VISUAL COGNITION, 2006, 14 (4/5/6/7/8), 863 876
Frontal control of attentional capture in visual search Nilli Lavie Department of Psychology, University College London, UK
Jan de Fockert Department of Psychology, Goldsmiths College, University of London, UK
Lavie and colleagues recently suggested that cognitive control functions that are mediated by frontal cortex provide goal-directed control of selective attention, serving to minimize interference by goal-irrelevant distractors. Here we provide new evidence for this claim from an attentional capture paradigm. An event-related fMRI experiment shows that the presence (vs. absence) of an irrelevant colour singleton distractor in a visual search task was not only associated with activity in superior parietal cortex, in line with a psychological attentional capture account, but was also associated with frontal cortex activity. Moreover, behavioural interference by the singleton was negatively correlated with frontal activity, suggesting that frontal cortex is involved in control of singleton interference. Behavioural tests confirmed that singleton interference depends on availability of cognitive control to the search task: Singleton interference was significantly increased by high working memory load. These results demonstrate the important role of frontal cognitive control of attention by working memory in minimizing distraction.
Focused goal-directed behaviour depends on top-down control of attention, so that attention is allocated to stimuli in accordance with current priorities. Lavie and colleagues (de Fockert, Rees, Frith, & Lavie, 2001; Lavie, 2000; Lavie, Hirst, de Fockert, & Viding, 2004) have recently suggested that cognitive control functions mediated by the frontal lobe, such as working memory, provide such goal-directed control of visual selective attention. Specifically, Lavie and colleagues proposed that working memory serves to actively maintain current processing priorities during task performance, and thus that availability of working memory for a selective attention task is critical for minimizing interference by goal-irrelevant distractors. Please address all correspondence to Nilli Lavie, Department of Psychology, University College London, Gower St, London WC1E 6BT, UK. E-mail:
[email protected] This work was supported by a Medical Research Council grant to the first author. http://www.psypress.com/viscog
# 2006 Psychology Press Ltd DOI: 10.1080/13506280500195953
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In support of this hypothesis, Lavie (2000; Lavie et al., 2004) showed that response competition effects produced by an irrelevant distractor in a selective attention task (consisting of a central target letter and a flanking distractor letter) are increased under conditions of high working memory load, when subjects were required to rehearse a set of six digits during performance of the selective attention task, compared to conditions of low working memory load when subjects had to rehearse just one digit or none. Moreover, in an fMRI study, de Fockert et al. (2001) manipulated working memory load during a Stroop-like selective attention task in which subjects were required to classify famous names while ignoring distractor faces (that could be either response-congruent with the target name, i.e., the face of the person named, or incongruent with the target, i.e., a face of a person from an opposite category). They found that working memory load was not only associated with increased activity in frontal cortex (a main effect), but also with increased activity related to the presence (vs. absence) of a distractor face in visual cortex (an interaction of working memory load and distractor face conditions), and with greater distractor interference effects on behaviour. However, although the convergence of behavioural findings with neuroimaging results seems to make an appealing case for the role of frontal cognitive control functions, such as working memory, in preventing interference by goal-irrelevant distractors, the evidence described so far was confined to flanker and Stroop-like tasks. In this paper we present new evidence from functional imaging and behavioural experiments using the paradigm of attentional capture in visual search, in support of the claim that the extent to which an irrelevant singleton distractor captures attention and produces interference effects on visual search performance depends on the availability of frontal control functions to the search task. Previous visual search studies have demonstrated that the presence of an irrelevant distractor with a unique feature that makes it a singleton in the visual search display (e.g., an irrelevant red distractor among green objects) will typically distract attention from focusing on the search target, producing the phenomenon of attentional capture (see Folk & Remington, 2006 this issue; Theeuwes, Reimann, & Mortier, 2006 this issue; Yantis, 2000). Such interruption of goal-driven attention is found even when the distractor object forms a singleton on a dimension that is never relevant to the task (e.g., a colour singleton will interfere with search on the basis of other features, such as search for a curved target among angular shapes), suggesting that the singleton distractor has captured attention, rather than that attention was allocated to the distractor at will (e.g., Theeuwes, 1996).
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FUNCTIONAL IMAGING STUDY The behavioural studies on attentional capture have led us to the following hypotheses about the neural correlates of attentional capture in visual search. First, as the singleton distractor attracts attention, we hypothesized that neural systems known to be involved in the allocation of attention to goal-relevant stimuli may also be associated with attentional capture by goalirrelevant singleton distractors. Specifically, activity in parietal cortex has been previously associated with the allocation of attention in a wide variety of tasks, including visual search (Corbetta, Shulman, Miezin, & Petersen, 1995) and spatial cueing (e.g., Corbetta, Miezin, Shulman, & Petersen, 1993; for reviews see Corbetta & Shulman, 2002; Wojciulik & Kanwisher, 1999). We therefore expected that capture of attention by an irrelevant singleton distractor during visual search would also be associated with parietal activity. Second, because attention was captured by a goal-irrelevant distractor, the competition between the target and the irrelevant singleton distractor was expected to impose a greater demand on top-down control mechanisms typically associated with the frontal lobe in order to resolve the competition (for review see Duncan & Owen, 2000). We thus anticipated that attentional capture by an irrelevant singleton would also implicate activity in frontal cortices associated with such top-down control. We tested these hypotheses in an fMRI study (de Fockert, Rees, Frith, & Lavie 2004), in which we scanned subjects during performance in Theeuwes’s (1991, 1992) task of visual search in the presence of an irrelevant colour singleton.
Method Subjects. Ten young adults with normal vision and normal colour vision gave informed consent and participated in the study. Stimuli and procedure. Subjects searched for a unique shape target (circle) among distractors of a different shape (diamonds) and were required to indicate the orientation (horizontal or vertical) of the line segment in the target by a speeded key press (see Figure 1A). We compared search performance and brain activity between conditions of presence and absence of an irrelevant colour singleton distractor. Since these conditions also vary the presence or absence of an odd colour in the array, we added another condition in which the colour singleton was present on the target shape. Comparing activity in the presence (vs. absence) of a colour singleton distractor to activity in the presence (vs. absence) of a colour singleton target (i.e., an interaction) would allow us to identify specifically the neural correlates of attentional capture by an irrelevant distractor. In order to produce a factorial design (as required for examining the interaction), we
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(A) Trial sequence. The example shows a colour singleton distractor present display and a colour singleton distractor absent display. (B) Colour singleton present (top row) and absent (bottom row) conditions were combined with distractor singleton (left column) and target singleton (right column) conditions to produce four display types. The colour singleton absent conditions included a reduced size (by 20%) singleton on the target or distractor. These displays, together with null events (fixation), were presented in equal proportions, in a random order during the experiment. In the figure, red colour singletons are printed in grey.
assigned colour singleton absent trials to a distractor condition or a target condition by presenting either one of the distractors or the target with a nonsalient but noticeable reduced size (by 20%) singleton, to produce colour singleton distractor absent or colour singleton target absent conditions, respectively (Figure 1B). In a behavioural pilot experiment we established that these size singleton distractors indeed do not produce any interference effects on response times (RTs) or errors. One fifth of all trials were null events, on which the fixation point was presented for the duration of a trial. Data acquisition and analysis. A 2T Siemens VISION system was used to acquire both T1 anatomical volume images and T2*-weighted echoplanar (EPI) images with blood oxygenation level dependent (BOLD) contrast. Each participant completed two blocks of 240 trials, chosen at random from the five trial categories, while being scanned. The MRI data acquired during these two blocks consisted of 216 volumes in each block, of which the first six volumes per run were discarded to allow for T1 equilibration effects. Volumes were acquired continuously with an effective repetition time (TR) of 2.4 s/volume. Prior to the scanning sessions, participants completed a practice block consisting of 20 trials. Statistical Parametric Mapping (SPM99; Wellcome Department of Imaging Neuroscience, University College London) was used for temporal
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and spatial data preprocessing, and data analysis. Data were time-corrected for slice acquisition times (using the middle slice as a reference). All volumes were then realigned to the first volume, and normalized to a standard EPI template volume (based on the MNI reference brain; Cocosco, Kollokian, Kwan, & Evans, 1997) in the space of Talairach and Tournoux (1988). These EPI volumes were then smoothed with an isotropic 10-mm FWHM Gaussian kernel. Statistical results were based on a fixed effects model with a pB/ .05 height threshold (corrected for the whole brain volume examined) for report of regions of significant activation. For the interaction contrast, we used a small-volume correction with spheres of 10 mm radius around the peak voxels in the areas of significant activity related to the simple main effect (presence vs. absence of colour singleton distractors). For the ANOVA with subjects as the random factor, the BOLD signal was extracted for all voxels contained within the three clusters of significant activation (at t / 4.54, corresponding to p B/ .05, corrected for multiple comparisons) in the presence (vs. absence) of distractor singletons. Next, for each participant, the averages BOLD signal across all voxels in each of the three clusters was entered into ANOVAs with colour singleton presence (present, absent) and singleton stimulus (target, distractor) as within-subjects factors and participants as the random factor.
Results and discussion Behavioural responses. The behavioural data collected during the scanning sessions showed that the presence of a colour singleton distractor produced significant interference (M /809 ms and 10% errors, for colour singleton distractor present trials compared with M /713 ms and 6% errors for colour singleton distractor absent trials, with a size distractor present instead), F (1, 9) /38.4, pB/ .001 for the RTs. This result is consistent with previous behavioural findings in similar visual search studies of attentional capture (e.g., Theeuwes, 1991, 1992, 1994). Interestingly, the size of the attentional capture effect is larger than the attentional capture effects in previous experiments using this task (these average less than 20 ms in Theeuwes’s, 1992, experiments) and appears in the range of attentional capture effects found in tasks in which on some trials or on some blocks the singleton feature is used to define the target (Theeuwes, 1991). The greater interference effect in our experiment is therefore likely to be due to the fact that we incorporated trials in which the target was also presented with the colour singleton. The presence of a colour singleton on the target only led to a small and nonsignificant trend for facilitation (M /730, 7% errors for colour singleton target present trials compared with M /739, 7% errors for colour target singleton absent trials, with a size singleton target instead),
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F B/1 for the RTs. This may be attributed to a floor effect, given that the target was a shape singleton so could pop out without the additional colour singleton on it. Imaging data. Brain activity time-locked to the individual trials was determined using an event-related analysis of the fMRI data. Figure 2 shows that activity associated with the presence (vs. absence) of a colour singleton distractor was found in bilateral superior parietal cortex (Brodmann area 7) and in left lateral precentral gyrus (BA 6) of the frontal cortex. Moreover, parietal and frontal cortices also showed significant interactions, such that activity in the presence (vs. absence) of a colour singleton distractor was greater than activity in the presence (vs. absence) of a colour singleton target. These interactions were consistent across subjects and were replicated in an ANOVA with participants as the random factor (see Figure 3). These findings confirm that activity in parietal and frontal cortices related to the presence (vs. absence) of a colour singleton distractor could not be attributed to the mere presence of an odd colour in the array. Parietal activation. As superior parietal cortex has been previously associated with spatial shifts of attention (see Corbetta & Shulman, 2002, for review), the present findings suggest that spatial attention was allocated to the singleton distractor, consistent with an attentional capture account of the behavioural interference effects.1 Specifically, many previous behavioural studies have shown that capture of attention by an irrelevant singleton involves spatial shifts of attention to the singleton position (e.g., Yantis & Jonides, 1990; see Yantis, 2000, for review). Although it has been shown that attentional set can under some conditions eliminate spatial cueing effects (e.g., Folk, Remington, & Johnston, 1992), this has only been shown in tasks that are very different from the task used in the present study (e.g., the singleton is presented before the search array and thus does not directly compete with the target). 1
It is worth noting that, although serial spatial shifts of attention may not be required for the search process in this feature-search task (Treisman, 1988), shifts of focused attention to the target position are required for the orientation discrimination aspect of this task (in order to discriminate the orientation of the small line (0.58 of visual angle) within the target shape, among the competing orientations in the nontarget shapes). Thus, in the absence of a singleton distractor, although the target will initially pop out, focused attention will be shifted to it in order to perform the orientation discrimination task. When the singleton distractor is present, however, it will pop out more readily than the target (due to its greater salience, see Theeuwes, 1992), and thus may be wrongly selected for a spatial shift of attention. Thus, the presence of a singleton distractor should involve an extra shift of spatial attention (as attention has to be shifted once more from the distractor to the target).
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Figure 2. Activity related to the presence (vs. absence) of a colour singleton distractor. Shown are left lateral (left panel) and dorsal (right panel) views of a T1-weighted anatomical template image in Talairach space (Talairach & Tournoux, 1988). For display purposes, activity is shown at pB/ .001, uncorrected, with an extent threshold of 200 voxels. Areas of significant activation (at pB/ .05, corrected) were left superior parietal cortex (peak activation: /24, /66, 50, t / 5.85, pB/ .001), right superior parietal cortex (peak activation: 26, /68, 50, t / 4.67, pB/ .030), and left lateral precentral gyrus (peak activation: /46, 4, 36, t / 4.79, pB/ .018).
It is important to note that although the superior parietal cortex has been associated with both covert shifts of attention (that do not involve eye movements) and overt shifts of attention (that involve eye movements as well, see Corbetta, 1998; Corbetta et al., 1998), the activations found in the present study cannot be attributed to eye movements since eye position was monitored during scanning, and eye position results confirmed that there were no significant differences in eye position between any of the experimental conditions: Fixation was consistently maintained within two 0.07
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Activity associated with the interaction between colour singleton presence (present, absent) and singleton stimulus (distractor, target). Bars represent BOLD signal change, averaged across voxels in each cluster, and across participants. Shown is the difference in mean activity between colour singleton present vs. absent, plotted separately for left superior parietal cortex (L SPL), right superior parietal cortex (R SPL), and left lateral precentral gyrus, and for distractor and target singletons. Error bars represent interparticipant standard error.
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degrees of fixation, less than the eccentricity of the search array (which subtended 3.18 from fixation to the centre of each display item). Frontal activation. The frontal activity found is in line with our expectation that target selection in the presence of a competing, attentioncapturing, singleton distractor would place a greater demand on top-down frontal control functions, as these are needed to resolve the competition between the target and capturing singleton distractor. Indeed, activity in BA 6 has often been implicated in attentional selection for action (e.g., Deiber et al., 1991; van Oostende, van Hecke, Sunaert, Nuttin, & Marchal, 1997) and in competition for responses in Stroop-like tasks (e.g., Hazeltine, Bunge, Scanlon, & Gabrieli, 2003; Schumacher & D’Esposito, 2002; Zysset, Mu¨ller, Lohmann, & von Cramon, 2001). Moreover, we found a significant negative correlation between activity in left frontal cortex (percentage signal change in the presence vs. absence of singleton distractor calculated as a proportion of the average fMRI signal per each individual) and the magnitude of the interference effect in RT (also calculated as a proportion of the average RT per individual), r / /.712, p / .021 (two-tailed). Whereas activity in the two clusters of activation in the bilateral superior parietal cortex showed no significant correlation with the RT effect, r /.247, p / .49, and r / /.103, p / .78 for the left and right superior parietal cortex, respectively, there was a significant negative correlation between activity in the left frontal cortex and the magnitude of the RT interference effect: Greater activity in frontal cortex (when a colour distractor was present vs. absent) was associated with smaller interference effects by the irrelevant distractor. This finding suggests that top down control functions mediated by frontal cortex serve to control against interference by irrelevant distractors. We discuss this suggestion in greater detail below. Contrast between parietal role and frontal role in attentional capture. In contrast with the strong negative correlation between the signal in frontal cortex and magnitude of behavioural interference, there was no significant correlation between activity in superior parietal cortex and behavioural interference. This contrast may indicate that these structures serve different functions in attentional capture. The activity in superior parietal cortex may reflect stimulus-driven shifts of attention towards the irrelevant distractor, as the irrelevant colour singleton we used was more salient than the shape target (see Theeuwes, 1996). As such, attention may always be captured by the more salient distractor, with little variation in the extent of attentional shifts and the strength of the associated signal in superior parietal cortex, thus precluding any correlation with behavioural interference effects. The extent to which the irrelevant singleton distractor (that has nevertheless
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captured spatial attention) will produce interference on behaviour, however, may be determined by the extent to which frontal cortex exerts a strong or weak top-down control signal (in order to resolve the competition between the target and the capturing distractor). Functional imaging conclusions. We conclude that the neural correlates of attentional capture by an irrelevant singleton during visual search are in parietal and frontal cortex. The superior parietal activity suggests allocation of spatial attention to the singleton distractor, in line with an attentional capture account for the singleton interference effects. The finding of a negative correlation between frontal activity and behavioural interference suggests a role for frontal cortex in control of interference by the irrelevant (yet attentional capturing) singleton. Top-down control of attentional capture by working memory: Behavioural tests. The suggestion of the correlation between fMRI signal and behaviour that attentional capture by an irrelevant singleton is controlled by frontal cortex is limited by the fact that correlations cannot inform about any causal role. We therefore examined the role of frontal cognitive control functions in attentional capture in new behavioural experiments (Lavie & de Fockert, 2005). As discussed in the introduction, Lavie and colleagues (e.g., Lavie et al., 2004) have recently suggested that top-down control of selective attention by frontal cortex involves active maintenance of priorities between goalrelevant targets and goal-irrelevant distractors in working memory throughout task performance. Importantly, the frontal areas involved in such control in de Fockert et al.’s (2001) study (i.e., areas associated with the main effect of working memory load) included the frontal area implicated in control against singleton distractor interference in the present study. However, these previous studies assessed distractor interference effects within a Stroop-like task. The hypothesis that working memory serves to provide goal-directed control of visual attention, allowing to prevent interference by goalirrelevant distractors, suggests that the extent to which goal-irrelevant singletons will capture attention, and thus interfere with search for the goalrelevant target, should also depend on the availability of working memory to control goal-directed performance of the visual search task. We therefore predicted that high working memory load during performance of a visual search task in the presence of an irrelevant singleton would result in greater attentional capture by the irrelevant singleton compared with no, or low, working memory load. We tested these predictions in experiments in which we interleaved a task of attentional capture in visual search (based on Theeuwes’, 1992, study) with a working memory task. The visual search task was very similar to that used in the imaging study except for the following changes: We did not
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include a condition of colour singleton on the shape target; instead a colour singleton distractor was present on 50% of the trials and absent on the remaining 50% of trials. We did not include a size singleton in the singleton absent condition conditions and we presented displays for 200 ms in order to prevent eye movements during search. High load on working memory was manipulated by requesting subjects to rehearse a set of six digits (Experiment 1) or four digits in exact order (Experiment 2) in order to recognize whether a memory probe following the visual search task was present or absent in the memory set of that trial low load
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Examples of a low load (left panel) and high load (right panel) trial. The memory set always consisted of the digits 0, 1, 2, 3, 4, which were either presented in sequential order on each trial (low working memory load), or in a different random order on each trial (high load). The digit 0 always occurred at the start of each memory set to ensure that both conditions of load had four possible memory probe responses (1, 2, 3, 4). In the visual search task, the target and all the nontarget shapes were green (white in the figure), except for the singleton distractor, which was presented in red (grey in the figure). This example shows colour singleton present displays. On half the trials (picked up at random) the singleton was absent.
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(Experiment 1) or in order to recall a digit that followed the probe digit in the memory set (Experiment 2). The interference effects produced by a colour singleton in the visual search task were compared between these high load conditions and a no load condition in which subjects did not perform the working memory task (Experiment 1), or a low load condition in which the memory set was always in the same order (the digit sequence ‘‘0, 1, 2, 3, 4’’ was always presented as the memory set; Experiment 2) (see Figure 4). Our hypothesis that the interference on visual search by an irrelevant singleton distractor depends on availability of working memory to control performance in the visual search task lead to the prediction of greater singleton interference effects in conditions of high working memory load than in conditions with no or low working memory load. The results provided support for this prediction: both in Experiment 1 and Experiment 2 we found that singleton interference effects were significantly greater (at p B/ .02) in conditions of high working memory load (mean singleton interference effects were 51 ms in Experiment 1 and 88 ms in Experiment 2) compared to no working memory load (the mean interference effect under no load was 15 ms in Experiment 1) or low working memory load (the mean interference effect under low load was 52 ms in Experiment 2). These results provide support for the hypothesis that attentional capture by task-irrelevant singletons in a visual search task depends on the availability of working memory for the search task.
GENERAL DISCUSSION The present experiments demonstrate that the extent to which an irrelevant colour singleton interferes with performance in a shaped-based visual search task depends on top-down cognitive control functions that are known to be mediated by frontal cortex. Our functional imaging study revealed that the presence (vs. absence) of such colour singleton distractors in the visual search display is not only associated with neural activity in bilateral superior parietal cortex, in line with the idea that the singleton captured spatial attention, but was also associated with activity in left precentral gyrus of the frontal cortex. Moreover, the strength of the neural signal in left frontal cortex was negatively correlated with the magnitude of singleton interference effects on behaviour. This finding suggests that frontal cortex is involved in control of interference by an irrelevant singleton distractor. An important cognitive control function that is mediated by frontal cortex is working memory. Behavioural tests confirmed a causal role for working memory in the control of singleton interference effects on visual search. Singleton interference on visual search was significantly increased under conditions of high working memory load compared with conditions of
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low or no working memory load. These experiments provide direct support for the hypothesis that goal-directed control of visual selective attention is mediated by frontal cognitive control functions such as working memory. The present findings may also enhance our understanding of the mechanism of attentional capture, specifically whether attentional capture is purely stimulus driven or may also be subject to top-down control. The finding that singleton interference is negatively correlated with neural signal strength in frontal cortex and is modulated by working memory load suggests that capture by the singleton is subject to top-down control. The finding, however, that an irrelevant colour singleton always captured attention in our task, and produced interference on behaviour and activity in superior parietal cortex (an area known to mediate spatial shifts of attention) even under conditions of low or no working memory load, when top-down control functions are fully available to the task, points to a stimulus-driven component of attentional capture. The feature differences we used for the target and singleton distractor meant that the irrelevant colour singleton was more salient than the shape target. Under these conditions, the singleton disrupted visual search despite being goalirrelevant, in line with stimulus-driven accounts for attentional capture (e.g., Theeuwes, 1996). It is important to note, however, that the stimulusdriven component of capture is dictated by the relative stimulus salience not the exact feature used. Thus it is highly likely that presenting a singleton with a very distinct shape or size differences during search for a target defined by a less distinct colour difference would produce similar effects of capture, with similar activations and similar effects of working memory load to those currently found. This could be an interesting question for further study. Regarding the role of top-down cognitive control of attentional capture, it is important to note that the present results converge on the same conclusion as that made following previous experiments using Stroop-like tasks (e.g., de Fockert et al., 2001; Lavie, 2000; Lavie et al., 2004). Since Stroop-like tasks measure distractor interference via effects of response-congruency on target RTs, whereas attentional capture in visual search is measured via the RT cost produced by the presence (vs. absence) of an irrelevant singleton, the convergence of these different measures on the same conclusion suggests a general role for frontal cognitive control functions in goal-directed control of visual selective attention.
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