exogenous control over spatial attention. Before we elaborate on our choice of control settings, we first develop our general theoretical and empirical approach. A benchmark Vorinostat price result in the task-switching literature is the so-called switch-cost asymmetry. When people switch between a dominant task, such as Stroop

word naming and a competing, non-dominant task, such as Stroop color naming, switch costs are larger when transitioning from the hard, non-dominant to the easy, dominant task than the other way round (e.g., Allport et al., 1984). This phenomenon is important here because carry-over models of task switching seem to be able to explain it in a straightforward manner: Non-dominant tasks require a particularly strong attentional setting to survive against the

competition from the dominant task and this strong setting is carried forward into the next trial where it needs to be overcome when switching back to the dominant task. In contrast, the dominant task requires only weak support from a task setting and therefore relatively speaking, less change in control settings is required when switching from the dominant to the non-dominant task. Critically, selleck kinase inhibitor for the carry-over account to work, trial-to-trial switching between the two competing tasks is a necessary condition for obtaining a switch cost asymmetry (Gilbert and Shallice, 2002, Yeung and Monsell, 2003a and Yeung and Monsell, 2003b). Even though this model adequately accounts for the not basic finding of the asymmetry in switch costs, there is also some initial evidence that directly contradicts the carry-over account. Obviously, the carry-over account can explain the task-selection cost asymmetry only for cases

in which the alternative task was performed in the immediately preceding trial––otherwise there would be no opportunity for carry-over. However, Bryck and Mayr (2008) have shown that a cost asymmetry can be obtained even in the absence of a task-switch transition (see also Allport & Wylie, 2000). This finding, which will be elaborated below, is important because it indicates that opportunity for trial-to-trial carry-over is not a necessary condition for the cost asymmetry to arise. A key tenet of our account is that interference comes not from the most recent past (i.e., the previous trial), but from any kind of previous experiences with the competing tasks that are stored in long-term memory. One long-term memory model that is particularly well-equipped to handle the influence of past task experiences is memory instance theory (Hintzman, 1986 and Logan, 1988). To fully explain task-selection costs, this theory needs to be augmented through additional assumptions about factors that affect encoding and retrieval of memory instances.