Visual crowding is altered during smooth pursuit eye movements
33.528, Sunday, May 12, 8:30 am - 12:30 pm, Vista Ballroom
William Harrison1, Roger Remington1, Jason Mattingley1,2; 1School of Psychology, The University of Queensland, 2Queensland Brain Institute, The University of Queensland
Visual crowding is normally measured while observers maintain fixation, leaving open the question of whether changing eye position signals can modify crowding effects. Here we measured the spatial extent of crowding during voluntary smooth pursuit eye movements, and compared the effects with those obtained at the same eccentricity during steady fixation. Observers pursued a fixation spot that translated at 10°/sec, either rightward or leftward, or they fixated a spot that remained stationary in the center of the display. A visual probe ("T", oriented up, down, left, or right) was presented for 47 ms to the left or right of fixation with equal probability. Other letters served as flankers, and, for pursuit trials, all stimuli translated across the screen with the pursuit target to minimize retinal motion. Probe-flanker separation was varied systematically, and the proportion of correct probe orientation discriminations across separations was modeled using two-parameter log functions to derive the critical spacing of crowding. Critical spacing was calculated separately for probes located contraversive and ipsiversive to the direction of the eye movement. Relative to the fixation condition, critical spacing increased when the probe was 3° from the pursuit target, but only when the probe was positioned contraversive to the direction of pursuit. By contrast, there was no change in critical spacing when the probe was positioned 7° from the pursued target. Control experiments ruled out any contribution from directional differences in either the perceived position of stimuli or visual attention. Thus, extra-retinal signals related to the generation of smooth pursuit eye movements increase the spatial extent of visual crowding contraversive to the direction of pursuit and relatively close (~3°) to the fovea. This selective expansion of the zone of crowding may attenuate the salience of background objects, and thus facilitate tracking of a target object during smooth pursuit.