Audiovisual Attentional Capture ( P 409 ) Attentional Capture in Serial Audiovisual Search Tasks

The phenomenon of attentional capture has typically been studied in spatial search tasks. Dalton and Lavie (2004) recently demonstrated that auditory attention can also be captured by a singleton item in a rapidly-presented tone sequence. In the experiments reported here, we investigated whether these findings extend crossmodally to sequential search tasks using audiovisual stimuli. Participants searched a stream of centrallypresented audiovisual stimuli for targets defined on a particular dimension (e.g. duration) in a particular modality. Task performance was compared in the presence versus absence of a unique singleton distractor presented. Irrelevant auditory singletons captured attention during visual search tasks, leading to interference when they coincided with distractors but to facilitation when they coincided with targets. These results demonstrate attentional capture by auditory singletons during nonspatial visual search. Audiovisual Attentional Capture (P409) 3 Attentional Capture in Serial Audiovisual Search Tasks The ability to focus attention selectively on certain stimuli at the expense of others is important for everyday cognitive functioning. However, it is also important that focused attention can be interrupted by stimuli that are likely to signal an unexpected (and potentially important) change in the environment. The phenomenon of attentional capture, in which focused attention is interrupted by the presence of certain types of distracting stimuli, has been the focus of much unimodal visual and auditory attention research (e.g. Dalton & Lavie, 2004; Folk, Leber, & Egeth, 2002; Jonides & Yantis, 1988; Theeuwes, 1992). However, studies looking at attentional capture between different sensory modalities are much less common and, when reported, have focused exclusively on spatial capture (or cuing) effects (see Spence, 2001, for a review). The present study investigates the possibility of non-spatial attentional capture between vision and hearing using sequential search tasks. Until recently, attentional capture research tended to focus on the effect as it occurred within spatial visual search arrays. In a typical experimental design, participants would be asked to search an array of spatially-distributed visual items for a target defined on a particular dimension (e.g. with a particular shape). The now well-established finding is that attention can be captured during this type of spatial visual search by unique ‘singletons’ (e.g. items with a unique colour, or items that appear with an abrupt-onset when the other items onset gradually), even when these singletons are unique on a dimension that is never relevant to the participant’s task (e.g. Jonides & Yantis, 1988; Pashler, 1988; Theeuwes, 1992). (Note 1). Audiovisual Attentional Capture (P409) 4 However, more recently, researchers have begun to ask whether attentional capture can also occur during sequential search tasks, in which items are presented one after another. Research on the attentional blink (AB) has demonstrated that allocation of attention to a first target item in a rapidly-presented stream of stimuli can impair processing of subsequent targets in the stream (e.g. Raymond, Shapiro, & Arnell, 1992). However, because participants in these studies are explicitly asked to attend to the first target, the results of AB experiments are not directly relevant with regard to the possibility of attentional capture by task-irrelevant items in sequential streams. Nevertheless, there is evidence that, under some circumstances, unique yet task-irrelevant items in the AB design (e.g. a colored box around a non-target letter) can interfere with the recall of a subsequent target, suggesting that the singleton item may have attracted attention despite its being irrelevant to the participants’ task (e.g. Chun, 1997; Folk, Leber, & Egeth, 2001; Maki & Mebane, in press; Wee & Chua, 2004). Note, however, that reliable effects in these studies were only elicited by singleton items that were defined on the same dimension as the targets (e.g. they were both color singletons). The results may therefore reflect deliberate attentional allocation toward these singletons because they possess a task-relevant feature. By contrast, Dalton and Lavie (in press) recently showed attentional capture during an RSVP size discrimination task by singletons defined on the task-irrelevant dimension of color, demonstrating that visual attentional capture by task-irrelevant singletons can be found in sequential search arrays. Because this kind of attentional capture involves attentional allocation toward an item that is separated in time (rather than in space) from the other search items, we have termed it temporal attentional Audiovisual Attentional Capture (P409) 5 capture, to distinguish it from spatial versions of the effect. Similar findings have also emerged from studies of audition. For example, Dalton and Lavie (2004) found temporal attentional capture by irrelevant auditory feature singletons in auditory detection and discrimination tasks using sequentially-presented stimuli. Thus there is evidence from unimodal visual and auditory studies that irrelevant singletons can capture attention during sequential search tasks. The present study investigated the possibility of crossmodal attentional capture by irrelevant singletons in sequential audiovisual search tasks. Although this possibility has not yet been addressed, previous research has demonstrated that, under certain conditions, the allocation of attention to an item in one modality can impair the processing of a subsequent item presented in a different modality. For example, some crossmodal AB studies have suggested that attentional allocation to a stimulus in one sensory modality can lead to an AB for a stimulus in a different modality (e.g. Arnell & Jenkins, 2004; Arnell & Jolicœur, 1999; Arnell & Larson, 2002; Soto-Faraco et al., 2002). Note, however, that the evidence in this area is fairly mixed, with other studies finding no evidence to support the existence of a crossmodal AB (e.g. Duncan, Martens & Ward, 1997; Potter, Chun, Banks, & Muckenhoupt, 1998; Soto-Faraco & Spence, 2002; see also Arnell & Duncan, 2002). In any case, as the AB research assesses the consequences of the deliberate allocation of attention to sequentially-presented targets, it cannot speak to the current issue of attentional capture by task-irrelevant singletons. There is, however, further evidence to suggest that the presentation of auditory deviants can interfere with behavioural performance in a subsequent visual task. For example, Escera, Alho, Winkler, and Näätänen (1998) showed that responses in a visual Audiovisual Attentional Capture (P409) 6 (odd vs. even number) discrimination task were less accurate when the number stimulus followed a sound of deviant frequency than when it followed a standard frequency sound. Similar effects have also been found following the presentation of intensity and duration deviants (e.g. Escera, Corral, & Yago, 2002). However, it is important to note that the auditory stimuli used in these studies predicted the target event, and the slow rates of stimulus presentation used mean that the overall demands on attentional resources were low. Taken together, these factors mean that there was little incentive for participants to attempt to ignore the auditory distractors in these studies (indeed, the fact that the deviants tended to predict the target should, if anything, have encouraged the participants to pay attention to them) and as such the results may be attributable to voluntary allocation of attention to the auditory stimuli, rather than to involuntary capture of attention by those stimuli. There is also evidence that auditory deviants can cause facilitation in simultaneous visual tasks. For example, participants in a study by Vroomen and de Gelder (2000; Experiment 1) had to search a rapidly-presented repeating sequence of four masked dot patterns for a target pattern (four dots arranged in a diamond shape) and indicate the corner of the screen in which the target diamond appeared. On half of the trials, each visual pattern was accompanied by a standard low frequency tone. On the other half of the trials, non-target visual patterns were accompanied by the standard whereas the visual target was accompanied by a high frequency tone. Responses were quicker and more accurate in the latter condition as compared with the condition in which the target was accompanied by the same frequency tone as the non-targets. This finding suggests that auditory perceptual organisation processes (which would have been Audiovisual Attentional Capture (P409) 7 expected to group the ‘odd-one-out’ high frequency tone separately from the lower standard tones) influenced visual perception, facilitating the processing of the visual stimulus that occurred at the same time as the high frequency tone (see also Watanabe & Shimojo, 2001, for a similar demonstration involving ambiguous visual motion judgments; Spence, Sanabria & Soto-Faraco, in press, for a review). However, the ‘oddone-out’ auditory tones that facilitated visual perception in these experiments always coincided with the visual target. Thus there was no incentive for participants to ignore the singleton stimuli (and in fact there may have been good reason for participants to attend to them). These results therefore cannot be characterised in terms of involuntary attentional capture by the singletons. They are nevertheless promising in suggesting that the auditory singletons in the present study might be expected to affect visual discrimination performance. The experiments reported here used sequential search tasks in which several audiovisual stimuli were presented one after another from the same location to form a search stream. In Experiment 1, we assessed whether auditory duration singletons (i.e., stimuli that are unique on the dimension of duration) would