Direct Electrophysiological Measurement of Attentional Templates in Visual Working Memory

When we look for our children on the playground, internal representations of these important targets must guide our search through the cluttered and chaotic scene. Several theories of attention propose that we hold target representations (i.e., attentional templates) in visual working memory (VWM) to control perceptual attention (Bundesen, Habekost, & Kyllingsbaek, 2005; Desimone & Duncan, 1995). Although this is a foundational theoretical assumption, there is no direct electrophysiological evidence from humans supporting this proposal and recordings from monkeys have yielded mixed results (Chelazzi, Miller, Duncan, & Desimone, 1993, 2001; Kusunoki, Sigala, Gaffan, & Duncan, 2009). This makes it difficult to rule out the classic hypothesis that visual search may operate like a prepared reflex, unguided by VWM representations (Logan, 1978). In the present study, we tested the attentional-template hypothesis by recording event-related potentials (ERPs) from subjects while they searched for targets in complex scenes. On each trial, subjects saw a target-cue array followed by a complex search array (see Figure 1A). We focused our analyses on the ERPs following the cue to determine whether the contralateral-delay activity (CDA) was present. The CDA indexes the maintenance of representations in VWM (e.g., Vogel & Machizawa, 2004), thus providing an ideal tool for testing the hypothesis that we maintain attentional templates in VWM during search. That is, unlike imaging studies demonstrating how brain areas modulate under different task demands (e.g., Soto, Humphreys, & Rotshtein, 2007), we can use the CDA component to definitely show that the same VWM mechanisms relied upon in explicit-memory tasks are engaged in maintaining attentional templates. If templates are maintained in VWM, then we should observe a cue-elicited CDA that continues until search is performed. Furthermore, if the cue-elicited CDA directly measures the attentional template, then CDA amplitude measured prior to the search task should predict subsequent performance. Figure 1 The stimuli, ERP findings, and relationship between the ERPs and behavior. A) Example of the stimulus sequence and the grand-average waveforms from electrodes T5/6, where the effect was maximal, contralateral (red) and ipsilateral (black) to the location ...