1,721,026 research outputs found
The translational vestibulo ocular reflex (TVOR) in humans: responses to abrupt, high-acceleration stimuli
No full text
Ocular motor responses to abrupt interaural head translation in normal humans
No full textWe characterized the interaural translational vestibulo-ocular reflex (tVOR) in 6 normal humans to brief (∼200 ms), high-acceleration (0.4-1.4g) stimuli, while they fixed targets at 15 or 30 cm. The latency was 19 ± 5 ms at 15-cm and 20 ± 12 ms at 30-cm viewing. The gain was quantified using the ratio of actual to ideal behavior. The median position gain (at time of peak head velocity) was 0.38 and 0.37, and the median velocity gain, 0.52 and 0.62, at 15- and 30-cm viewing, respectively. These results suggest the tVOR scales proportionally at these viewing distances. Likewise, at both viewing distances, peak eye velocity scaled linearly with peak head velocity and gain was independent of peak head acceleration. A saccade commonly occurred in the compensatory direction, with a greater latency (165 vs. 145 ms) and lesser amplitude (1.8 vs. 3.2 deg) at 30- than 15-cm viewing. Even with saccades, the overall gain at the end of head movement was still considerably undercompensatory (medians 0.68 and 0.77 at 15- and 30-cm viewing). Monocular viewing was also assessed at 15-cm viewing. In 4 of 6 subjects, gains were the same as during binocular viewing and scaled closely with vergence angle. In sum the low tVOR gain and scaling of the response with viewing distance and head velocity extend previous results to higher acceleration stimuli. tVOR latency (∼20 ms) was lower than previously reported. Saccades are an integral part of the tVOR, and also scale with viewing distance
Human responses to vestibular and visual stimuli moving in depth: similarities and differences
No full textBinocular coordination in fore/aft motion
No full textStabilization of images on the fovea during either fore/aft translation of a subject or fore/aft movement of a visual target in front of a stationary observer imposes complex geometrical requirements that depend upon the eccentricity of the object of interest with respect to the eyes. Each eye needs to be rotated independently with varying proportions of conjugate (version) and disconjugate (vergence) eye movements to maintain fixation of the target. Here, we describe binocular coordination in the early response to translational movements of normal subjects along their naso-occipital axis. We recorded the responses evoked by small (about 4 cm), abrupt (about 0.7 g), fore/aft translations in four normal subjects while they viewed a near target. In the for-ward and backward starting positions the target was 15 or 10.5 cm away, respectively. Each subject was tested with the target centered between the eyes, aligned on the right eye, and placed to the right of the right eye by ∼3 cm. The three conditions differed only in the lateral eccentricity of the target, yet the geometrical requirements for image stabilization are very different: pure vergence, one eye still, or mostly version. We found that the eye-movement responses closely matched what was needed for visual stabilization of the target, though responses to stimuli calling for divergence were less accurate than those for convergence. The latency of these responses ranged from 40 to 65 ms and achieved about 80% of the ideal response by 90 to 100 ms after the onset of the stimulus. Next, we asked whether these eye movements were generated by the vestibular system or by high-level strategies for image stabilization, such as pursuit. Thus, in a second set of experiments we used the mean profile of fore\aft body motion computed for each subject to drive a small visual target across the same distances and in the same eccentricities used during body translations. We found that visually driven responses had longer latencies (by at least 80 ms, ranging from 144 to 155 ms) and slower dynamics (with significantly lower peak eye velocities), highlighting the different subsystems producing the two types of responses. Saccades were also an important component of the response to both visual and vestibular stimuli, less frequent during the centered-target configuration and more frequent during viewing of eccentric targets. Visual stimuli evoked saccadic corrections more often and at shorter latencies than did vestibular stimuli. Both smooth and saccadic eye movements were appropriately disconjugate and their pattern depended on whether the eyes were converging or diverging. © 2005 New York Academy of Sciences
Corrective saccades in acute vestibular neuritis: studying the role of prediction using automated passively-induced head impulses.
Full text linkWhen the demands for visual stabilization during head rotations overwhelm the ability of the vestibulo-ocular reflex (VOR) to produce compensatory eye movements, the brain produces corrective saccades that bring gaze towards the fixation target, even without visual cues (covert saccades). What triggers covert saccades and what might be the role of prediction in their generation are unknown. We studied 14 subjects with acute vestibular neuritis. To minimize variability of the stimulus, head impulses were imposed using a motorized torque generator with the subject on a bite-bar. Predictable and unpredictable (timing, amplitude, direction) stimuli were compared. Distributions of covert corrective saccade latencies were analyzed with a 'LATER' (linear approach to threshold with ergodic rate) approach. On the affected side, VOR gain was higher (0.47±0.28 vs. 0.39±0.22, p<<0.001) with predictable than unpredictable head impulses, and gaze error at the end of the head movement was less (5.4±3.3 vs 6.9±3.3 deg p<<0.001). Analyzing trials with covert saccades, gaze error at saccade end was significantly less with predictable than unpredictable head impulses (4.2 ±2.8 vs 5.5 ±3.2 deg, p<<0.001). Furthermore, covert corrective saccades occurred earlier with predictable than unpredictable head impulses (140±37 vs. 153±37 ms p<<0.001). Using a LATER analysis with reciprobit plots, we were able to divide covert corrective saccades into two classes - early and late - with a break point in the range of 88-98ms. We hypothesized two rise-to-threshold decision mechanisms for triggering early and late covert corrective saccades, with the first being most engaged when stimuli are predictable
- …
