Combined Electrophysiological and Behavioral Evidence for the Suppression of Salient Distractors

Abstract

Researchers have long debated how salient-but-irrelevant features guide visual attention. Pure stimulus-driven theories claim that salient stimuli automatically capture attention irrespective of goals, whereas pure goal-driven theories propose that an individual's attentional control settings determine whether salient stimuli capture attention. However, recent studies have suggested a hybrid model in which salient stimuli attract visual attention but can be actively suppressed by top-down attentional mechanisms. Support for this hybrid model has primarily come from ERP studies demonstrating that salient stimuli, which fail to capture attention, also elicit a distractor positivity (P_D) component, a putative neural index of suppression. Other support comes from a handful of behavioral studies showing that processing at the salient locations is inhibited compared with other locations. The current study was designed to link the behavioral and neural evidence by combining ERP recordings with an experimental paradigm that provides a behavioral measure of suppression. We found that, when a salient distractor item elicited the P_D component, processing at the location of this distractor was suppressed below baseline levels. Furthermore, the magnitude of behavioral suppression and the magnitude of the P_D component covaried across participants. These findings provide a crucial connection between the behavioral and neural measures of suppression, which opens the door to using the P_D component to assess the timing and neural substrates of the behaviorally observed suppression.

Figure 1.

Schematic illustration of the capture-probe paradigm used in Experiment 1. On search trials, participants search for a target shape (e.g., green diamond) in a display of heterogeneous shapes and respond (via a speeded button press) to indicate whether a small black dot is on the left or right side of the target. On infrequent probe trials, an array of probe letters is superimposed on the search array for a brief period. Participants then report (via unspeeded mouse click) as many letters as they can recall. No response is made to the search array on probe trials.

Figure 2.

Behavioral results from Experiment 1. (A) Mean response time on the search task by singleton presence. (B) Percentage of probe letters reported on probe trials by probed search item. Errors bars represent the within-subject 95% confidence interval (Loftus & Masson, 1994).

Figure 3.

Electrophysiological results from search trials in Experiment 1. In the schematics of the search displays, the target was the green diamond and the singleton was the uniquely colored item. The waveforms in this and all subsequent figures were low-pass filtered to improve the visibility of the effects (Butterworth noncausal filter, half-amplitude cutoff = 30 Hz, slope = 12 dB/octave).

Figure 4.

Scatterplot of P_D positive area on singleton lateral/target midline trials and probe suppression effects for each participant (gray dots). The dashed line is a regression line.

Figure 5.

Schematic illustration of search displays used in Experiment 2. The target shape was held constant throughout the experimental session (e.g., diamond). The distractors shapes were homogenous (e.g., circles). This allowed participants to use either singleton-detection mode or feature-search mode to locate the target.

Figure 6.

Behavioral results from Experiment 2. (A) Mean response time on the search task by singleton presence. (B) Percentage of probe letters reported on probe trials by probed search item. Errors bars represent the within-subject 95% confidence interval (Loftus & Masson, 1994).

Figure 7.

Electrophysiological results from Experiment 2. In the schematics of the search displays, the target was the green diamond and the singleton was the uniquely colored item. Difference waveforms were created by subtracting contralateral waveforms from ipsilateral waveforms.

Figure 8.

Relationship between ERP and behavioral measures of suppression in Experiment 2. (A) A scatter plot of P_D positive area and probe suppression effects for each participant (gray dots). The dashed line is a regression line. (B) A scatter plot of N2pc negative area and probe suppression effects for each participant. (C) P_D difference waveforms for two groups created by a median split of probe suppression effects. Difference waveforms were created by subtracting contralateral waveforms from ipsilateral waveforms on trials where the singleton was lateralized and the target was on the midline. The high probe suppression group shows a large early positivity (P_D), but the low suppression group shows no such positivity.

Figure 9.

Electrophysiological results from Experiment 3. The target was always one of the nonsingleton items in the singleton distractor condition (a), whereas the target was always the singleton in the singleton target condition (b). Difference waveforms (c) were created by subtracting ipsilateral waveforms from contralateral waveforms.