Inhibition as a potential resolution to the attentional capture debate

Highlights

• A simple suppressive mechanism could resolve the attentional capture debate.

• Distractors with known features can be preemptively inhibited to prevent capture.

• Suppression is triggered by recent experience rather than an act of will.

• Suppression can be observed in psychophysics, eye movements, and ERPs.

Physically salient stimuli, such as uniquely colored objects, seem to have an inherent power to capture our attention, but formal research on this topic has produced conflicting results and theories. Here, we review evidence that the attentional capture debate can be resolved by positing a new suppressive process. This suppressive process can occur before attentional shifting to prevent salient items from attracting attention. In the current article, we review converging evidence that salient items are suppressed to avoid attentional capture comes from studies of psychophysics, eye movements, single-unit recordings, and event-related potentials (ERPs). Crucially, the ability to inhibit salient distractors seems to be learned as participants gain experience with the simple features of the to-be-ignored stimuli.

Introduction

When we search visual scenes, physically salient items seem to automatically attract our attention, even when they are completely irrelevant to our goals. For this reason, brightly colored signs and flashing lights are commonly used as visual warning signals to alert people to important information. From neon traffic signs to flashing beacons on police cars to fluorescent advertisements in storefront windows, people frequently encounter salient stimuli (see Figure 1). But exactly how these salient stimuli are handled by the visual system has been heatedly disputed.

Traditionally, research on attentional capture has been divided into two opposing theoretical positions (see Table 1). Bottom-up theories propose that salient stimuli automatically and inevitably capture attention, independent of our knowledge and goals [1,2]. These models predict rampant distraction in the real world because the visual system is at the mercy of the most salient item in a scene. Top-down theories, however, propose that salient items have no special influence on attentional allocation unless they match the anticipated features of a search target [3, 4, 5] or match previous experience (i.e., selection history; see Refs. [6,7]). Thus, these models predict a kind of ‘tunnel vision’ – whereby salient warning signals that fall outside of one’s attentional template will go unnoticed.

The dispute between these two theoretical positions has now lasted for decades. Both positions are supported by numerous studies, which has led to a theoretical stalemate. Recently, however, several researchers have provided evidence that bottom-up capture can be eliminated by means of top-down inhibitory mechanisms [8, 10, 11,12^••], providing a potential bridge between bottom-up and top-down theories. The signal suppression hypothesis [10,13], proposes that salient items automatically produce a priority signal that attracts attention, consistent with bottom-up theories, but that the salient items can be suppressed before capturing attention, consistent with top-down theories. It is important to emphasize that this suppression occurs before the initial shift of visual attention (an issue that we will discuss in-depth later in this paper). Because this inhibitory mechanism makes it possible to explain why capture is observed in some experiments and not in others, we believe that it provides a plausible resolution to the debate between bottom-up and top-down theories of attention capture, as well as adding a new dimension to general theories of visual search. Note that the idea that inhibition may play a role in attentional guidance is not new (e.g., [14, 15, 16, 17]), but the idea that inhibition plays a role in the attentional capture has recently gained considerable traction.

Much has been learned about the suppression of salient stimuli over the past few years. For example, there is growing evidence that suppression of salient stimuli is not a reactive process that is triggered by a salience signal per se (as originally proposed by Ref. [10]). Instead, it seems to be the result of a proactive feature-based attention process that downweights objects containing a to-be-ignored feature value, which must be known in advance ([18^••,19,20]; but see Ref. [21]). Second, although suppression is top-down (as traditionally defined [22]), it now seems likely that it is a result of recent experience rather than an act of will [23,24]. Indeed, if the to-be-ignored feature value varies from trial to trial and is indicated with a precue, attention is initially attracted to this color [25,26,27^•]. However, much is still unknown. For example, most of the research on inhibition of salient stimuli has focused on color singletons (a uniquely colored object amongst homogenously colored search items – as in Figure 1), but it is unclear if all types of physically salient stimuli (especially sudden onsets and other dynamic stimuli) can be suppressed.

In the current article, we will review the recent empirical evidence supporting the idea that salient distractors can be inhibited to prevent attentional capture, discuss how this idea has evolved with new findings, and point to important areas for future research.