Selective attention and internal constraints: There is more to the flanker effect than biased contingencies

n both natural and laboratory settings, people are often required to fo- I cus their attention on a single stimulus or spatial location and ignore everything else that is simultaneously in view. The experimental evidence suggests, however, that when attempting to focus their attention, people can seldom completely succeed. For example, when subjects are asked to view three-item displays, restricting their attention to the center stimulus (hereafter, the target), the identity of the task-irrelevant left and right stim-uli (the flankers) have consistent effects on performance. In particular, if subjects are required to press the left response key when the target letter is an A and the right key when the target is a 2, then left-key responses are faster (and more accurate) to displays of AAA than to ZAZ (e.g., B. A. Eriksen & Eriksen, 1974). Similarly, under this particular target-to-response mapping, right-key responses are faster to ZZZ than to M A. In summary, selective-attention tasks of this sort have shown that perfor-mance depends not only on the task-relevant target, but also on the task