Josef Schönhammer

Salient Stimuli Capture Attention and Action

Reaction times in a visual search task increase when an irrelevant but salient stimulus is presented. Recently, the hypothesis that the increase in reaction times was due to attentional capture by the salient distractor has been disputed. We devised a task in which a search display was shown after observers had initiated a reaching movement toward a touch screen. In a display of vertical bars, observers had to touch the oblique target while ignoring a salient color singleton. Because the hand was moving when the display appeared, reach trajectories revealed the current selection for action. We observed that salient but irrelevant stimuli changed the reach trajectory at the same time as the target was selected, about 270 ms after movement onset. The change in direction was corrected after another 160 ms. In a second experiment, we compared manual selection of color and orientation targets and observed that selection occurred earlier for color than for orientation targets. Salient stimuli support faster selection than do less salient stimuli. Under the assumption that attentional selection for action and perception are based on a common mechanism, our results suggest that attention is indeed captured by salient stimuli.

Saliency Changes Appearance

Numerous studies have suggested that the deployment of attention is linked to saliency. In contrast, very little is known about how salient objects are perceived. To probe the perception of salient elements, observers compared two horizontally aligned stimuli in an array of eight elements. One of them was salient because of its orientation or direction of motion. We observed that the perceived luminance contrast or color saturation of the salient element increased: the salient stimulus looked even more salient. We explored the possibility that changes in appearance were caused by attention. We chose an event-related potential indexing attentional selection, the N2pc, to answer this question. The absence of an N2pc to the salient object provides preliminary evidence against involuntary attentional capture by the salient element. We suggest that signals from a master saliency map flow back into individual feature maps. These signals boost the perceived feature contrast of salient objects, even on perceptual dimensions different from the one that initially defined saliency.

Attentional guidance by relative features: Behavioral and electrophysiological evidence.

Our ability to select task-relevant information from cluttered visual environments is widely believed to be due to our ability to tune attention to the particular elementary feature values of a sought-after target (e.g., red, orange, yellow). By contrast, recent findings showed that attention is often tuned to feature relationships, that is, features that the target has relative to irrelevant features in the context (e.g., redder, yellower). However, the evidence for such a relational account is so far exclusively based on behavioral measures that do not allow a safe inference about early perceptual processes. The present study provides a critical test of the relational account, by measuring an electrophysiological marker in the EEG of participants (N2pc) in response to briefly presented distractors (cues) that could either match the physical features of the target or its relative features. In a first experiment, the target color and nontarget color were kept constant across trials. In line with a relational account, we found that only cues with the same relative color as the target were attended, regardless of whether the cues had the same physical color as the target. In a second experiment, we demonstrate that attention is biased to the exact target feature value when the target is embedded in a randomly varying context. Taken together, these results provide the first electrophysiological evidence that attention can modulate early perceptual processes differently; in a context-dependent manner versus a context-independent manner, resulting in marked differences in the range of colors that can attract attention.

Detection costs and contingent attentional capture

Peripheral cues reduce reaction times (RTs) to targets at the cued location with short cue-target SOAs (cueing benefits) but increase RTs at long SOAs (cueing costs or inhibition of return). In detection tasks, cueing costs occur at shorter SOAs and are larger compared with identification tasks. To account for effects of task, detection cost theory claims that the integration of cue and target into an object file makes it more difficult to detect the target as a new event, which is the principal task-requirement in detection tasks. The integration of cue and target is expected to increase when cue and target are similar. We provided evidence for detection cost theory in the modified spatial cueing paradigm. Two types of cues (onset, color) were paired with two types of targets (onset, color) in separate blocks of trials. In the identification task, we found cueing benefits with matching (i.e., similar) cue-target pairs (onset-onset, color-color) and no cueing effects with nonmatching cue-target pairs (onset-color, color-onset), which replicates previous work. In the detection task, cueing effects with matching cues were reduced and even turned into cueing costs for onset cues with onset targets, suggesting that cue-target integration made it more difficult to detect targets at the cued location as new events. In contrast, the results for nonmatching cue-target pairs were not affected by task. Furthermore, the pattern of false alarms in the detection task provides a measure of similarity that may explain the size of cueing benefits and costs.

Which kind of attention is captured by cues with the relative target colour

ABSTRACT Most theories of visual search maintain that attention is selectively tuned to the attributes of the search target (e.g., orange). Conversely, according to the relational account, attention is biased to the relative feature of the target (e.g., redder). However, previous studies that supported the relational account mainly measured mean response times. Hence, the results might not reflect early, perceptual mechanisms (e.g., signal enhancement) but later, decision-based mechanisms (channel selection). The current study tested the relational account against feature-specific theories in a spatial cueing task, in which the targets were backward-masked, and target identification accuracy was measured. The first experiment corroborated earlier results, demonstrating that relational effects are due to signal enhancement. In the second experiment, we chose highly discriminable colours along the blue–red continuum, and obtained results that were more consistent with broad feature-specific rather than relational tuning. The implications of these findings for current theories of attention are discussed.

Optimal task-sets override attentional capture by rare cues.

Many studies converge on the conclusion that spatially irrelevant precues do not capture attention when cue and target features do not match. However, a recent study reported that rare onset cues captured attention even though observers searched for a nonmatching color target. Hence, attentional capture by rare onsets cues might be cognitively impenetrable (Folk & Remington, 2015). Although we replicated these findings (Experiment 1), we hypothesized that capture by rare onset cues can be prevented when the target display promotes a stronger task-set for the target or suppression of the cue features (i.e., a white onset). Therefore, we presented the color target together with a single white nontarget, and indeed found that the rare onset cues failed to capture (Experiment 2). Moreover, we examined the previous suggestion that frequency effects are limited to onset cues by presenting rare color cues with nonmatching color targets (Experiment 3). We observed capture by rare color cues, but again, capture could be prevented when a nontarget in the cue color was added to the target display (Experiment 4). Overall, these findings suggest that capture by rare cues can be prevented when the suppressive bias against nontarget features is optimally directed against the cue properties.