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Biomedical subjects

I Israël

Publications and source records attributed to I Israël.

31 records · Page 2Linked to original sources
PubMed1995

Cortical control of vestibular-guided saccades in man.

Memory-guided saccades, made to a remembered location to which gaze was directed before a passive body rotation (i.e. with a vestibular input), were electro-oculographically recorded in 24 patients with various cortical lesions and in 18 control subjects. Anticipation and latency, direction errors and accuracy of the first saccade, stability of eye position in darkness and final eye position were quantified. Patients were divided into small groups, each with lesions affecting one of the following cortical areas: left or right frontal eye field (FEF), left or right prefrontal cortex (area 46 of Brodmann) (PFC), left supplementary eye field (SEF), left or right posterior parietal cortex (PPC) and right parieto-temporal cortex (PTC). There were some abnormalities in the results of the right FEF group, concerning anticipation, direction errors and latency of the first saccade, but no abnormality in the accuracy of the first saccade or of the final eye position. Results in the left FEF group were normal. Accuracy of the first saccade was impaired in the SEF group, bilaterally. Final eye position was also inaccurate in the SEF group. In both PFC groups, significant and, in general, bilateral abnormalities existed for all tested parameters. Accuracy of the first saccade was impaired in the PTC group, leftwards. In contrast, the results in both PPC groups were not significantly different from those of control subjects. Our results suggest that (i) the PFC is involved in the memorization of saccade goals probably encoded in spatiotopic coordinates; (ii) the SEF, but not the FEF, is involved in the control of accuracy of these vestibular-derived goal-directed saccades; (iii) the PTC (i.e. the vestibular cortex), but not the PPC, is involved in the control of such saccades. Therefore, a cortical network different from that involved in the control of memory-guided saccades made to visual targets, with only the PFC in common, could control vestibular-derived goal-directed saccades.

Adult↗
PubMed1995

Otolithic thresholds influence the perception of passive linear displacement.

The vestibular sensors are the necessary source of information for estimating self-displacement during passive linear transport. Displacement has to be computed from the otolith signal by means of an integration process (path integration), which provides a measure of linear acceleration. However, the onset of self-motion perception is delayed due to perceptual thresholds. We investigated the effect of these thresholds on the estimation of passive displacement. Subjects seated on a linear acceleration device (ESA-SLED) were displaced to their left or right. Two tasks were performed: (1) subjects pressed a button when they perceived self-motion; (2) subjects pressed a button when they thought they had reached a previously seen visual target located above the sled rail. Displacement estimation (task 2) was found to depend upon the acceleration magnitude and on the individual motion perception threshold measured in task 1. The results can be explained by assuming that self-displacement computation starts at the onset of self-motion perception and is initialised by a value that is independent of the amplitude of acceleration, thereby compensating for the displacement information lost due to the thresholds.

Acceleration↗
PubMed1995

Self-rotation estimate about the vertical axis.

The aim of this paper was to examine the accuracy of passive whole-body self-rotation estimate around the earth-vertical (yaw) axis. Subjects were required to reach an imposed angle of +/- 90 degrees, 180 degrees or 360 degrees (outward way), and then to rotate back to the initial position (return way), through passive rotation controlled by the subject, in darkness. During these rotations, only the semi-circular canals were stimulated. On the outward way, subjects were required to elaborate the correct imposed angle, without any external reference. In average, subjects undershot the expected angles on outward way, which suggests an overestimate of self-rotation, and the error increased with increasing expected angle. On the return way, subjects had to rotate back to the starting point, through a path integration process. There was no effect of the imposed angle on the error, and the variability was lower than on the outward way. The data suggests that the subjects could construct the internal representation of a virtual starting point: the initial body position, which became a goal to reach clearer than the outward rotation angle, which the subjects also have defined, in an environment deprived of any external spatial reference.

Adult↗
PubMed1993

Vestibular perception of passive whole-body rotation about horizontal and vertical axes in humans: goal-directed vestibulo-ocular reflex and vestibular memory-contingent saccades.

This study was aimed at complementing the existing knowledge about vestibular perception of self-motion in humans. Both goal-directed vestibulo-ocular reflex and vestibular memory-contingent saccade (VMCS) tasks were used, respectively as concurrent and retrospective magnitude estimators for passive whole-body rotation. Rotations were applied about the earth-vertical and earth-horizontal axes to study the effect of the otolith signal in self-rotation evaluation, and both in yaw and pitch to examine the horizontal and vertical semi-circular canals. Two different magnitudes of constant angular acceleration (50 degrees/s2 and 100 degrees/s2) were used. The main findings were (1) strong correlation between both oculomotor responses of both tasks, (2) greater accuracy with rotations about the earth-vertical than the earth: -horizontal axis, (3) greater accuracy for yaw than for pitch rotations, (4) greater accuracy for high acceleration than for low, and (5) no effect of the delay (2 s or 12 s) in the VMCS task. Adequacy of both tasks as subjective magnitude estimators of vestibular perception of self-motion is discussed.

Adult↗
PubMed1993

Role of the different frontal lobe areas in the control of the horizontal component of memory-guided saccades in man.

Two paradigms of memory-guided saccades were studied in 14 patients with focal vascular lesions affecting either the frontal eye field (FEF), or the supplementary eye field (SEF) or Brodmann's area 46 in the prefrontal cortex (PFC), and in 13 age-matched control subjects. In the first paradigm, the subject had to remember the position of a visual target with the body immobile and, in the second, the position towards which gaze was directed before a body rotation, i.e. with a vestibular input. In control subjects, the percentage of error in saccade accuracy (horizontal component) was greater in the second than in the first paradigm (37% and 14% on average, respectively). Compared with controls, amplitude error was significantly increased in the FEF group for the first paradigm only, in the SEF group for the second paradigm only, and in the PFC group for both paradigms. These results are consistent with (1) the PFC providing an improvement in the utilization by the saccade system of the visual and vestibular signals used in the two paradigms, and (2) the FEF and SEF providing an improvement in the utilization of visual signals in the first paradigm and vestibular signals in the second paradigm, respectively. Furthermore, from these findings and experimental data, it may be hypothesized (1) that the PFC is a part of the network contributing to short-term memorization of both visual and vestibular signals, and (2) that the FEF and SEF control two different types of memory-guided saccades, with separate calculation modes to determine their amplitude.

Brain Mapping↗
PubMed1993

Estimation of passive horizontal linear whole-body displacement in humans.

1. Passive linear self-motion estimation along the X and Y axes was investigated in human subjects. 2. A target was viewed from a distance of 0.8 or 2.4 m from the starting point. Subjects were then blindfolded and transported toward the target on a distance of 3.2 m and back to the start. Acceleration was constant: +/- 0.2 or +/- 1 m/s2. The subjects pushed a button on both outward and return paths, when they passed the previously seen target. 3. The results showed anticipation of the target on the outward path, and not on the return. This was identical for both axes and both accelerations. 4. The data are in accord with a model of double integration of the otolith signal, suggesting that linear path integration is a basic sensory mechanism.

Acceleration↗
PubMed1993

Gaze control in microgravity. 1. Saccades, pursuit, eye-head coordination.

During the long-duration spaceflight Aragatz on board the Mir station, an experiment exploring the different oculomotor subsystems involved in gaze control during orientation to a fixed target or when tracking a moving target was executed by two cosmonauts. Gaze orientation: with head fixed, the "main sequence" relationships of primary horizontal saccades were modified, peak velocity was higher and saccade duration was shorter in flight than on earth, latency was decreased and saccade accuracy was better in flight. With head free, gaze orientation toward the target was achieved by coordinated eye and head movements, their timing was maintained in the horizontal plane; when gaze was stabilized on the target, there was a trend of a larger eye than head contribution not seen in preflight tests. Pursuit: Horizontal pursuit at 0.25 and 0.5 Hz frequency remained smooth with a 0.98 gain and minor phase lag, on earth and in flight. In the vertical plane, the eye did not track the target with a pure smooth pursuit eye movement, but the saccadic system contributed to gaze control. Upward tracking was mainly achieved with a succession of saccades, whereas downward tracking was due to combined smooth pursuit and catch-up saccades. This asymmetry was maintained during flight in head fixed and head free situations. On earth, head peak velocity was maximal upward, and in flight it was maximal downward.

Electrooculography↗
PubMed1993

Gaze control in microgravity. 2. Sequences of saccades toward memorized visual targets.

The reproduction, in complete darkness, of sequences of 5 horizontal saccades towards previously presented visual targets has been investigated in human subjects on the ground (control subjects) and one cosmonaut in microgravity. The incidence of corrective saccades during the execution of the memory-guided saccades in darkness has been examined. It was quite large for the control subjects (more than half of all saccades), and increased during the flight, while the corrective visually guided saccades incidence decreased. Direction errors occurred in about the third of all sequences on the ground, and this parameter also increased in microgravity. Memory-guided sequences were mostly hypermetric. Whereas the absolute error continuously increased with the target rank, it was not the case with the amplitude ratio, which presented a peak at the third rank, that is, at the middle of the sequence. The accuracy of the reproduction of the sequences did depend on the sequence pattern as much as on the subject. Some learning was observed in repeated reproduction of the same pattern. Although the average error did not change in microgravity, the linear regression coefficient between the visually guided and memory-guided saccades decreased.

Adult↗
PubMed1992

Memory-guided saccades: what is memorized?

In order to find out whether extraretinal (oculomotor, internal) input suffices to provide the oculomotor system with the information necessary for saccadic control, two subjects were asked to make memory-guided saccades in complete darkness, after three different location acquisition conditions. These conditions were visually-guided saccades (SA), providing retinal (external) and extraretinal input, visual peripheral target presentation during central target fixation (FI) (external input only), and smooth pursuit (PU) (internal input only). Either 2 or 12 s (delay) after locating the target, the subjects had to make a memory-guided saccade toward it in complete darkness. The results show that whereas these memory-guided saccades were quite accurate for trials with preceding external input, this was not the case with acquisition through internal input alone. Moreover, the accuracy of memory-guided saccades decreased when the delay increased from 2 to 12 s for both conditions with retinal input, whereas the accuracy increased for the one condition without retinal input, i.e., the smooth pursuit location acquisition. Furthermore, when both retinal and oculomotor inputs were provided, better accuracy of the memory-guided saccades was observed than with single input.

Adult↗
PubMed1989

Contribution of the otoliths to the calculation of linear displacement.

1. The present work is a quantitative study of the eye movements induced by linear translation when the subject is instructed to stabilize his gaze on a memorized earth-fixed target. These experiments may allow a better understanding of the central processing of otolithic signals. 2. Human subjects were submitted to either sinusoidal or step-like horizontal linear displacements along the interaural (Y)-axis in darkness, seated in a cart moving along a linear track. Each subject's head was fixed by a helmet secured to the cart. They were asked to keep their eyes on an earth-fixed memorized target at 63 cm from them on the X-axis. 3. During sinusoidal motion, a combination of low smooth compensatory eye movements and of compensatory saccades allowed the subjects to track the memorized target. The linear model of the responses of five subjects (seven sessions) exhibited a near-ideal slope of 1.14 (range 0.84-1.58). Two subjects did not compensate properly for their displacement. The mean "vestibular-saccadic" (VS) gain (ratio of overall eye movement peak-peak amplitude versus head displacement amplitude) was 1.52 +/- 0.80 (SD), showing an overestimation of head displacement. 4. The otolith-ocular reflex (OOR) mean gain values (ratio of slow phase cumulated peak-peak amplitude versus head displacement amplitude) were about 0.13 degrees/cm. This value is 5 times higher than what has been reported in the literature, probably due to the fact that the target was at a short distance. 5. The number of saccades occurring during sinusoidal stimulations varied according to the different subjects. They were obviously compensatory saccades and not quick phases. They indicate that although the gain of the OOR was small, the brain has computed the adequate desired eye position. 6. During steplike head displacements in darkness, although the OOR gain was also small, seven of the eight subjects could stabilize their gaze with a mean VS gain of 1.01 +/- 0.70. The linear model for the pooled responses of these subjects exhibited a slope of 0.82. 7. When subjects were instructed not to move their eyes during the translation, three of the five examined could still correctly reproduce the head movement amplitude with saccades, even as late as 50 s after motion had stopped. This indicates that head displacement was stored with the adequate metrics and could be used to drive the saccadic system. 8. Bilabyrinthectomized patients could not perform any adequate gaze stabilization. This shows that the observed performance was of vestibular origin.

Adult↗
PubMed1987

Linear head displacement measured by the otoliths can be reproduced through the saccadic system.

Normal subjects seated on a moving cart have been submitted to horizontal linear accelerations in the frontal plane. They were shown an earth-fixed target at a distance of 63 cm before the onset of cart motion, which occurred in total darkness. They were asked to keep their gaze on the imagined target. The main result is that, in spite of a low gain otolith-ocular reflex, they could reproduce their motion with great accuracy by saccades (a small overestimation was, however, observed). It is concluded that otolith information is used by probably cortical spatial analysers which can derive a precise measurement of linear displacement and feed this information in the saccadic system.

Acceleration↗