Capture versus suppression of attention by salient singletons: electrophysiological evidence for an automatic attend-to-me signal

Abstract

There is considerable controversy about whether salient singletons capture attention in a bottom-up fashion, irrespective of top-down control settings. One possibility is that salient singletons always generate an attention capture signal, but this signal can be actively suppressed to avoid capture. In the present study, we investigated this issue by using event-related potential recordings, focusing on N2pc (N2-posterior-contralateral; a measure of attentional deployment) and Pd (distractor positivity; a measure of attentional suppression). Participants searched for a specific letter within one of two regions, and irrelevant color singletons were sometimes present. We found that the irrelevant singletons did not elicit N2pc but instead elicited Pd; this occurred equally within the attended and unattended regions. These findings suggest that salient singletons may automatically produce an attend-to-me signal, irrespective of top-down control settings, but this signal can be overridden by an active suppression process to prevent the actual capture of attention.

Figure 1

Example of search displays used in Experiments 1 and 2. In the target, target-similar, and standard arrays (left panel), all letters were the same color (either red or green). In the salient distractor arrays (right panel), one letter was red and the other letters were green (or vice versa). The central fixation and the two outlined rectangles were gray. All stimuli were presented against a black background.

Figure 2

Grand average waveforms from Experiment 1 for each stimulus category at contralateral versus ipsilateral PO7/PO8 electrode sites, along with the difference between the contralateral and ipsilateral waveforms (which isolates the N2pc and Pd components).

Figure 3

Grand average waveforms from Experiment 2 for each stimulus category at PO7/PO8 and the difference between contralateral and ipsilateral waveforms.

Figure 4

Example of search displays used in Experiment 3 (left) and square stimuli around fixation in close up (right). All squares were gray, three had a gap on their right or left sides but one had a gap on its upper or lower side.

Figure 5

Grand average waveforms from Experiment 3 for salient distractors at PO7/PO8 and the difference between contralateral and ipsilateral waveforms.

Figure 6

Grand average waveforms from Experiment 4 for each stimulus category at PO7/PO8 and the difference between contralateral and ipsilateral waveforms.

Figure 7

Topographic maps from Experiment 4 for each stimulus category. The top and middle maps show the topography of the N2pc component for targets and target-similar distractors, respectively, measured as the mean amplitude between 225 and 300 ms. The bottom map shows the topography of the Pd component for salient distractors measured as the mean amplitude between 115 and 225 ms. The data are arranged so that the left and right sides of the outline head represent ipsilateral and contralateral electrode sites, respectively.