Dependence of saccadic eye-movements on stimulus luminance, and an effect of task

Abstract

Various aspects of saccadic eye-movements are related to stimulus luminance for a small lit stimulus that steps 10° horizontally in complete darkness. The relations depend on whether the stimulus is the target for a foveating saccade, or is the cue for an “anti” saccade which peripheralizes the retinal image of the cue: (1) at scotopic luminances the differences between foveating and anti saccades are diminished, largely because foveation is the more severely affected. Latencies are long, amplitudes are scattered, and direction errors are not infrequent in both tasks; (2) the latency-luminance relation for foveating saccades shows an abrupt discontinuity at the perceptual rod-cone transition. Above the cone threshold corrective secondary saccades appear in greater numbers; (3) the corresponding latency transition for anti saccades is anomalous and protracted. Latency remains constant for mesopic cue luminances up to 1.0 log unit brighter than the perceptual rod-cone threshold. Direction errors are especially common in this mesopic luminance range. Mechanisms are discussed.