Reaction Times of Smooth Pursuit Initiation in Normal Subjects and in “Express-Saccade Makers”

Abstract

Introducing a temporal gap between extinction of fixation point (FP) and occurrence of a visual target (T) is known to reduce the reaction time (RT) of saccades performed from FP to T. In this ‘gap paradigm’, as it is called, the frequency distribution of saccadic RTs is typically bimodal with an early peak of very short latency saccades (express sac-cades, ES) and a second peak of regular saccades (Fischer and Boch 1983). There are considerable inter-individual differences. A large proportion of ES has been taken to indicate a weak fixation system which normally inhibits the generation of reflexive saccades, a finding in a subpopulation of dyslexic children (Biscaldi et al. 1996). Moreover, repeated practice of this paradigm may increase the percentage of ES in some normal subjects. Also, target eccentricity plays a role (ES are missing with eccentricities <1°;Weber et al. 1992). Finally, the phenomenon depends on gap duration; for instance, if FP extinction coincides with T appearance (‘simultaneous paradigm’) ES become rare and the RT distribution becomes unimodal.