Abstract
Our eyes are in constant motion even when we attempt to hold our gaze steady. The precision in eye positions during fixation — fixation stability — is an index of oculomotor control, and has been proposed as a biomarker for early diagnosis of certain neurological and visual disorders. However, it is difficult to compare fixation stability across studies because of the different methodologies used for data collection and quantification. Our goal was to provide a single metric to quantify fixation stability. We measured eye positions during fixation of a 1° cross for multiple trials of 70s for 11 observers, using Eyelink II (250 Hz), Eyelink 1000 Plus (1000 Hz) and a scanning laser ophthalmoscope (eye positions sampled at 540 Hz). For each trial, fixation stability was calculated for epochs that grew in length, starting from the first 0.5s, in steps of 0.5s (0-0.5s, 0-1s, … 0-70s), using the bivariate-contour ellipse area (BCEA) that assumes normality of eye positions, and the iso-line area (ISOA) that does not assume normality. In general, both BCEA and ISOA increased with epoch length until they reached saturation. These data were modeled using an exponential function from which the time constant and the saturation level were derived. BCEA was always larger than ISOA. The BCEA/ISOA ratio differed with epoch lengths and eye-trackers but became identical (~1.1) across eye-trackers for epoch lengths beyond 20s. For 95% ISOA, time constants were statistically indistinguishable across eye-trackers (16.6–21.8s); while saturation levels differed by 3× (1525–4595 arcmin2). Normalizing these values by the imprecision (e.g. standard deviation) of the estimates across observers and trials for each eye-tracker removed the statistical differences. These findings suggest that a single metric can be used to unify the quantification of fixation stability for data obtained using different eye-trackers, facilitating the comparison of fixation stability across studies.
Meeting abstract presented at VSS 2018