Spatially Guided Distractor Suppression during Visual Search

Abstract

Past work has demonstrated that active suppression of salient distractors is a critical part of visual selection. Evidence for goal-driven suppression includes below-baseline visual encoding at the position of salient distractors (Gaspelin and Luck, 2018) and neural signals such as the distractor positivity (Pd) that track how many distractors are presented in a given hemifield (Feldmann-Wüstefeld and Vogel, 2019). One basic question regarding distractor suppression is whether it is inherently spatial or nonspatial in character. Indeed, past work has shown that distractors evoke both spatial (Theeuwes, 1992) and nonspatial forms of interference (Folk and Remington, 1998), motivating a direct examination of whether space is integral to goal-driven distractor suppression. Here, we use behavioral and EEG data from adult humans (male and female) to provide clear evidence for a spatial gradient of suppression surrounding salient singleton distractors. Replicating past work, both reaction time and neural indices of target selection improved monotonically as the distance between target and distractor increased. Importantly, these target selection effects were paralleled by a monotonic decline in the amplitude of the Pd, an electrophysiological index of distractor suppression. Moreover, multivariate analyses revealed spatially selective activity in the θ-band that tracked the position of the target and, critically, revealed suppressed activity at spatial channels centered on distractor positions. Thus, goal-driven selection of relevant over irrelevant information benefits from a spatial gradient of suppression surrounding salient distractors.SIGNIFICANCE STATEMENT Past work has shown that distractor suppression is an important part of goal-driven attentional selection, but has not yet revealed whether suppression is spatially directed. Using behavioral data, event-related potentials (ERPs) of the EEG signal [N2pc and distractor positivity (Pd) component], as well as a multivariate model of EEG data [channel tuning functions (CTF)], we show that suppression-related neural activity increases monotonically as the distance between targets and distractors decreases, and that spatially-selective activity in the θ-band reveals depressed activity in spatial channels that index distractor positions. Thus, we provide robust evidence for spatially-guided distractor suppression, a result that has important implications for models of goal-driven attentional control.

Keywords: EEG; Pd; attentional capture; multivariate models; suppression; visual attention.

Figure 1.

Visual search displays used for experiment 1a (A), experiments 1b and 2 (B), and experiment 3 (C). Participants had to find the diamond-shaped target and report the orientation of its embedded line with a key press. In half the trials, all items were gray (distractor absent trials). In the other half of the trials, a color singleton (red/green ring), serving as a distractor, was presented equally likely in one of the six positions. This means the singleton was in a distractor position in 5/6 of singleton-present trials (distance > 0) and in a target position in 1/6 of singleton-present trials (distance = 0). In experiment 3, different color were used for distance > 0 and distance = 0 trials.

Figure 2.

Trial procedure. Each trial started with a ready display. Participants had to fixate the central dot and press a key when ready. Upon key press a fixation cross followed for 400–600 ms (500 ms in experiment 1) before the search display was presented for 200 ms. The subsequent response display only showed a fixation cross. Participants had to respond during the search display or response display presentation (i.e., within 1200 ms). The response display was followed by an intertrial interval showing a blank screen for 1000 ms (experiment 1) or 800–1200 ms (experiments 2 and 3). The ready display was not used in experiment 1.

Figure 3.

Behavioral results in experiments 1a (A), 1b (B), 2 (C), and 3 (D). The upper panels shows RT as a function of trial type (distractor absent, distractor-target distance = 0, distractor-target distance > 0). The lower panels show RT as a function of distance between target and distractor in distractor-present trials (blue line depicts RT in distractor-absent trials for comparison). Negative numbers represent counterclockwise distances, positive numbers clockwise distances. Distance 0 refers to trials in which target and distractor appeared at the same location (experiments 1b, 2 and 3) or the target was the color singleton itself (experiment 1a). Error bars denote SEMs, corrected for within-subjects variability (Cousineau, 2005).

Figure 4.

Grand average ERPs collapsed across experiments 2 and 3. The left panels show trials in which the target was presented laterally and the distractor was presented on the vertical midline or was absent (black line). The right panels show trials in which the distractor was presented laterally and the target was presented on the vertical midline (or target was at distractor location, red lines). The lowest row shows the difference waves (contra minus ipsilateral) for the same conditions as shown in the upper rows. Signal is pooled across PO7/8, P7/8, and PO3/4. For display purposes, signal was filtered with a 30-Hz lowpass filter. All statistical analyses were conducted on unfiltered data. Yellow shaded areas show the time windows used for statistical purposes.

Figure 5.

CTFs for evoked α-band (8–12 Hz) activity (left panels) and evoked θ-band (4–8 Hz) activity (right panels). All CTFs are trained on target position in distractor-absent trials and tested on (A) target position and (B) distractor positions in distractor-present trials. The slope of the CTFs in (A) and (B) is show in (C) as blue lines (target) and red lines (distractor). Thin blue lines show time point where the target-CTF slope is different from zero, thin red lines show where the distractor-CTF is different from zero. Thin green lines show where target-CTF and distractor-CTF differ from one another. Only time points in clusters of at least 50 subsequent time points (= 50 ms) with p < 0.05 are highlighted. Shades indicate SEMs, corrected for individual differences (Cousineau, 2005).