Saccadic reaction times: a statistical analysis of multimodal distributions

The distributions of saccadic reaction times (SRT) often deviate from unimodal normal distributions. An excess-mass procedure was used to detect peaks in 963 data sets containing 90,927 reaction times from 170 subjects. About 55% showed one, 30% two, 12% three and 3% four peaks. According to their clustering along the reaction time scale the modes could be classified into express (90-120 msec), fast regular (135-170 msec) and slow regular (200-220 msec) modes. Among the unimodal distributions 29% had peaks in the range of the express mode and 46% had peaks in the range of the fast regular mode. Therefore, 87% of the data sets support the notion of saccadic reaction time distributions being the superposition of three modes. All experimental distributions were fitted by as many gamma distributions as determined by the excess-mass test. The significance of the multimodality for saccade generation processes is discussed.