Biomedical subjects

V E Das

Publications and source records attributed to V E Das.

22 records · Page 2Linked to original sources

PubMed1995

Modulation of high-frequency vestibuloocular reflex during visual tracking in humans.

1. Humans may visually track a moving object either when they are stationary or in motion. To investigate visual-vestibular interaction during both conditions, we compared horizontal smooth pursuit (SP) and active combined eye-head tracking (CEHT) of a target moving sinusoidally at 0.4 Hz in four normal subjects while the subjects were either stationary or vibrated in yaw at 2.8 Hz. We also measured the visually enhanced vestibuloocular reflex (VVOR) during vibration in yaw at 2.8 Hz over a peak head velocity range of 5-40 degrees/s. 2. We found that the gain of the VVOR at 2.8 Hz increased in all four subjects as peak head velocity increased (P < 0.001), with minimal phase changes, such that mean retinal image slip was held below 5 degrees/s. However, no corresponding modulation in vestibuloocular reflex gain occurred with increasing peak head velocity during a control condition when subjects were rotated in darkness. 3. During both horizontal SP and CEHT, tracking gains were similar, and the mean slip speed of the target's image on the retina was held below 5.5 degrees/s whether subjects were stationary or being vibrated at 2.8 Hz. During both horizontal SP and CEHT of target motion at 0.4 Hz, while subjects were vibrated in yaw, VVOR gain for the 2.8-Hz head rotations was similar to or higher than that achieved during fixation of a stationary target. This is in contrast to the decrease of VVOR gain that is reported while stationary subjects perform CEHT.(ABSTRACT TRUNCATED AT 250 WORDS)

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PubMed1995

Convergent-divergent pendular nystagmus: possible role of the vergence system.

We used the magnetic search coil technique to measure horizontal, vertical, and torsional components of convergent-divergent pendular nystagmus in three patients. All showed phase shifts of approximately 180 degrees between the two eyes in the horizontal and torsional planes, but the vertical components were conjugate. Viewing a near target increased the oscillations threefold in one patient and by 60% in a second patient. The waveform was sinusoidal in one patient, but in the other two it was complex, resembling either a sum of several sine waves or a cycloid. When the predominant frequency of the nystagmus was low (1.8 Hz), oscillation of visually mediated vergence might have been responsible; when the frequency was high (6 Hz), the nystagmus might have arisen from an internal instability in connections between nucleus reticularis tegmenti pontis and cerebellar nucleus interpositus, which are important for vergence control.

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PubMed1995

Head perturbations during walking while viewing a head-fixed target.

BACKGROUND: Inexpensive, head-fixed computer displays are now available that subjects can wear during locomotion. HYPOTHESIS: Viewing a head-fixed visual display will change the characteristics of rotational head perturbations during natural walking. METHODS: Using a 3-axis angular rate sensor, we measured head rotations during natural or treadmill walking, in 10 normal subjects and 2 patients with deficient vestibular function, as they attempted to view (A) a stationary target at optical infinity; and (B) a target at a distance of 20 cm rigidly attached to the head. RESULTS: Normal subjects and patients showed no significant change in the predominant frequency of head rotations in any plane (ranging 0.7-5.7 Hz) during the two different viewing tasks (p > 0.1). Mean peak head velocities (ranging 6-36 degrees.s-1) also showed no difference during the two viewing conditions except in the yaw plane, in which values were greater while viewing the near target (p < 0.005). Predominant frequencies of head rotations were similar in the pitch plane during natural or treadmill walking; however, peak velocities of pitch head rotations were substantially greater during natural walking (p < 0.05). One vestibular patient showed modest increases of head velocity during natural walking compared with normal subjects. CONCLUSIONS: Rotational head perturbations that occur during natural walking are largely unaffected when subjects view a head-fixed target. There is need to study how such perturbations, which induce vestibular eye movements, affect vision of head-fixed displays.

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PubMedDate not supplied

Measuring eye movements during locomotion: filtering techniques for obtaining velocity signals from a video-based eye monitor.

Video-based eye-tracking systems are especially suited to studying eye movements during naturally occurring activities such as locomotion, but eye velocity records suffer from broad band noise that is not amenable to conventional filtering methods. We evaluated the effectiveness of combined median and moving-average filters by comparing prefiltered and postfiltered records made synchronously with a video eye-tracker and the magnetic search coil technique, which is relatively noise free. Root-mean-square noise was reduced by half, without distorting the eye velocity signal. To illustrate the practical use of this technique, we studied normal subjects and patients with deficient labyrinthine function and compared their ability to hold gaze on a visual target that moved with their heads (cancellation of the vestibulo-ocular reflex). Patients and normal subjects performed similarly during active head rotation but, during locomotion, patients held their eyes more steadily on the visual target than did subjects.

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