When disruptions episodically occurred in our experiment (Experiments 2 and 3), a conflict would have arisen been the target anticipated by the observer and the one actually presented. There is evidence that the anterior cingulate cortex is involved in detecting such information processing conflicts (Botvinick, Nystrom, Fissell, Carter, & Cohen,
1999, although see Mayr, Awh, & Laurey,
2003 for an alternative interpretation), and that neurons in the pre-supplementary motor area (Isoda & Hikosaka,
2007) and the supplementary eye fields (Stuphorn, Taylor, & Schall,
2000; Sumner et al.,
2007) may perform similar roles. After a disruption, but before the reappearance of the low-frequency target, average latencies were significantly increased (Experiment 2). This suggests that latencies did not simply default to a 50:50 expectation level once the pattern sequence was lost, but rather were actively delayed, presumably reflecting inhibition of this habituated response that is no longer appropriate once the sequence was disrupted. Indeed, recent evidence shows that neurons within the pre-supplementary motor area in monkeys are involved in actively suppressing a habituated response that is no longer appropriate (Isoda & Hikosaka,
2007) in a task-switching paradigm: such experiments are probably most analogous to our Experiment 3 in which the task required was known with a high degree of confidence on both sides of a disruption. Our results for this experiment showed a transient increase only at the time of the disruption (
Figure 4), with latency distributions appearing shifted relative to saccades away from the disruption. Similar task-switching experiments, also showing a lengthening of latencies at the time of the switch, have been performed in human observers (Sinha et al.,
2006). Sinha et al. (
2006) have shown that the transient latency increase in task switching is compatible with the operation of two LATER units in series: the first to interpret the instruction for a given trial prior to a second LATER unit deciding the appropriate response. It is therefore not altogether surprising that such a compound mechanism does not produce a simple swivel in distributions. More recent work has suggested that task-switching costs only occur when an instruction alters a response programmed by the appearance of a stimulus (Cameron, Watanabe, & Munoz,
2007), in concordance with the fact that no switching cost was found by Sinha et al. when the switching instruction preceded the target onset by a criterion amount. Viewed within the context of a two-stage LATER model, deciding the task instruction is already completed by the first stage and so no additional delay occurs when, upon appearance of the stimulus, the second stage must decide upon an appropriate motor response. However, Cameron et al. (
2007) found that merely being able to predict where a target would appear was insufficient to program a response and so generate a switching cost, which appears at odds with our results. There are, however, several important differences between these two studies. First, the bias used by Cameron et al. to allow the location of the target to be predicted was relatively low at 75% and so is unlikely to generate the sorts of habituated responses seen in our experiments. Second, switching occurred at a dramatically higher rate than in our experiment (50% of the time, as opposed to our 3%) and so their subjects' expectation that a switch would occur would be dramatically increased.