Predicting the duration of ocular pursuit in humans

This study examines the effects of expectation on the timing of ocular pursuit termination. Human subjects pursued repeated, constant velocity (15 or 30 degrees/s) target motion stimuli (ramps), moving left or right. Ramps were of constant duration (RD = 240, 480, 720 or 960 ms), resulting in anticipatory slowing of eye velocity prior to ramp termination and target extinction. At unexpected intervals RD was increased or decreased, but velocity remained constant. When RD increased eye velocity continued to decline, even though the target remained visible and continued to move. It took approximately 180 ms before eye velocity started to recover towards the steady state velocity level for the continued target motion. When RD decreased, eye velocity continued as if for a longer ramp duration, again taking approximately 180 ms before eye velocity started to decrease. These results suggest that timing of the response to the expected ramp duration had been pre-programmed on the basis of prior experience of ramp duration. Moreover, adjustments to timing occurred rapidly, within the second presentation of the new RD. Responses were compared to control conditions with randomised ramp duration. Eye velocity declined later in the controls for RD < or = 720 ms, as expected, but exhibited similar decline in predictable and randomised conditions for RD = 960 ms. Further controls established that eye velocity could only be reliably maintained until the end of the ramp when the target was expected to continue in motion after the end of the ramp. The results suggest that estimates of stimulus duration are made continuously in all conditions, based on expectancy of target termination.