The Role of Inhibition in Avoiding Distraction by Salient Stimuli

Abstract

Researchers have long debated whether salient stimuli can involuntarily 'capture' visual attention. We review here evidence for a recently discovered inhibitory mechanism that may help to resolve this debate. This evidence suggests that salient stimuli naturally attempt to capture attention, but capture can be avoided if the salient stimulus is suppressed before it captures attention. Importantly, the suppression process can be more or less effective as a result of changing task demands or lapses in cognitive control. Converging evidence for the existence of this suppression mechanism comes from multiple sources, including psychophysics, eye-tracking, and event-related potentials (ERPs). We conclude that the evidence for suppression is strong, but future research will need to explore the nature and limits of this mechanism.

Keywords: attention capture; inhibition; suppression; visual attention.

Figure 1

Examples of attention capture. Salient stimuli, such as the uniquely colored bird (A), seem to automatically attract visual attention. For this reason, salient stimuli are often used as warning signals (B). However, researchers disagree about whether such stimuli attract attention in a truly automatic manner.

Figure 2

Conflicting results in a typical attention capture task. In both versions, participants search for the green circle and make a speeded response indicating the tilt of a line inside (left or right). On half of trials, a red singleton appears at a nontarget location. (A) Stimulus-driven theorists frequently use a version of the task where the target circle appears amongst homogenous distractors (i.e., all diamonds). This leads a large singleton presence cost, indicating capture. (B) Goal-driven theorists use a version of the task where the circle target appears amongst heterogeneous distractors. This leads to no singleton presence cost, indicating no capture. These stimuli and data are illustrative, based on a combination of several studies.

Figure 3

Mechanisms underlying the guidance of visual attention. The stimulus is first represented by a series of feature maps, which denote the location of a simple feature in the visual field. These feature maps are used to construct an attentional priority map. When singletons fail to capture attention, the signal suppression hypothesis predicts that the attentional priority at the singleton will be lower than baseline levels. This inhibition will result in relatively poor processing at the singleton location.

Figure 4

Converging evidence for the suppression of salient-but-irrelevant singletons. (A) In an ERP capture task, participants searched for a specific target (e.g., large green A) and attempted to ignore a salient color singleton (red O). Salient singletons elicited an electrophysiological index of suppression called the P_D component [19]. (B) In the capture-probe paradigm, participants search for a green circle and report the location of a dot. On a random subset of probe trials, letters briefly appear in each search location and participant tried to recall as many letters as possible. Probe recall accuracy was impaired at the singleton location compared to other nontarget locations [69]. (C) In an oculomotor capture task, participants searched for a green circle. As shown in the heat map, first eye movements were biased away from the singleton distractor, suggesting that it was inhibited [68].

Figure 5

An eye-tracking task in which the singleton color is blocked. The left panel shows the target color and the singleton colors, each of which was used for one block of 120 trials. The right panel shows the amount of oculomotor suppression at the singleton location over the course of a block of trials (running average across sets of 10 trials). In the first few trials with a new singleton color, the eyes are captured by the singleton. As participants gain experience with the singleton color value, the eyes are biased away from the singleton [25].