Saccadic latency effects of progressively deleting stimulus offsets and onsets.

We designed two extensions of Saslow's well-known gap and overlap conditions that require increased voluntary effort because of the progressive elimination of target onsets and fixation point offsets, and obtained repeatable data obeying simple numerical relations. For each of the five stimulus lighting conditions, saccadic latency was measured as a function of the retinal eccentricity or displacement of the target. Latencies were fitted by a serial processing model in which the visually guided minimum tracking latency VGL(min) is supplemented by two types of delay, dubbed 'unlock' and 'target', that can be either short or long ('direct' or 'indirect'), depending on the conditions. There are two findings: (1) The model has utility. The rank order of saccadic latencies for the five stimulus lighting conditions was constant across all subjects, sessions and eccentricities in the range 7.5'-6 degrees left or right. For pooled data, and the saccadic latency plateau (1-6 degrees), the model was also within +/-3 ms of the mean latencies. (2) Latencies of tiny saccades to intrafoveolar stimulation (7.5-45') were invariably long in all five stimulus conditions. One factor here must be the experimentally measured local prolongation of VGL(min).