A brain-potential study of preparation for and execution of a task-switch with stimuli that afford only the relevant task.

Behavioural and neurophysiological studies of task-switching have tended to employ 'bivalent' stimuli (which afford responses in two tasks). Using brain potential recordings, we investigated task-switching with 'univalent' stimuli affording responses in only one of the tasks, and compared the outcomes to those recently obtained with bivalent stimuli (Lavric et al. [2008]: Eur J Neurosci 1-14), in order to examine two phenomena. First, when only univalent stimuli are presented, the processing of task cues becomes optional. Our results showed that in these circumstances linguistic (but not pictorial) cues were still effective in eliciting at least some degree of preparation for a task-switch, as evidenced by the reduction in the error cost of switching at the longer preparation interval and by a posterior switch-induced ERP positivity at about 450-800 ms in the cue-stimulus interval. Second, single affordance stimuli not only reduced behavioural switch costs relative to bivalent stimuli; they also produced a smaller post-stimulus switch-induced negativity, consistent with the latter being a marker of conflict between task-sets. However, using stimuli not associated with responses in the alternative task did not completely eliminate the negativity. We speculate that the residue reflects other sources of conflict: attention to the irrelevant perceptual dimension and/or persistence of task goals.

No negative priming without cognitive control.

There is evidence that the efficiency of selective attention depends on the availability of cognitive control mechanisms as distractor processing has been found to increase with high load on working memory or dual task coordination (Lavie, Hirst, de Fockert, & Viding, 2004). We tested the prediction that cognitive control load would also affect the negative priming effect produced when a distractor from 1 trial appears as a target on the next trial. We measured priming on trials that involved either high or low cognitive control load, and found that under high control load, negative priming was eliminated, and could even be reversed to positive priming, suggesting that the negative priming effect depends on the availability of cognitive control resources.

Imaging the impossible: an fMRI study of impossible causal relationships in magic tricks.

Understanding causal relationships and violations of those relationships is fundamental to learning about the world around us. Over time some of these relationships become so firmly established that they form part of an implicit belief system about what is possible and impossible in the world. Previous studies investigating the neural correlates of violations of learned relationships have focused on relationships that were task-specific and probabilistic. In contrast, the present study uses magic-trick perception as a means of investigating violations of relationships that are long-established, deterministic, and that form part of the aforementioned belief system. Compared to situations in which expected causal relationships are observed, magic trick perception recruited dorso-lateral prefrontal cortex (DLPFC) and anterior cingulate cortex (ACC), brain regions associated with the detection of conflict and the implementation of cognitive control. These activations were greater in the left hemisphere, supporting a role for this hemisphere in the interpretation of complex events. DLPFC is more greatly activated by magic tricks than by surprising events, but not more greatly activated by surprising than non surprising events, suggesting that this region plays a special role in causality processing. The results suggest a role for cognitive control regions in the left hemisphere in a neurobiology of disbelief.

Neurophysiological signature of effective anticipatory task-set control: a task-switching investigation.

Changing between cognitive tasks requires a reorganization of cognitive processes. Behavioural evidence suggests this can occur in advance of the stimulus. However, the existence or detectability of an anticipatory task-set reconfiguration process remains controversial, in part because several neuroimaging studies have not detected extra brain activity during preparation for a task switch relative to a task repeat. In contrast, electrophysiological studies have identified potential correlates of preparation for a task switch, but their interpretation is hindered by the scarcity of evidence on their relationship to performance. We aimed to: (i) identify the brain potential(s) reflecting effective preparation for a task-switch in a task-cuing paradigm that shows clear behavioural evidence for advance preparation, and (ii) characterize this activity by means of temporal segmentation and source analysis. Our results show that when advance preparation was effective (as indicated by fast responses), a protracted switch-related component, manifesting itself as widespread posterior positivity and concurrent right anterior negativity, preceded stimulus onset for approximately 300 ms, with sources primarily in the left lateral frontal, right inferior frontal and temporal cortices. When advance preparation was ineffective (as implied by slow responses), or made impossible by a short cue-stimulus interval (CSI), a similar component, with lateral prefrontal generators, peaked approximately 300 ms poststimulus. The protracted prestimulus component (which we show to be distinct from P3 or contingent negative variation, CNV) also correlated over subjects with a behavioural measure of preparation. Furthermore, its differential lateralization for word and picture cues was consistent with a role for verbal self-instruction in preparatory task-set reconfiguration.

Can the task-cuing paradigm measure an endogenous task-set reconfiguration process?

In 6 task-cuing experiments, with 2 cues per task, the authors varied cue-stimulus interval to investigate G. D. Logan and C. Bundesen's (2003) claim that when cue repetition is controlled for, task-switch cost and its reduction with preparation are largely eliminated and hence cannot index an endogenous control process. Experiment 1 replicates their result, but Experiments 2 and 3, with similar designs, demonstrate a substantial task-switch cost, reducing with increasing cue-stimulus interval. Experiments 4 to 6 show that the critical difference is the probability of a task change: If it is kept low enough to discourage reconfiguration of task set unless and until the cue signals a task change, robust evidence for anticipatory task-set reconfiguration is obtained, even in Experiment 6, modeled closely on Logan and Bundesen's.