Distinct Mechanisms for Distractor Suppression and Target Facilitation

Abstract

It is well established that preparatory attention improves processing of task-relevant stimuli. Although it is often more important to ignore task-irrelevant stimuli, comparatively little is known about preparatory attentional mechanisms for inhibiting expected distractions. Here, we establish that distractor inhibition is not under the same top-down control as target facilitation. Using a variant of the Posner paradigm, participants were cued to either the location of a target stimulus, the location of a distractor, or were provided no predictive information. In Experiment 1, we found that participants were able to use target-relevant cues to facilitate target processing in both blocked and flexible conditions, but distractor cueing was only effective in the blocked version of the task. In Experiment 2, we replicate these findings in a larger sample and leveraged the additional statistical power to perform individual differences analyses to tease apart potential underlying mechanisms. We found no evidence for a correlation between these two types of benefit, suggesting that flexible target cueing and distractor suppression depend on distinct cognitive mechanisms. In Experiment 3, we use EEG to show that preparatory distractor suppression is associated with a diminished P1, but we found no evidence to suggest that this effect was mediated by top-down control of oscillatory activity in the alpha band (8-12 Hz). We conclude that flexible top-down mechanisms of cognitive control are specialized for target-related attention, whereas distractor suppression only emerges when the predictive information can be derived directly from experience. This is consistent with a predictive coding model of expectation suppression.

Significance statement: If you were told to ignore a white bear, you might find it quite difficult. Holding something in working memory is thought to automatically facilitate feature processing, even if doing so is detrimental to the current task. Despite this paradox, it is often assumed that distractor suppression is controlled via similar top-down mechanisms of attention that prepare brain areas for target enhancement. In particular, low-frequency oscillations in visual cortex appear especially well suited for gating task-irrelevant information. We describe the results of a series of studies exploring distractor suppression and challenge this popular notion. We draw on behavioral and EEG evidence to show that selective distractor suppression operates via an alternative mechanism, such as expectation suppression within a predictive coding framework.

Keywords: alpha; attention; distractor inhibition; expectation suppression; prediction.

Figure 1.

Task schematic and RT results for Experiment 1. A, Each trial began with a fixation cross. In both tasks, a cue was presented in the corner of the fixation cross, which predicted the forthcoming cued stimuli in Target and Distractor blocks. In Neutral cue trials, the cue was presented randomly. Subjects were instructed the presence or absence of the distractor at the beginning of each block. Targets were a patchwork pattern of two superimposed Gabor patches. Distractors were randomly oriented Gabors. For illustration purposes, color-coded borders represent the spatial information provided in each condition. Stimuli are not shown to scale. B, The mean of the individual median RTs for the Blocked (B) and Flexible (C) versions of the task for the 3 different cueing conditions (green represents target; red represents distractor; blue represents neutral) in the distractor present and absent conditions. Error bars indicate ±1 SEM. Capital letters indicate the presence of stimuli (T, Target only; TD, target and distractor present). Lowercase letters indicate condition (t, Target cue; d, distractor cue; n, neutral cue). Tt, Target only, target location cued; Td, Target only, distractor location cued; Tn, Target only, neutral cues providing no valid spatial information for either stimulus location; TDt, Distractor present, target location cued; TDd, Distractor present, distractor location cued; TDn Distractor present, neutral cues providing no valid spatial information for either stimulus location. *Indicates significance paired t-test effects.

Figure 2.

Illustrative task schematic and results for Experiment 2. A, Each trial began with a fixation cross, randomly jittered in duration. The fixation cross then turned from gray to white, signaling the onset of the trial. In the Flexible task, a cue was presented in the corner of the fixation cross, which predicted the forthcoming cued stimuli in Target and Distractor cued blocks. In Neutral cued trials, the cue was presented randomly. In the Blocked task, no explicit cue was presented as such fixation-target stimulus-onset asynchrony (SOA) is 1000 ms. Subjects were instructed the presence or absence of the distractor the beginning of each block. In the Blocked task, subjects were also instructed to the cued stimulus location. Location was valid for all trials within a block. Targets were either triangles or squares, whereas distractors were the two superimposed shapes. B, Mean of the individual median RTs (error bars indicate ±1 SEM) for Blocked version of the task for the 3 different Cue Types (green represents target; red represents distractor; blue represents neutral) in the distractor present and absent conditions. C, The same as in B, but for the Flexible version of the task. D, Mean of the individual median RTs (error bars indicate ±1 SEM) for pooled Blocked and Flexible neutral cueing condition for the trials in which the target stimulus (left) or distractor stimulus (right) was repeated in the same location relative to nonrepeat trials. E, Correlations between blocked distractor cueing effects and flexible target enhancement effects (left and middle) and between flexible target enhancement effects on blocks of trials with and without the distractor stimuli (right). F, Correlations between even and odd trials for each subcomponent of correlations shown in E. TrepTDn, Target repeats and distractor present on neutral trials; DrepTDn, Distractor repeats and distractor present on neutral trials; nrepTDn, Neither target nor distractor stimuli repeats; FlexTnTt, distractor absent target cueing RT effects in the flexible task; FlexTDnTDt, distractor present target cueing RT effects in the flexible task; BlockTDnTDd, distractor present distractor cueing RT effects in the Blocked task. *Indicates significance of statistical tests.

Figure 3.

Behavioral and ERP results from Experiment 3. A, Mean of the individual median RTs (error bars indicate ±1 SEM) for the Blocked version of the task for the 3 different Cue Types (green represents target; red represents distractor; blue represents neutral) in the distractor present and absent conditions. B, Schematic illustration of lateralization of stimuli and electrodes used in all ERP analyses. C–F, ERP waveforms for contraT (left panels, dashed lines) and ipsiT (right panels, solid lines) for distractor absent (C, E) and distractor present (D, F) blocks, when blocked stimuli were presented in the upper (C, D) or lower (E, F) visual field. Inset, Illustrations in each quadrant represent the nature of the stimulus configurations (relative to electrode side) that compose each analysis and highlight the corresponding contraT (dashed) or ipsiT (solid) side of space. Significance bar indicates cluster-corrected (p < 0.05) t tests between cueing and neutral trials (green represents neutral vs target; red represents neutral vs distractor). Gray panels represent a 20 ms window centered over the peak of the ERP components C1, P1, and N1. P1 components were not readily identified in the lower visual field because of overlap with C1. *Significance bars represent significant pairwise comparisons between cueing conditions and neutral trials (same color convention as before). Inset, Bar plot represents the mean P1 amplitude for 3 Cue Types for the significant TDnTDd contrast TOI (±1 SEM).

Figure 4.

Lateralized waveforms as a function of condition for distractor absent (A) and distractor present (B) blocks. ERPs were averaged across upper and lower visual fields for the difference between contralateral to target and ipislateral to target as a function of cue condition: target (green), distractor (red), and neutral (blue). Significance bars indicate cluster-corrected (p < 0.05) t tests between contraT and ipsiT per Cue Type. Light green significance bar indicates a significant difference between target cueing and neutral cueing. Inset, Illustrations represent the nature of the stimulus configurations (after right-sided targets are flipped into the left hemifield) that compose each analysis and highlight the contraT (dashed) or ipsiT (solid) spatial relationship.

Figure 5.

Lateralization alpha (8–12 Hz) power. A, Power spectrum averaged over all trials, time-points, and subjects. A peak at 10 Hz is clearly visible. Gray shading represents the frequency range used in all alpha-band analyses. Time frequencies are plotted for each Cue Type separately (target, B; distractor, C), for distractor absent (top) and distractor present (bottom) blocks for all trials. In Target cueing analysis, trials are aligned relative to the spatial location of the target. Dashed lines indicate electrodes contralateral to the target location. In Distractor cueing analysis, trials are aligned relative to the spatial location of the distractor. Solid lines indicate electrodes contralateral to distractor location. Significance bars indicate cluster-corrected (p < 0.05) differences. Top right insets, Corresponding stimulus configurations. Bold black letters indicate blocked stimulus location. Gray letters indicate that trials in which the location-variable stimuli occupied one of the other quadrants were included in the analysis. Bottom left insets, Time-frequency spectrogram representing raw power values for contralateral minus ipsilateral electrodes. Time scale is between −800 and 200 ms for frequencies between 1 and 30 Hz. Red square represents the analyzed alpha band. Black vertical line indicates stimulus onset.