Fixation and saccade control in an express-saccade maker.

In express-saccade makers a large incidence of express saccades (latencies around 100 ms) is paralleled by a reduced ability to suppress saccade generation when required. Such a behavior occurs frequently in dyslexics. We studied the latencies and the metrical properties of saccades in the very rare case of an adult, nondyslexic express-saccade maker (male, age 29 years). The subject produced 65-95% express saccades in the gap (fixation point removed 200 ms before target onset) as well as in the overlap (fixation point not removed) paradigm, which qualified the subject as the most clear case of an express-saccade maker found so far. The number of express saccades increased rather than decreased when fixation foreperiod, gap duration, and target location were randomized from trial to trial as compared to when they remained constant. In the memory-guided saccade and in the antisaccade paradigms in which immediate saccade execution to a visual target had to be suppressed, the subject often reacted to the target with express saccades in an involuntary way. The amplitudes of express saccades were--in some conditions--found to progressively decrease with increasing latency, giving rise to amplitude transition functions. The present findings disprove the notion that express saccades are generated based on the prediction of the time and location of target appearance and support the notion that they are the result of an optomotor reflex. It is argued that the operation of the reflex is gated by a separate fixation system. Express-saccade makers are described as subjects with a dysfunction of the fixation system. Recent neurophysiological findings suggest that the subject studied in the present study has a selective dysfunction of the fixation system at the level of the superior colliculus.