The remembered pursuit task: evidence for segregation of timing and velocity storage in predictive oculomotor control

Regular, repeated presentation of identical constant-velocity target motion stimuli tramps) appears to allow build up of an internal store, release of which can be used to generate anticipatory smooth pursuit prior to subsequ ent target onset. Here, we examine whether release of the anticipatory resp onse can be controlled by timing cues unrelated to the motion stimulus itse lf. In experiment 1, the target moved in alternate directions and was expos ed for 480 ms as it passed through centre; otherwise subjects were in darkn ess. Inter-stimulus interval (ISI) was either regular (3.6 s) or randomized (2.7-4.3 s). Presentations were given with or without audio cues that occu rred at a constant cue time (CT) prior to target appearance. Even when ISI was randomized, cues could be used to generate anticipatory smooth pursuit. Eye velocity (V100) measured 100 ms after target onset (just prior to visu al feedback influence) was greater with cues than without and decreased sig nificantly as CT increased from 240-960 ms. In experiment 2, we assessed th e effects of fixation between presentations and eccentricity of target star ting position, using unidirectional ramps. The target was visible for 400 m s and started on, ended on or straddled the midline. Subjects held fixation on the midline until an audio cue signalled that preparation for ensuing t arget appearance could begin. There was no difference in V100 between start ing positions or between presence/absence of fixation. In experiment 3, we compared the effects of using audio, visual or tactile cues. All types of c ue evoked anticipatory smooth pursuit, but the response to the visual cue w as significantly delayed compared with the others. However, V100 was not si gnificantly different between cues. In all experiments, V100 was scaled in proportion to target velocity over the range 12.5-50 degrees/s, showing tha t this was a truly predictive response. The results provide evidence that t iming and velocity storage can be independently controlled through differen t sensory channels and suggest that the two functions are probably carried out by separate neural mechanisms.