Observers keep track of a moving target by combining two types of eye movements: pursuit eye movements that smoothly follow the moving target, and catch-up saccades that rapidly correct the gaze position when errors accrue. Here, we investigated if observers can learn to detect and control reflexive catch-up saccades during smooth pursuit eye movements.
In each trial, we displayed a target moving horizontally at one of three constant speeds (3, 6, or 9 dva/s). In half of the trials, we additionally displayed two horizontally-extended stimulus bands above and below the target trajectory consisting of vertically oriented gratings. These stimulus bands were rendered invisible during the slow pursuit movement by rapid temporal phase shifts (>60 Hz). The bands, however, could become visible when a sudden slowing of the phase shift—either because of a catch-up saccade or its replayed retinal consequence—briefly stabilized the stimulus band on the retina. We asked observers to report whether they detected (1) the stimulus bands (visual sensitivity) and (2) the generation of a catch-up saccade (motor sensitivity). Observers were instructed to utilize the immediate visual feedback of the stimulus to suppress catch-up saccades.
Observers showed high visual but low motor sensitivity, which did not improve significantly over four experimental sessions. Moreover, the number of catch-up saccades did not decrease with increasing session number and saccade sensitivity remained independent of detecting the stimulus bands throughout. Overall, we conclude that observers lack the ability to detect or control their reflexive catch-up saccades during smooth pursuit eye movements.