showed a decrease of the Surface and Variance of Speed and a more posterior position of the CoP with either stimulation compared with the control condition. The fractal analysis showed a decrease with MAS. Wavelet analysis in the time-frequency domain revealed an increase in the Cancelling Time of the low frequency band with MAS. These results suggest a better stability for a lower energy cost. Concerning eye movements, the inserts influenced only vergence (not saccades): MAS caused an increase of the phasic amplitude of divergence, and conversely a decrease of the tonic amplitude. In contrast, LAS caused an increase of the tonic amplitude of convergence. Thus, MAS renders divergence less visually driven, while LAS renders convergence more visually driven. We conclude that the CNS uses the podal signal for both postural and vergence control via specific mechanisms. Plantar inserts have an influence upon posture and vergence movements in a different way according to the part of the foot sole being stimulated. These results can be useful to clinicians interested in foot or eye.

Eye movement abnormalities in somatic tinnitus: fixation, smooth pursuit and optokinetic nystagmus.

OBJECTIVE: Smooth pursuit (SP), optokinetic nystagmus (OKN) and fixation were investigated in five subjects with somatic tinnitus modulated by eye movements, jaw or neck. METHODS: Eye movements were recorded with the EyeLink II video system. RESULTS: (1) Fixation was characterized by high frequency and amplitude of saccade intrusions; (2) SP had low gain particularly in the vertical direction, and it was characterized by high frequency of catch-up saccades with high amplitude, including predictive saccades; (3) OKN also had low gain particularly in the vertical direction. Each subject showed abnormality for more than one type of eye movement, and for specific directions. CONCLUSIONS AND SIGNIFICANCE: The results suggest mild dysfunction of cortical-subcortical and cerebellar structures involved in the control of these eye movements. Particularly deficits for vertical pursuit eye movements and fixation instability in line with cerebellar signs. Further studies of more patients with or without modulated tinnitus are in progress.

Rare express saccades in elderly fallers.

OBJECTIVE: To examine horizontal saccades in elderly subjects with falling history; prior extensive screening was done to recruit subjects with falling history in the absence of pathology. METHODS: Twelve elderly with falling history were tested. Two testing conditions were used: the gap (fixation target extinguishes prior to target onset) and the overlap (fixation stays on after target onset) paradigms. Each condition was run at three viewing distances--20 cm, 40 cm, and 150 cm, corresponding to convergence angle at 17.1 degrees, 8.6 degrees, and 2.3 degrees, respectively. Eye movements were recorded with the photoelectric IRIS (Skalar medical). RESULTS: (i) like in healthy elderly subjects, elderly with falling history produce shorter latencies in the gap paradigm than in the overlap paradigm; (ii) their latencies are shorter at near distances (20 and 40 cm) relative to 150 cm for both paradigms; (iii) the novel result is that they fail to produce express latencies even in the conditions (near viewing distance and the gap task) known to promote high rates of express in adults (25%) or in healthy elderly (20%). Seven from the 10 healthy elderly produced express saccades at rates >12%, while 9 of the 12 older subjects with falling history showed no express saccades at all; the remaining 3 subjects showed low rates <12%. CONCLUSION: The quasi paucity of express saccades could be due to the disequilibrium of complex cortical/subcortical networks needed for making express saccades. The results support models suggesting specific network for express saccades; missing of such optomotor reflex may go along with missing other reflexes as well increasing the chances of falling.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.