The top-down control of visual selection and how it is linked to the N2pc component.

Is the spatial selection of visual objects fully under the control of top-down factors such as current tasks sets? In his admirably clear and systematic review (Theeuwes, 2010), Jan Theeuwes claims that the selection of visual objects is determined solely by bottom-up mechanisms that are independent of observers' selection intentions, and that top-down control only comes into play at later stages of attentional processing. His argument starts with the assumption that visual information is initially processed in a parallel and feedforward fashion, and that this early stage is entirely stimulus-driven. That much is not contentious — in fact, many models of visual processing and attentional selectivity assume the existence of an early fast feedforward sweep that is essentially non-selective. Theeuwes' central and controversial claim concerns the factors that drive the subsequent attentional selection of visual objects. He argues that this selection is determined by bottom-up salience maps that are computed during the initial parallel visual processing stage. Crucially, this salience-driven attentional object selection cannot be prevented or even modulated by top-down task set. Top-down factors can only influence what happens after an object has been selected; for example, attention can be rapidly disengaged from objects that do not possess currently task-relevant attributes. But intentional factors have no role whatsoever in the attentional selection of visual objects. It is unusual to define a psychological concept (bottom-up visual selection) by applying a negative criterion (the absence of intentional modulation). As a result, there is a surprisingly wide range of cases that appear to meet this criterion. For example, Theeuwes considers phenomena such as intertrial feature priming or memory-driven attentional capture to be instances of bottom-up selection, even though here the selection of visual objects is not determined by their salience. Such an extension of the concept of bottom-up selection beyond situations where attentional selectivity is demonstrably and exclusively driven by visual salience comes at a price: if we no longer have a straightforward empirical criterion on which to base the distinction between bottom-up versus top-down attentional selection, we are likely to find cases that could be categorized either way. Theeuwes argues that intertrial priming phenomena such as the priming-of-popout effect described by Maljkovic & Nakayama (1994) are bottom-up, because they are unrelated to top-down selection intentions. But is this strictly true? Such intertrial priming effects owe their existence to the fact that on preceding trials, observers