Head Movement Amplitude and Velocity During a Common Visual Task

Abstract

Abstract: : Purpose:The angular extent and angular velocity of the head movement component of gaze shift was investigated in a common visual task. This data was collected to establish parameters for head movements in subsequent experiments investigating motion detection thresholds in the presence of head movements. Methods:Head position of 12 subjects was recorded with an InsideTrack (Polhemus USA) system, with a 10 Hz sampling frequency. Head position was recorded during copy typing of a ten line passage with a PC based wordprocessor. Subjects had normal distance and near visual acuity, stereopsis of at least 60 seconds of arc, and were untrained typists. Horizontal angular extent of gaze shifts for the text passage ranged from 22° to 38° to the left of the computer monitor for fixation distances of 50 to 60 cm. Total gaze shift range for computer monitor and keyboard was 12 to 15°. Angular extent of head movement (HM) in azimuth, averaged HM velocity (angular extent/time in movement) and peak HM velocity (maximum HM in one sampling interval) were calculated for each HM. The effect of both direction and angular extent of HM on averaged and peak velocity was evaluated by multivariate analysis of variance. Results:Horizontal angular HMs ranged from 1° to 34.5°, with a mean angular head movement of 6.5°± 6.26°. Mean averaged velocity of horizontal HM was 11 ± 8.7 deg/s. Mean peak velocity was 25.7 ± 23.1 deg/s. There was a significant difference in HM velocity between HMs directed toward or away from the copy task (averaged velocity: F = 155.1, p< 0.001; peak velocity: F = 218.5, p < 0.001). A significant interaction of HM direction with HM angular extent was found for HM velocity (averaged velocity: F=22.42, p<0.001; peak velocity: F = 36.56, p <0.001). Toward task HMs were slower than HMs away from task for both averaged and peak velocity for HMs larger than 11°. Conclusion:Both direction and the angular extent of the HM affect averaged and peak velocity of HM. HMs towards a demanding visual task are significantly slower than HMs away from the task. This is consistent with the visual demands of the visual task. Gaze shifts to the copy material would be expected to be slower as a specific landing site is required from each gaze shift; whereas returning gaze to the keyboard or monitor can be less specific in landing site selection. This implies the existence of perceptual control over the head movement component of gaze shifts.

Keywords: 495 ocular motor control • 406 eye movements