Expectations can modulate the frequency and timing of multiple saccades: a TMS study.

This study was undertaken to determine if target predictability could modulate saccadic planning and timing at the level of the frontal eye fields (FEF). To this end, healthy participants performed two gap saccade tasks in which the targets were displaced left or right of the midline in either a predictable or a random fashion. Additionally, half of the participants were informed about this manipulation. Single pulse transcranial magnetic stimulation (TMS) was applied to the left FEF before, during, or after the target onset. We examined both the saccade latency and the frequency of multiple saccades (MS) (i.e., saccades that covered <90 % of the distance to the target and were subsequently followed by a corrective saccade). Findings revealed that saccadic reaction times were quickest to the more predictable target side and also confirmed that MS were released more quickly than single saccades. Further, the frequency of MS differed between target locations; higher frequencies of MS were found on the unpredictable side, showing more vulnerability to TMS disruption. In conclusion, we have demonstrated that target predictability modulates saccade planning and that this modulation takes place at least in part in the FEF. The influence of FEF in these processes is observed both in the latencies with which saccades are executed and in the timing and characteristics of the multiple saccades that are observed under different task constraints. Finally, the timing of the FEF contribution also appears to be influenced by the manipulation of target predictability. Each of these observations serves to further clarify the role of the human FEF in saccade planning.