Attention capture by contour onsets and offsets: No special role for onsets

In five experiments, we investigated the power of targets defined by the onset or offset of one of an object’s parts (contour onsets and offsets) either to guide or to capture visual attention. In Experiment 1, search for a single contour onset target was compared with search for a single contour offset target against a static background of distractors; no difference was found between the efficiency with which each could be detected. In Experiment 2, onsets and offsets were compared for automatic attention capture, when both occurred simultaneously. Unlike in previous studies, the effects of overall luminance change, new-object creation, and number of onset and offset items were controlled. It was found that contour onset and offset items captured attention equally well. However, display size effects on both target types were also apparent. Such effects may have been due to competition for selection between multiple onset and offset stimuli. In Experiments 3 and 4, single onset and offset stimuli were presented simultaneously and pitted directly against one another among a background of static distractors. In Experiment 3, we examined “guided search,” for a target that was formed either from an onset or from an offset among static items. In Experiment 4, the onsets and offsets were uncorrelated with the target location. Similar results occurred in both experiments: target onsets and offsets were detected more efficiently than static stimuli which needed serial search; there remained effects of display size on performance; but there was still no advantage for onsets. In Experiment 5, we examined automatic attention capture by single onset and offset stimuli presented individually among static distractors. Again, there was no advantage for onset over offset targets and a display size effect was also present. These results suggest that, both in isolation and in competition, onsets that do not form new objects neither guide nor gain automatic attention more efficiently than offsets. In addition, in contrast to previous studies in which onsets formed new objects, contour onsets and offsets did not reliably capture attention automatically.     

Contingent capture for onsets and offsets: attentional set for perceptual transients

Four experiments were conducted to examine whether attentional set affects the ability of visual transients (onsets and offsets) to capture attention. In the experiments, visual search for an identity-defined target was conducted. In the first 3 experiments, the target display either onset entirely or was revealed by offsetting camouflaging line segments to reveal letters. Prior to the target display, there was a noninformative cue, either an onset or an offset, at one of the potential target locations. Cues that shared the same transient feature as the target display captured attention. The lack of predictable target transients led to attentional capture by all forms of transients. The final experiments with luminance changes without offsets or onsets showed attentional capture when the luminance changes were large. The results suggest that attentional set can be broadly or narrowly tuned to detect changes in luminance.     

Oculomotor consequences of abrupt object onsets and offsets: onsets dominate oculomotor capture

Previous research has shown that the appearance of an object (onset) and the disappearance of an object (offset) have the ability to influence the allocation of covert attention. To determine whether both onsets and offsets have the ability to influence eye movements, a series of experiments was conducted in which participants had to make goal-directed eye movements to a color singleton target in the presence of an irrelevant onset/offset. In accord with previous research, onsets had the ability to capture the eyes. The offset of an object demonstrated little or no ability to interrupt goal-directed eye movements to the target. Two experiments in which the effects of onsets and offsets on covert attention were examined suggest that offsets do not capture the eyes, because they have a lesser ability to capture covert attention than do onsets. A number of other studies that have shown strong effects of offsets on attention have used offsets that were uncorrelated with target position (i.e., nonpredictive), whereas we used onsets and offsets that never served as targets (i.e., antipredictive). The present results are consistent with a new-object theory of attentional capture in which onsets receive attentional priority over other types of changes in the visual environment.     

Actions modulate attentional capture

The purpose of the present study was to determine whether the action system plays a role in determining which stimulus characteristics capture attention by assessing the interference effects caused by dynamic discontinuities that have similar attention-capturing properties, but different action affordances. Results revealed that offset distractors caused significant interference when onset targets were presented in a keypress task, but did not increase reaction times when the same onset targets were presented in a goal-directed aiming task In contrast, onset distractors caused interference when offset targets were presented in both keypress and aiming tasks. This pattern of results suggests that the attentional set, and thus the properties of stimuli that capture attention, is modulated by the interaction between stimulus and response expectations.     

Actions modulate attentional capture

The purpose of the present study was to determine whether the action system plays a role in determining which stimulus characteristics capture attention by assessing the interference effects caused by dynamic discontinuities that have similar attention-capturing properties, but different action affordances. Results revealed that offset distractors caused significant interference when onset targets were presented in a keypress task, but did not increase reaction times when the same onset targets were presented in a goal-directed aiming task. In contrast, onset distractors caused interference when offset targets were presented in both keypress and aiming tasks. This pattern of results suggests that the attentional set, and thus the properties of stimuli that capture attention, is modulated by the interaction between stimulus and response expectations.     

Attentional capture by motion onsets is modulated by perceptual load

The onset of motion captures attention during visual search even if the motion is not task relevant, which suggests that motion onsets capture attention in a stimulus-driven manner. However, we have recently shown that stimulus-driven attentional capture by abruptly appearing objects is attenuated under conditions of high perceptual load. In the present study, we examined the influence of perceptual load on attentional capture by another type of dynamic stimulus: the onset of motion. Participants searched for a target letter through briefly presented low- and high-load displays. On each trial, two irrelevant flankers also appeared, one with a motion onset and one that was static. Flankers defined by a motion onset captured attention in the low-load but not in the high-load displays. This modulation of capture in high-load displays was not the result of overall lengthening of reaction times (RTs) in this condition, since search for a single low-contrast target lengthened RTs but did not influence capture. These results, together with those of previous studies, suggest that perceptual load can modulate attentional capture by dynamic stimuli.     

Attentional misguidance in visual search

Previous research has shown that a task-irrelevant sudden onset of an object will capture an observer’s visual attention or draw it to that object (e.g., Yantis & Jonides, 1984). However, further research has demonstrated the apparent inability of an object with a task-irrelevant but unique color or luminance to capture attention (Jonides & Yantis, 1988). In the experiments reported here, we reexplore the question of whether task-irrelevant properties other than sudden onset may capture attention. Our results suggest that uniquely colored or luminous objects, as well as salient though irrelevant boundaries, do not appear to capture attention. However, these irrelevant features do appear to serve as landmarks for a top-down search strategy which becomes increasingly likely with larger display set sizes. These findings are described in terms of stimulus-driven and goal-directed aspects of attentional control.     

Perceptual load modulates attentional capture by abrupt onsets

The abrupt appearance of a new object captures attention, even when the object is task irrelevant. These findings suggest that abrupt onsets capture attention in a stimulus-driven manner and are not susceptible to top-down influences on attentional control. However, previous studies examining the ability of abrupt onsets to capture attention have used search displays that lacked significant complexity. Because attention is a limited capacity mechanism, it is possible that increasing the complexity, or perceptual load, of the search arrays may modulate capture by abrupt onsets. We used a flanker task to examine the effect of perceptual load on attentional capture by abruptly appearing objects. Subjects searched for a target letter through low-load (set size=1) and high-load (set size=6) displays. On each trial, irrelevant flankers also appeared, one as an onset and the other as an offset. Onset flankers affected search in low-load but not high-load displays. This modulation of attentional capture was not caused by generalized slowing when subjects searched through high-load displays; search for a single perceptually degraded target slowed response times but did not affect attentional capture. These findings demonstrate that attentional capture by an abrupt onset is attenuated when people search through high-load scenes.     

Visual marking: a convergence of goal- and stimulus-driven processes during visual search

Watson and Humphreys (1997) proposed that visual marking is a goal-directed process that enhances visual search through the inhibition of old objects. In addition to the standard marking case with targets at new locations, included in Experiment 1 was a set of trials with targets always at old locations, as well as a set of trials with targets varying between new and old locations. The participants' performance when detecting the target at old locations was equivalent to their performance in the full-baseline condition when they knew the target would be at old locations, and was worse when the target appeared at old locations on 50% of the trials. Marking was observed when the target appeared at new locations. In Experiment 2, an offset paradigm was used to eliminate the influence of the salient abrupt-onset feature of the new objects. No significant benefits were found for targets at new locations in the absence of onsets at new locations. The results suggest that visual marking may be an attentional selection mechanism that significantly benefits visual search when (1) the observer has an appropriate search goal, (2) the goal necessitates inhibition of old objects, and (3) the new objects include a salient perceptual feature.     

The time course of attentional and oculomotor capture reveals a common cause

Eye movements are often misdirected toward a distractor when it appears abruptly, an effect known as oculomotor capture. Fundamental differences between eye movements and attention have led to questions about the relationship of oculomotor capture to the more general effect of sudden onsets on performance, known as attentional capture. This study explores that issue by examining the time course of eye movements and manual localization responses to targets in the presence of sudden-onset distractors. The results demonstrate that for both response types, the proportion of trials on which responses are erroneously directed to sudden onsets reflects the quality of information about the visual display at a given point in time. Oculomotor capture appears to be a specific instance of a more general attentional capture effect. Differences and similarities between the two types of capture can be explained by the critical idea that the quality of information about a visual display changes over time and that different response systems tend to access this information at different moments in time.     

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.     

Characteristics of covert and overt visual orienting: Evidence from attentional and oculomotor capture

Five visual search experiments found oculomotor and attentional capture consistent with predictions of contingent orienting, contrary to claims that oculomotor capture is purely stimulus driven. Separate saccade and attend-only conditions contained a color target appearing either singly, with an onset or color distractor, or both. In singleton mode, onsets produced oculomotor and attentional capture. In feature mode, capture was absent or greatly reduced, providing evidence for top-down modulation of both types of capture. Although attentional capture by color abstractors was present throughout, oculomotor capture by color occurred only when accompanied by transient change, providing evidence for a dissociation between oculomotor and attentional capture. Oculomotor and attentional capture appear to be mediated by top-down attentional control settings, but transient change may be necessary for oculomotor capture. ((c) 2003 APA, all rights reserved)     

Change blindness and the primacy of object appearance

A large body of work suggests that the visual system is particularly sensitive to the appearance of new objects. This is based partly on evidence from visual search studies showing that onsets capture attention whereas many other types of visual event do not. Recently, however, the notion that object onset has a special status in visual attention has been challenged. For instance, an object that looms toward an observer has also been shown to capture attention. In two experiments, we investigated whether onset receives processing priority over looming. Observers performed a change detection task in which one of the display objects either loomed or receded, or a new object appeared. Results showed that looming objects were more resistant to change blindness than receding objects. Crucially, however, the appearance of a new object was less susceptible to change blindness than both looming and receding. We argue that the visual system is particularly sensitive to object onsets.     

Modulation of oculomotor capture by abrupt onsets during attentionally demanding visual search

The influence of abrupt onsets on attentionally demanding visual search (i.e., search for a letter target among heterogeneous letter distractors), as indexed by performance and eye movement measures, was investigated in a series of studies. In Experiments 1 and 2 we examined whether onsets would capture the eyes when the appearance of an onset predicted neither the location nor the identity of the target. Subjects did direct their eyes to the abrupt onsets on a disproportionate number of trials in these studies. Interestingly, however, onset capture was modulated by subjects' scan strategies. Furthermore, onsets captured the eyes less frequently than would have been predicted by paradigms showing attentional capture when no eye movements are required. Experiment 3 examined the question of whether onsets would capture the eyes in a situation in which they never served as the target. Capture was observed in this study. However, the magnitude of capture effects was substantially diminished as compared to previous behavioural studies in which the onset had a chance probability of serving as the target. These data are discussed in terms of the influence of top-down constraints on stimulus-driven attentional and oculomotor capture by abrupt onset stimuli.     

Searching for stimulus-driven shifts of attention

Several types of dynamic cues (e.g., abrupt onsets, motion) draw attention in visual search tasks even when they are irrelevant. Although these stimuli appear to capture attention in a stimulus-driven fashion, typical visual search tasks might induce an intentional strategy to focus on dynamic events. Because observers can only begin their search when the search display suddenly appears, they might orient to any dynamic display change (Folk, Remington, & Johnston, 1992; Gibson & Kelsey, 1998). If so, the appearance of capture might result from task-induced biases rather than from the properties of the stimulus. In fact, such biases can even create the appearance of stimulus-driven capture by stimuli that typically do not capture attention (Gibson & Kelsey, 1998). The possibility of task-induced, topdown biases plagues the interpretation of all previous studies claiming stimulus-driven attention capture by dynamic stimuli. In two experiments, we attempt to eliminate potential task-induced biases by removing any need to monitor for display changes. In the first experiment, search displays did not change on most trials. In the second experiment, although new search displays appeared on each trial, we ensured that observers never saw the changes, by making them during large saccades. In both cases, dynamic events still received search priority, suggesting that some dynamic stimuli capture attention in a stimulus-driven fashion.     

Differential redundancy gain in onset detection versus offset detection

In a visual simple reaction time paradigm with attention divided between the left and right visual fields, redundancy gain refers to the finding of faster responses to stimuli presented in both fields than to single stimuli. The present study investigated whether the effects of low-level perceptual processing affect the redundancy gain by comparing the detection of onsets versus offsets. In different blocks, participants responded to left and right visual field stimuli that either appeared (onset) or disappeared (offset), with a single stimulus change in some trials and two redundant stimulus changes in others. With onset stimuli, the results replicated previous redundancy gain effects. In contrast, there was much less redundancy gain when participants responded to stimulus offsets. This finding suggests that redundancy gain is sensitive to low-level perceptual characteristics, such as onset versus offset presentation of stimuli.     

Better late than never: how onsets and offsets influence prior entry and exit

The three experiments presented in the paper examine visual prior entry (determining which of two stimuli appeared first) and prior exit (determining which of two stimuli disappeared first) effects with a temporal order judgment (TOJ) task. In addition to using onset and offset targets, the preceding cues also consisted of either onset or offset stimuli. Typical, and equivalent, prior entry effects were found when either onset or offset cues preceded the onset targets. Unexpectedly large prior exit effects where found with the offset targets, with offset cues producing greater capture effects than onset cues. These findings are consistent with the notion that more attention is allocated to searching the visual field when targets are more difficult to find. In addition, the results indicate that attentional control settings may be more likely to occur with more difficult searches. In addition, these findings demonstrate that TOJ tasks provide extremely precise measures of the allocation of attention and are very sensitive to a range of task manipulations.     

Attentional capture under high perceptual load

Attentional capture by abrupt onsets can be modulated by several factors, including the complexity, or perceptual load, of a scene. We have recently demonstrated that observers are less likely to be captured by abruptly appearing, task-irrelevant stimuli when they perform a search that is high, as opposed to low, in perceptual load (Cosman & Vecera, 2009), consistent with perceptual load theory. However, recent results indicate that onset frequency can influence stimulus-driven capture, with infrequent onsets capturing attention more often than did frequent onsets. Importantly, in our previous task, an abrupt onset was present on every trial, and consequently, attentional capture might have been affected by both onset frequency and perceptual load. In the present experiment, we examined whether onset frequency influences attentional capture under conditions of high perceptual load. When onsets were presented frequently, we replicated our earlier results; attentional capture by onsets was modulated under conditions of high perceptual load. Importantly, however, when onsets were presented infrequently, we observed robust capture effects. These results conflict with a strong form of load theory and, instead, suggest that exposure to the elements of a task (e.g., abrupt onsets) combines with high perceptual load to modulate attentional capture by task-irrelevant information.     

Suppression history of distractor location biases attentional and oculomotor control

Past selection experience greatly affects the deployment of attention such that targets are more readily selected if their features or locations were more frequently selected in the past. Crucially, recent studies have shown similar experience-dependent effects also for salient task irrelevant stimuli: distractors exerted less interference if they appeared at a location where they were presented more often, relatively to other possible locations. Here we investigated the effects of such suppression history on the immediate behavioural correlates of attentional deployment, i.e., eye movements. Participants were to make saccadic eye movements to a target stimulus, while ignoring a highly distracting irrelevant visual onset appearing abruptly on the screen in a proportion of trials. Crucially, this irrelevant onset occurred more frequently in two locations on the visual display and our results showed that, relatively to distractors elsewhere, onsets presented at these locations became easier to ignore, giving rise to reduced oculomotor capture. Consistent with the notion that experience can alter attentional deployment towards spatial locations, these findings indicate that, through learning, the priority of high frequency locations becomes suppressed, attenuating the intrinsic saliency of distractors appearing therein. Traces left by individual events of attentional suppression decrease the processing priority of coordinates within topographic maps of the visual space.     

Object onset and parvocellular guidance of attentional allocation

The parvocellular visual pathway in the primate brain is known to be involved with the processing of color. However, a subject of debate is whether an abrupt change in color, conveyed via this pathway, is capable of automatically attracting attention. It has been shown that the appearance of new objects defined solely by color is indeed capable of modulating attention. However, given evidence suggesting that the visual system is particularly sensitive to new onsets, it is unclear to what extent such results reflect effects of color change per se, rather than effects of object onset. We assessed attentional capture by color change that occurred as a result of either new objects appearing or already-present "old" objects changing color. Results showed that although new object onsets accrued attention, changing the color of old objects did not. We conclude that abrupt color change per se is not sufficient to capture attention.