Strategic modulation of the fixation-offset effect: dissociable effects of target probability on prosaccades and antisaccades.

In several previous experiments examining the effects of participants' expectations on oculomotor performance, the manipulation of target probability has been confounded with factors such as target occurrence and saccade frequency. We report results from three experiments that manipulated target probability in isolation from systematic variations in such bottom-up factors. We present evidence for trial-by-trial, top-down modulation of the fixation-offset effect in prosaccade latency. Furthermore, fixation-stimulus offset and target-probability manipulations had additive effects on antisaccade latency, suggesting that these factors influence separable neural processes engaged for antisaccade performance. Based on these findings, we suggest that cognitive processes utilizing target-probability information influence task processes engaged for prosaccades that differ from those engaged for antisaccades.