Localization of speed differences of context stimuli during fixation and smooth pursuit eye movements

The visual system can detect speed changes of moving objects only by means of alterations of retinal image motion, which is also subject to changes induced by head or eye movements. Here we investigated whether smooth pursuit eye movements affect the ability to localize short speed perturbations of large context stimuli. Psychophysical thresholds for localization, discrimination and detection of speed perturbations in one of two context stimuli were measured under two main conditions: in fixation trials subjects fixated a central stationary spot, in pursuit trials they followed a horizontally moving target with their eyes. Context stimuli were vertically oriented sine wave gratings moving simultaneously above and below the fixation or pursuit target for one second in the same direction at the same or a different speed as the pursuit target. During the movement one of the gratings suddenly changed its speed for 500 ms and returned to its original speed. Observers were asked to discern the location of the speed change (two-alternative spatial forced choice task). While detection (two-interval forced choice) and discrimination thresholds for the kind of speed perturbation were in the normal range of Weber fractions of 10-15%, thresholds for the location of the speed perturbation were dramatically increased to 30-50%. Localization thresholds were particularly high when the retinal motion was mainly due to the context movements as during fixation or slow pursuit and significantly reduced when the retinal motion was mainly due to pursuit. This result indicates that the origin of retinal motion, whether it is caused by object motion or by voluntary pursuit is important. We conclude that the localization of speed perturbations affecting one of two peripheral moving objects is exceedingly complicated for the visual system probably due to the dominance of relative motion. During smooth pursuit the ability to localize speed perturbations of non-foveated objects seems to be improved by additional information gained from pursuit such as corollary discharge.