Abstract
Optic flow is an important cue for human perception and locomotion and naturally triggers eye movements. Many aspects of visual perception benefit from eye movements, but it is unclear whether the perception of naturalistic optic flow is affected by eye movements. Here we investigate whether the perception of optic flow direction is limited or enhanced by eye movements by manipulating the instruction to observers (free viewing vs. fixation) during a direction discrimination task. Observers (n=23) viewed an optic flow pattern for 1 second and reported in which of the four quadrants (top left/right, bottom left/right) the focus of expansion (FOE) was located using a keypress; in all observers, the task was performed during fixation and free-viewing. Task difficulty was varied by manipulating the coherence of radial motion from the FOE (4%-64%). Threshold and slope measures of a psychometric function were compared across eye movement conditions. During free-viewing, observers tracked the optic flow pattern with a combination of saccades and smooth pursuit eye movements. During fixation, observers nevertheless made small-scale saccades and low-velocity pursuit. Despite differences in spatial scale eye movements during free viewing and fixation were similarly directed toward the FOE (saccades) and away from the FOE (smooth pursuit). Perceptual direction discrimination thresholds (p=.22, d=.26) and psychometric function slopes (p=.15, d=.30) were similar during free-viewing and fixation, indicating that FOE direction judgments were comparable across eye movement conditions. These findings suggest that optic flow patterns generate small and large-scale eye movements, but that their occurrence might not influence optic flow perception in a task that requires coarse localization of the FOE. Preliminary data of a control experiment requiring observers to discriminate luminance changes within the FOE differ between free viewing and fixation, indicating that the influence of eye movements on optic flow perception might be task-specific.