The frontal eye fields (FEF) have typically been predominantly investigated in terms of their role in the generation of eye movements. Lesions to this area, either accidental or experimental, disrupt saccades and electrical stimulation elicits eye movements. Recently there has been increasing interest in the involvement of this area in visual processes, including in tasks where eye movements were either not required or were precluded. In addition to being involved in a range of visual tasks, evidence from visual search paradigms has suggested that this area might be important when the defining quality of the target is unpredictable or that it may be involved in priming. We investigated the role of FEF in a task requiring localisation of a target defined by colour, in which the target colour was either maintained or switched across trials. Disruption of performance was seen on the task when transcranial magnetic stimulation (TMS) was delivered over the left FEF, specifically elevating response times on trials when the target and distracter colours were switched rather than affecting any benefit of repetition of the target attribute (priming). This result is consistent with altered modulation of extrastriate areas, consequently affecting the speed with which a switch of the target colour could be detected. This both offers an explanation for effects seen in unpredictable feature search and is consistent with other TMS and microstimulation studies showing that FEF modulates responses of extrastriate cortex. (C) 2009 Published by Elsevier Srl.