Effect of Telescopic Spectacles on Head Stability in Normal and Low Vision

Abstract

Telescopic spectacles, highly magnifying visual aids mounted in spectacle frames, markedly alter the visual consequences of head movements. To evaluate the effect of this altered visual feedback on head stability, angular head velocity of normally sighted and low vision subjects was measured in the roll, pitch, and yaw axes. Measurements were made under two postural conditions: (1) quiet standing; and (2) walking in place, as well as three visual conditions: (1) eyes closed; (2) unmagnified vision; and (3) vision with 4× binocular telescopic spectacles. For normal subjects during quiet standing, both unmagnified vision and vision with telescopic spectacles tended to reduce spontaneous head velocity in all axes as compared to the eyes-closed condition. However, in low vision subjects neither unmagnified vision nor vision with telescopic spectacles produced significant changes in values of head velocity relative to those measured with eyes closed. Spontaneous head velocities for standing low vision subjects tended to be higher than in normal subjects, although not all differences were statistically significant. During walking in place, Fourier analysis demonstrated prominent frequency components related to harmonics of the walking frequency under all viewing conditions. In normal subjects, vision with telescopic spectacles, to a greater degree than unmagnified vision, reduced head velocity during walking in the roll and yaw, but not the pitch, axes. For low vision subjects, significant reductions in head velocity during walking were observed only during vision with telescopic spectacles. These findings indicate that vision reduces angular instability of the head during standing and walking. Magnification produced by telescopic spectacles further improves head stability under some conditions, although the effect of vision is least evident in the pitch axis. The stabilizing effect of vision is reduced in low vision subjects.

Evaluation of vestibular and visual oculomotor function

“…When subjected to Fourier analysis, these movements during standing typically have peak components up to 5 Hz and significant components up to 10 Hz. Much larger head velocities are encountered in all three axes during gentle walking in place, 2 but maximum head velocities during running are up to 90 degrees/second, with predominant frequencies up to 2.7 Hz for yaw and 8.2 Hz for pitch 3 , 4 . The frequency of involuntary head movements in pitch tends to be substantially higher and often at a higher velocity than head movements in the other two rotational axes.…”

“…Natural head movements are ubiquitous consequences of transmitted heartbeat, tremor, ambulation, vehicular travel, and (in California) seismic activity 1 . Even when attempting to stand completely at rest, people exhibit significant involuntary head movements about the vertical axis (yaw), about the interaural axis (pitch), and about the anteroposterior axis (roll) 2 . When subjected to Fourier analysis, these movements during standing typically have peak components up to 5 Hz and significant components up to 10 Hz.…”

Vision and vestibular adaptation

“…Trials with visible targets in the light (VVOR) were interleaved with VOR trials with instructions to the subjects to visualize in darkness the remembered targets at the distances previously viewed. Two motion profiles were used: sinusoids at a constant peak velocity of 30 degrees/second and frequencies ranging between 0.8 and 2.0 Hz, and unpredictable, pseudorandom step changes in head position having typical head accelerations in pitch of 900 to 1100 degrees/second2 (146 N‐M = 108 ft‐lb rotator), and in yaw of 200 to 300 degrees/second2 (27 N‐M rotator).…”

“…Angular head movements occur constantly in daily life because of transmitted heartbeat, tremor, sway, ambulation, and vehicular travel 1. Even when a person is attempting to stand completely at rest, involuntary angular head movements occur about the vertical axis (yaw), the interaural axis (pitch), and the anteroposterior axis (roll) 2. Such head movements during quiet standing have root‐mean‐square (RMS) velocities of around 1 degree/second and commonly occur at frequencies above 1.0 Hz, where smooth visual‐tracking mechanisms have significant performance limitations 3,4.…”

“…1 Even when a person is attempting to stand completely at rest, involuntary angular head movements occur about the vertical axis (yaw), the interaural axis (pitch), and the anteroposterior axis (roll). 2 Such head movements during quiet standing have root-mean-square (RMS) velocities of around 1 degree/second and commonly occur at frequencies above 1.0 Hz, where smooth visual-tracking mechanisms have significant performance limitations. 3,4 During quiet standing these movements typically have peak components at frequencies exceeding 5 Hz.…”