DETECTING MULTIMODALITY IN SACCADIC REACTION-TIME DISTRIBUTIONS IN GAP AND OVERLAP TASKS

in many cases the distribution of saccadic reaction times (SRT) deviat es considerably from a unimodal distribution and may often exhibit sev eral peaks. We present a statistical approach to determining the numbe r and form of the individual peaks. The overall density of the reactio n times f(i)(t), i = 1...M obtained in M different experiments with th e same subject is described as the sum of K basis functions X-k(t), k = 1...K with different weights and an error term. A change in the expe rimental conditions is assumed to cause a change in the weights, not i n the basis functions. We minimize the square of the difference (measu red data minus approximation), divided by the error of the data. Incre menting K step by step we determine the necessary number of basis func tions. This method is applied to data of six subjects tested in differ ent saccade tasks. We detect five different modes: two in the range 80 -140 ms (express modes), two in the range 145-190 ms (fast-regular mod e) and one at about 230 ms (slow-regular mode). These modes are locate d at about the same positions for different subjects. The method prese nted here not only proves statistically the existence of several modes in SRT distributions but also allows the distributions to be describe d by a few characteristic numbers that go beyond the mean values and s tandard deviations.