Latency of smooth pursuit under conditions of stimulus-response uncertainty

Abstract

Hick's law predicts that response latencies increase as a log2 function of the number of stimulus-response (S-R) alternatives. However, earlier findings by our group demonstrate that visually guided saccades (prosaccades) violate Hick's law. Prosaccadic response latencies do not increase as the number of S-R alternatives rises logarithmically from 1 to 8. In the present study, we investigated the applicability of Hick's law to smooth pursuit eye movements (SPEM). Subjects were asked to pursue a moving dot which moved from the center of the screen outwards 6 degrees. We varied S-R uncertainty by varying the number of possible movement trajectories the dot could move within a block of trials to either 1, 2, 4, or 8 possible directions. The level of S-R uncertainty was made salient before each trial by presenting 1, 2, 4 or 8 arrows indicating the potential movement trajectories for that particular trial. We found that the latency to initiate smooth pursuit does not depend on the number of S-R alternatives and thus violates Hick's law. Our finding suggests that probabilistic knowledge of target movement direction is not employed by the smooth pursuit system to reduce SPEM latency.