Biomedical subjects

J L Demer

Publications and source records attributed to J L Demer.

94 records · Page 6Linked to original sources

PubMedDate not supplied

Visual-vestibular interaction with telescopic spectacles.

Vestibularly and visually driven eye movements interact to compensate for head movements to maintain the necessary retinal image stability for clear vision. The wearing of highly magnifying telescopic spectacles requires that such compensatory visual-vestibular interaction operate in a quantitative regime much more demanding than that normally encountered. We employed electro-oculography to investigate the effect of wearing of 2x, 4x, and 6x binocular telescopic spectacles on visual-vestibular interactions during sinusoidal head rotation in 43 normal subjects. All telescopic spectacle powers produced a large, immediate increase in the gain (eye velocity/head velocity) of compensatory eye movements, called the visual-vestibulo-ocular reflex (VVOR). However, the amount of VVOR gain augmentation became limited as spectacle magnification and the amplitude of head velocity increased. Optokinetic responses during wearing of telescopic spectacles exhibited a similar nonlinearity with respect to stimulus amplitude and spectacle magnification. Computer simulation was used to demonstrate that the nonlinear response of the VVOR with telescopic spectacles is a result of nonlinearities in visually guided tracking movements. Immediate augmentation of VVOR gain by telescopic spectacles declined significantly with increasing age in the subject pool studied. Presentation of unmagnified visual field peripheral to the telescopic spectacles reduced the immediate VVOR gain-enhancing effect of central magnified vision. These results imply that the VVOR may not be adequate to maintain retinal image stability during head movements when strongly magnifying telescopic spectacles are worn.

Adaptation, Ocular↗

PubMedDate not supplied

Effect of aging on vertical visual tracking and visual-vestibular interaction.

We investigated the effect of aging on vertical smooth pursuit, small-field optokinetic nystagmus (OKN), the vestibulo-ocular reflex (VOR), and visual-vestibular interactions (VVI). Telescopic spectacles were used to magnify the visual effects of head movements and accentuate the demand on VVI. Eleven normally sighted young subjects (age 30 +/- 6 y, mean +/- SD), and 9 normally sighted elderly subjects (age 70 +/- 8 y) were studied. Three types of stimulus motion were used: 1) predictable single sinusoidal frequencies from 0.8 to 3.2 Hz, 2) poorly predictable motion consisting of a sum of multiple harmonics of a 0.4 Hz fundamental, and 3) unpredictable velocity impulses. Tracking gains declined with frequency and stimulus unpredictability but were always greater for young than elderly subjects. Pursuit gain was always less than OKN gain for subjects tested under comparable conditions of frequency and velocity. In both subject groups, pursuit and OKN tracking exhibited phase lags at higher frequencies, particularly for poor predictable motion. Tracking phase lags were greater for pursuit than OKN and were consistently greater in the elderly than in young subjects. In young subjects, stimulus dimming using a 2 log neutral density filter reduced tracking gains and increased phase lags. In both young and elderly subjects, VOR gain was < 1.0 at low frequencies and increased slightly with increasing frequency. Visually enhanced VOR gain during normal vision was the ideal value of 1.0 at all frequencies in both subject groups. During wearing of telescopic spectacles, the young subjects exhibited marked gain enhancement that was greatest at lower frequencies and greater during predictable than unpredictable motion. Young subjects were able to suppress the VOR by fixation of real or imaginary targets moving with the head; this suppression was better at the lowest frequencies and for predictable motion. Phase errors were substantial for significant VVI in young subjects but were minimal in the elderly. Visual enhancement of VOR gain, as well as suppression by real and imaginary head-fixed targets, were attenuated in the elderly subjects. During rotations in which young subjects wore 4x telescopic spectacles, 2 log unit dimming of the visual environment reduced gain and increased phase lags. The deficiencies in VVI exhibited by elderly subjects are attributable both to deficits in the visual tracking component, as well as to nonvisual mechanisms, because even changes in mental set are less effective in modifying gain in older subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMedDate not supplied

The linear vestibulo-ocular reflex in normal subjects and patients with vestibular and cerebellar lesions.

We measured the horizontal linear vestibulo-ocular reflex (LVOR) in normal human subjects and patients with abnormal angular vestibulo-ocular reflexes (AVOR) and abnormal smooth pursuit. Eye movements were induced by sinusoidal linear acceleration along the interaural axis (0.8 Hz, 0.5 g peak acceleration) on a parallel swing. Horizontal movement of each eye was recorded with an infrared limbus tracking device. Normal subjects increased LVOR sensitivity as real or imagined targets moved closer. Perceived target distance was more important than the vergence angle since changing the vergence angle alone with prisms resulted in only a slight change in LVOR sensitivity. Subjects suppressed the LVOR with real or imagined head-fixed targets. Patients with decreased horizontal AVOR responses had decreased horizontal LVOR responses with imagined earth-fixed targets in the dark. They were able to generate normal velocity LVOR responses with real earth-fixed targets. Patients with increased AVOR responses and poor smooth pursuit due to cerebellar atrophy had low LVOR responses that were minimally affected by real or imagined earth-fixed or head-fixed targets. We conclude that the smooth pursuit system and the cerebellum are critical for generating the eye movements required as subjects fixate a near target during translational head movements.

Adult↗

PubMedDate not supplied

Visual-vestibular interaction during standing, walking, and running.

In artificial laboratory situations where subjects undergo repetitive self-generated or externally imposed head rotations, visual-vestibular interaction during the wearing of telescopic spectacles can markedly augment gain of the vestibulo-ocular reflex (VOR). The present study was conducted to determine whether the wearing of these aids for the visually impaired is associated with similar visual-vestibular interaction during more natural activities. Angular eye and head movements of unrestrained normal volunteers were measured using magnetic search coils. In some subjects, head translations and rotations were also monitored by a flux gate magnetometer array. Measurements were performed of the VOR in darkness, and of the visually enhanced VOR (VVOR) in lit conditions, during three natural activities: 1) standing quietly; 2) walking in place; 3) running in place. These data were compared with similar measurements during repetitive voluntary head oscillations at 0.8 Hz in pitch or yaw. During VVOR, subjects viewed a target placed 6 to 10 m away and remembered this target during VOR trials in darkness. To assess the effects of altering visual-vestibular interactions, VVOR testing during normal vision was augmented by wearing of binocular telescopic spectacles of 2X, 4X, and 6X powers. Dorsoventral and mediolateral head translations were consistently phase-locked with pitch and yaw head rotations, respectively, such that head translation at least partially compensated for rotational disturbances of gaze. Angular velocity of the head was greater during walking than during standing, and was greater still during running, with a greater increase in each case for pitch as compared with yaw. Eye movements were phase compensatory for head movements. VOR gain (eye velocity divided by head velocity) was near 1.0 in both pitch and yaw during standing and during actively generated head rotation. During walking and running there was a significant decrease in angular VOR gain in pitch to approximately 0.75 (P < 0.0005). During ambulatory activities, normal and magnified vision were associated with VVOR gain enhancement in pitch and yaw that was statistically significant, but substantially less than was telescope magnification and markedly lower than was the corresponding VVOR gain measured during active head rotation. Measurements of unmagnified VOR and VVOR during walking and running showed that gain was lower than the "ideal" value of 1.0. However, since translational head perturbations during these activities partially offset the visual effects of angular disturbances, lower gains may nevertheless be associated with retinal image stability at typical indoor target distances. In contrast with performance during repetitive, uniplanar motion, vision has very limited influence on VOR gain during natural activities.

Adult↗