Oblique saccades in visually normal human observers

Abstract

Introduction: Recent evidence suggests that oblique eye movements are differentially affected by some systemic diseases (for example Graves' disease). Previous studies of eye movements have tended to concentrate on horizontal or vertical eye movements. The aim of this study is to establish baseline data (latency, duration and peak velocity) for oblique eye movement in normal human observers.Methods: Eye movements were measured using an IRIS 6500 infrared limbal eye tracker (Skalar Medical, Delft, The Netherlands) customized for the direct recording of oblique eye movements. Data were collected at a sampling rate of 205 Hz and a resolution of 0.05° using a Gould 1604 digital storage oscilloscope. Data were subsequently transferred via an IEEE488 interface bus to a PC for analysis. Ten visually normal observers recruited from the student and patient population of the University of Bradford's Eye Clinic participated in the study. Subjects made saccadic eye movements in 5° to 15° steps in horizontal, vertical and oblique orientations. Stimuli were presented in random order to ensure there was no predictable component to the response.Results: Saccadic responses to vertical and horizontal step stimuli were consistent with previous published work. The modification to the eye tracker allowed direct measurement of oblique saccades. The duration of the saccade increased as a function of stimulus amplitude and conformed to the relationship: D = D0 + dA (where D is the duration of saccadic eye movement; A is the amplitude of saccadic eye movement; d is the rate of duration increase per degree of amplitude and D0 is the y‐axis intercept of the duration vs amplitude line). Calculation of latency (∼200 ms), duration (∼80–100 ms) and peak velocity (∼300 ms−1) for oblique saccades gave responses which were similar to those reported using indirect measurement.Conclusion: A veridical measure of oblique saccadic eye movements was obtained using the modified eye tracker. Oblique responses were generally slower than for horizontal measures of saccadic eye movements. These baseline data will allow a comparison in age‐matched subjects with early onset ocular motor anomalies.