Optokinetic potential and the perception of head-centred speed.

Extra-retinal information about eye velocity is thought to play an important role in compensating the retinal motion experienced during an eye movement. Evidently this compensation process is prone to error, since stimulus properties such as contrast and spatial frequency have marked effect on perceived motion with respect to the head. Here we investigate the suggestion, that 'optokinetic potential' [Perception 14 (1985) 631] may contribute to an explanation of these errors. First, we measured the optokinetic nystagmus induced by each stimulus so as to determine the optokinetic potential. Second, we determined the speed match between two patches of Gaussian blobs presented sequentially. Observers pursued the first pattern and kept their eyes stationary when viewing the second. For stimuli with identical contrast or spatial frequency, the pursued pattern was perceived to move slower than the non-pursued pattern (the Aubert-Fleischl phenomenon). Lowering the contrast or the spatial frequency of the non-pursued pattern resulted in a systematic decrease of its perceived speed. A further condition in which the contrast or spatial frequency of the pursued pattern was varied, resulted in no change to its perceived speed. Pursuit eye movements were recorded and found to be independent of stimulus properties. The results cast doubt on the idea that changing contrast or spatial frequency affects perceived head-centred speed by altering optokinetic potential.