Oregon State University, Corvallis, OR, USA b University of New Mexico, Albuquerque, NM, USA PLEASE SCROLL DOWN FOR ARTICLE

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1 This article was downloaded by: [National Cheng Kung University] On: 24 January 2 Access details: Access Details: [subscription number ] Publisher Psychology Press Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, 37-4 Mortimer Street, London WT 3JH, UK Visual Cognition Publication details, including instructions for authors and subscription information: Controlling spatial attention without central attentional resources: Evidence from event-related potentials Mei-Ching Lien a ; Khara Croswaite a ; Eric Ruthruff b a Oregon State University, Corvallis, OR, USA b University of New Mexico, Albuquerque, NM, USA First published on: 9 July 2 To cite this Article Lien, Mei-Ching, Croswaite, Khara and Ruthruff, Eric(2) 'Controlling spatial attention without central attentional resources: Evidence from event-related potentials', Visual Cognition, 9:, 37 78, First published on: 9 July 2 (ifirst) To link to this Article: DOI:.8/ URL: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 VISUAL COGNITION, 2, 9 (), 3778 Controlling spatial attention without central attentional resources: Evidence from event-related potentials Mei-Ching Lien and Khara Croswaite Oregon State University, Corvallis, OR, USA Eric Ruthruff University of New Mexico, Albuquerque, NM, USA The present study examined whether the control of spatial attention requires central attentional resources using a modified Psychological Refractory Period paradigm. We varied across experiments whether Task was a two- or four-choice speeded task and whether it was auditory or visual. Task 2 (unspeeded response) was to identify a visual letter in a specific target colour, while ignoring letters in other colours. We measured the N2pc effect (reflecting lateralized attentional allocation) elicited by Task 2 as a function of the stimulusonset asynchrony (SOA) between Task and Task 2. The question was whether spatial attention could shift to the Task 2 stimulus at short SOAs, while central attention was still allocated to Task. For the twochoice Task, Task 2 elicited a strong N2pc effect (indicating capture) even at short SOAs, regardless of whether Task was auditory (Experiment ) or visual (Experiment 2). But for the four-choice Task, the N2pc effect elicited by Task 2 was attenuated strongly at short SOAs, both for the visual Task (Experiment 3) and the auditory Task (Experiment 4). N2pc attenuation was also observed in Experiment 5, which mapped four auditory stimuli onto two responses for Task. This finding suggests that the attenuation is due to the difficulty of stimulus classification on Task, not the number of responses. Experiment 6 showed that the attenuation of N2pc effect on Task 2 was due to central operations on Task, not the mere presence of the Task stimulus. We propose that controlling spatial attention without central resources is possible, but the quality of attentional settings degrades if concurrent tasks impose a sufficiently great load on working memory (especially the load related to stimulus classifications). Keywords: Central attention; N2pc effect; Psychological refractory period; Spatial attention. Please address all correspondence to Mei-Ching Lien, Department of Psychology, Oregon State University, Corvallis, OR , USA. mei.lien@oregonstate.edu We thank Andrew Morgan at Oregon State University Enterprise Computing Service for providing technical support and building the customized response box for this study. We also thank Benoit Brisson, Charles Folk, and Robert McCann for their helpful comments on earlier drafts of this paper. # 2 Psychology Press, an imprint of the Taylor & Francis Group, an Informa business DOI:.8/

3 38 LIEN, CROSWAITE, RUTHRUFF In the course of everyday activities, we typically face an enormous number of visual stimuli at any given moment. Visual-spatial attention, also known as the mind s eye, is critical for selectively processing the most relevant visual stimuli. How do we control our spatial attention to optimize performance in the face of multiple stimuli and multiple tasks? In particular, can spatial attention be directed to other objects while our central attentional resources are already occupied by another stimulus? As a concrete example, would a road sign warning of icy conditions capture a driver s attention while the driver is already busy responding to the break light of the car slowing in front of him? Spatial attention has been assumed to influence early stages of perception such as detection and identification (e.g., Treisman & Gelade, 98), whereas central attention has been assumed to influence later central stages such as decision making and response selection (e.g., Pashler, 984). In addition, several studies have provided evidence that spatial attention and central attention operate independently and are mediated by different processing mechanisms (e.g., Johnston, McCann, & Remington, 995; McCann & Johnston, 992; Pashler, 989, 99). Thus, one might expect that spatial attention could be shifted to a stimulus even when central attention is already devoted to another stimulus for another task. On the one hand, there is behavioural evidence consistent with this claim (e.g., Pashler, 989, 99; Ruthruff, Lien, Johnston, & Adamic, 28). On the other hand, electrophysiological evidence presented by Brisson and Jolicœur (27a, 27b, 27c) suggests the opposite conclusion. We present here new evidence to resolve the controversy, using electrophysiological measures of spatial attention in a modified Psychological Refractory Period (PRP) paradigm. THE PSYCHOLOGICAL REFRACTORY PERIOD PARADIGM The PRP paradigm has been widely used to study whether particular mental processes require central attention, by determining whether this process can proceed in parallel with central operations on another task. In this paradigm, two distinct tasks (Task and Task 2) are separated by various stimulusonset asynchronies (SOAs). Participants are typically instructed to make speeded responses to both Task and Task 2, with the emphasis on In our study, we measured the N2pc effect with respect to target stimuli that participants intended to attend. We therefore deliberately use the term shift of spatial attention rather than attention capture in this paper to avoid the controversial implication that the shifts were involuntary. Note, however, that some researchers have argued that involuntary attention capture (both capture by salient but irrelevant objects and contingent capture) shares some underlying processing mechanism with voluntary deployment of attention (e.g., Hickey, McDonald, & Theeuwes, 26; Leblanc, Prime, & Jolicœur, 28).

4 CONTROLLING SPATIAL ATTENTION 39 Task. At long SOAs, the two tasks are performed more or less independently. At short SOAs, however, the two tasks compete for central attentional resources at the same time. The typical finding is that response time for Task (RT) is roughly constant across SOAs, but response time for Task 2 (RT2) increases as the SOA decreases. The latter is known as the PRP effect (Telford, 93). The PRP effect has been attributed to a central processing bottleneck*an inability to perform central operations, such as response selection and decision making, for two tasks simultaneously (see Figure ; for reviews, see e.g., Lien & Proctor, 22; Pashler & Johnston, 989; but see also Meyer & Kieras, 997; Tombu & Jolicœur, 23). In other words, the PRP effect occurs because central attentional resources are not available for Task 2 until central operations for Task have been completed. Because Task 2 central operations are postponed at short SOAs, this central bottleneck creates a period of cognitive slack between the perceptual and central stages of Task 2 (e.g., Pashler, 984; Schweickert, 978). Any Task 2 processes that do not require central resources can proceed during this slack period. SPATIAL ATTENTION VERSUS CENTRAL ATTENTION Although there is a consensus that people have difficulty carrying out central operations for two tasks simultaneously (structurally or strategically; for a review, see Lien & Proctor, 22), there is some evidence that spatial S SOA A 2A B Slack C 2B R 2C S2 R2 Figure. The temporal relations between processing stages of Tasks and 2 at a short SOA in the psychological refractory period paradigm, as suggested by the central bottleneck model. This model assumes that perceptual and response initiation/execution stages of Task 2 can operate in parallel with any stage of Task, but that central stages of Task 2 cannot start until central stages of Task have been completed. A, B, and C are the perceptual, central, and response initiation/execution stages of Task, respectively. 2A, 2B, and 2C are the corresponding stages for Task 2. S: Stimulus for Task ; S2: Stimulus for Task 2; R: Response for Task ; R2: Response for Task 2; SOA: Stimulusonset asynchrony.

5 4 LIEN, CROSWAITE, RUTHRUFF attention is not subject to this bottleneck (i.e., can proceed without central resources). In a PRP paradigm, Pashler (989, 99), had participants make a speeded response to a Task tone (high vs. low pitch) and an unspeeded response to a Task 2 target letter embedded within two rows of letters. The Task 2 target letter was defined by the presence of a simultaneous line (a probe) that appeared above or below the letter. The target display was presented briefly (2 ms initially, then adjusted between blocks based on the participant s accuracy), and then replaced with a mask display. Pashler reasoned that if shifts of spatial attention to the Task 2 stimulus are postponed while central resources are occupied by Task (see Figure 2b), one would expect a large decrement in Task 2 accuracy at short SOAs. On the other hand, if spatial attention can be allocated to the Task 2 stimulus without central resources, then the allocation should occur immediately at all SOAs (see Figure 2c). As a consequence, Task 2 identification accuracy should be just as high when the Task 2 probe appears shortly after Task onset (i.e., at short SOAs) as when it appears long after Task onset. His results were generally consistent with the latter prediction, leading Pashler to conclude that spatial attention to the Task 2 target was not delayed while central attentional resources were occupied by Task. Furthermore, Johnston et al. (995) provided evidence that spatial attention constitutes an early bottleneck, whereas central attention constitutes a late bottleneck. Although Johnston et al. did not actually manipulate both kinds of attention in the same experiment, and thus might have been unable to detect an interaction, their results add to the evidence for the separation of spatial attention and central attention. Taken together, these studies seem to suggest that spatial attention can be directed to a task even when central attention is occupied by another one (at least when it is a nonvisual task). Recently, Jolicœur and his colleagues (Jolicœur, Sessa, Dell Acqua, & Robitaille, 26a, 26b) pointed out that previous studies supporting the independence of spatial and central attentions used abrupt onsets. Capture by abrupt onsets is sometimes assumed to be stimulus driven, based entirely on stimulus salience rather than top-down goals (e.g., Theeuwes, 99, 994; Yantis & Jonides, 984, 99; but see also Folk, Remington, & Johnston, 992; Lien, Ruthruff, Goodin, & Remington, 28). Hence, it would seem to follow that capture by abrupt onsets would require no central attentional resources and would be unaffected by the presence of concurrent Task (but see Boot, Brockmole, & Simons, 25). Brisson and Jolicœur (27c) noted that although there was no SOA effect on Task 2 accuracy when abrupt onset (a probe) was used in Pashler s (99) Experiment, there was an SOA effect when colour was used as a guide for selecting the Task 2 target (a green target letter among coloured distractors) in Experiment 7. Thus, it is possible that salience-based capture (e.g., by abrupt onsets) is not subject to the

6 CONTROLLING SPATIAL ATTENTION 4 (a) A B C SOA 2A 2B 2C Spatial Attention Central Attention (b) (c) SOA SOA A B C 2A 2A 2B 2C Spatial Attention A B C Spatial Attention Central Attention 2A 2B 2C Central Attention Figure 2. Hypothesized temporal relationship between spatial attention and central attention in the Psychological Refractory Period paradigm. In this example, we assumed that a shift of spatial attention is required for Task 2 and the shift influences later substages of perceptual processing. We also assume that some features of objects (e.g., colour) can be extracted without spatial attention (e.g., Treisman & Gelade, 98). (a) The temporal relations between the processing stages of Task and Task 2 at a long SOA, where spatial attention is required for later perceptual processing and central attention is required for central operations. (b) The hypothesis that the shift of spatial attention for Task 2 (Stage A ) requires central resources and can proceed only after the central operations of Task have been completed. (c) The hypothesis that the shift of spatial attention for Task 2 is not subject to the central bottleneck and can proceed during central operations of Task. A, B, and C are the perceptual, central, and response initiation/execution stages of Task, respectively. 2A, 2B, and 2C are the corresponding stages for Task 2. SOA: Stimulusonset asynchrony. central capacity limitations, but shifts of attention contingent on top-down control settings are (e.g., capture by stimuli you are looking for). To critically evaluate whether shifts of spatial attention contingent on top-down control settings requires central resources, Brisson and Jolicœur (27a, 27c) asked participants to perform a tone Task, followed by a two-choice colour-discrimination Task 2 in a PRP paradigm. There were

7 42 LIEN, CROSWAITE, RUTHRUFF four different tone stimuli (S) mapped to four different responses (R) in their Task ; we refer this kind of mapping as a 4S4R mapping. In their studies, participants were asked to respond to the pitch of the tone for Task and the location of the gap in the uniquely coloured square for Task 2. They used electrophysiological measures (event-related potentials; ERPs) to provide online indices of attentional allocation. The specific ERP component they used is the N2pc effect (short for N2-posterior-contralateral), which is an increased negativity over posterior scalp contralateral to an attended stimulus, usually starting roughly 8 ms after stimulus onset and lasting for roughly ms (for a review, see Eimer, 996; Luck, 25; Luck & Hillyard, 99, 994). That is, the ERP at a given electrode in the left hemisphere becomes more negative when spatial attention is directed to a right-hemifield stimulus (contralateral) than to a left-hemifield stimulus (ipsilateral), and vice versa. A nonzero N2pc effect, quantified as the average difference in amplitudes between contralateral and ipsilateral electrode sites relative to some lateralized stimulus, indicates that spatial attention was allocated to that stimulus (e.g., Luck & Hillyard, 99, 994; Woodman & Luck, 23). The N2pc effect can provide both temporal (when) and spatial (where) information regarding an attentional shift, which behavioural measures cannot easily provide. Brisson and Jolicœur (27a, 27c) found large N2pc effects at long SOAs but small N2pc effects at short SOAs, implying that the shift of spatial attention to the Task 2 stimulus (a square in a specific colour) was impeded when central resources were occupied by Task. Similar findings were also obtained when Task 2 involved coloured letters and digits (Brisson & Jolicœur, 27b) and when coloured distractors matched the Task 2 target feature (Brisson, Leblanc, & Jolicœur, 29). Their findings suggest that the shift of spatial attention is not independent of whatever caused the PRP effect, contrary to Pashler (99). Brisson and Jolicœur s (27a, 27c) findings are intriguing given that previous studies have suggested that shifting of spatial attention constitutes an initial processing bottleneck that is independent from a later responseselection bottleneck (or a more general central bottleneck; e.g., Johnston et al., 995; McCann & Johnston, 992; Pashler, 989, 99). Interestingly, a recent behavioural study by Ruthruff et al. (28) failed to find evidence for Brisson and Jolicœur s claim. In Ruthruff et al. s study, selection was based on colour, but they used a masked accuracy PRP paradigm as in Pashler s (989, 99) studies. Task was a 2S2R speeded task; depending on the experiment, participants were to make a tone/noise judgement (auditory) or a digit magnitude judgement (visual) for Task. After a variable SOA, a noninformative cue display appeared, containing one red box, one green box, and two white boxes. Finally, the Task 2 letters appeared very briefly (75 ms) and then were masked. Participants identified the letter

8 CONTROLLING SPATIAL ATTENTION 43 in a particular colour (red or green), with no speed pressure. The main question was whether the cue containing the target colour would capture attention; if so, Task 2 accuracy would be higher when the target appeared in that same location than when it appeared in a different location (a cue validity effect). Indeed, a substantial cuing validity effect was found and it was nearly the same at all SOAs, implying that shifts of spatial attention occurred independently from central attention. It is unclear why Brisson and Jolicœur (27a, 27b, 27c) failed to find evidence of shifts of spatial attention at short SOAs with electrophysiological measures (the N2pc effect), whereas Ruthruff et al. (28) did find evidence of shifts of spatial attention with behavioural measures (the cue validity effect on RT). One speculation is that the critical difference is the overall difficulty of Task central operations. Whereas Brisson and Jolicœur s studies used a 4S4R Task, Ruthruff et al. used a 2S2R Task (see also Johnston et al., 995; McCann & Johnston, 992; Pashler, 989, 99). This contrast suggests that only relatively difficult Task central operations prevent the shift of spatial attention to the Task 2 stimulus. Because of many methodological differences between the studies (different paradigms), however, no firm conclusion can yet be drawn. In a relevant study, Brisson and Jolicœur (27c; Experiment 2) reported that N2pc effects elicited by Task 2 were much larger at a short SOA when Task was an extremely easy simple reaction task (a 4SR task) than when it was a 4S4R task. However, simple reaction tasks lie at the extreme end of the difficulty continuum, requiring little or no central processing. Meanwhile, unpractised 4S4R mapping tasks fall near the other extreme, with respect to the tasks typically used to study basic attentional processes (the mean RT, 87 ms, was more than three times as large as that obtained with the simple reaction task, 288 ms). There is huge gap in difficulty between these two extremes that has not yet been adequately explored. It is unclear whether any task requiring an extended period of central operations is sufficient to attenuate the N2pc effect (e.g., a typical 2S2R task used in most PRP studies), or whether only especially demanding tasks are capable of doing so. THE PRESENT STUDY The present study examined whether shifts of spatial attention can occur while central attentional resources are occupied by another task. In Experiment, Task required a speeded two-choice response to two auditory stimuli (i.e., a 2S2R task). Task 2 required an unspeeded response to a letter in a particular colour (red or green across participants) within the masked target display. The SOAs were, 2, and 9 ms intermixed within blocks. This method closely resembles Ruthruff et al. s (28) design

9 44 LIEN, CROSWAITE, RUTHRUFF but without the cue display or cue validity manipulation. Instead, we measured the N2pc effect elicited by Task 2 target letter, as in Brisson and Jolicœur (27a, 27c). To foreshadow the results, the N2pc effect was similar across SOAs in Experiment. This finding suggests that the shift of spatial attention to the Task 2 target location occurred even when central resources were occupied by Task, supporting Ruthruff et al. s (28) conclusions. Experiment 2 used a 2S2R visual, rather than auditory, Task. Surprisingly, there was little evidence that central operations of visual Task block shifts of spatial attention towards a visual Task 2. Subsequent experiments then examined whether much more difficult Task central operations (4S4R or 4S2R mapping) would prevent the visual Task 2 stimulus from capturing spatial attention. EXPERIMENT As a first step, Experiment examined whether shifts of spatial attention to visual Task 2 letters can occur while central resources are devoted to a 2S2R auditory Task. The particular Task we chose is the high/low pitch judgement that has been used in numerous PRP studies (e.g., Johnston et al., 995; McCann & Johnston, 992; Pashler, 989, 99; Pashler & Johnston, 989). Although this task is not especially easy (it does produce large dualtask costs in the PRP paradigm), it is considerably easier than the 4S4R Task used by Brisson and Jolicœur (27c). On each trial, participants first made a speeded response to the Task tone. After a variable SOA (, 2, or 9 ms), four coloured letters (one red, one green, and two white) appeared on the screen for 75 ms and then were masked (see Figure 3). The red and green letters always appeared in opposite hemifields. All participants received the same displays, but were given different instructions regarding the target colour. Previous single-task studies using similar displays have shown that stimuli in the target colour capture attention away from other stimuli, producing an N2pc effect (e.g., Lien, Ruthruff, & Cornett, 2; Lien et al., 28). The main question was whether the N2pc effect would also be elicited in a dual-task condition at a short SOA. Method Participants. Eighteen undergraduate students from Oregon State University participated in exchange for extra course credit. Half of the participants were instructed to respond to the red letters for Task 2 and the other half to the green letters. Their mean age was 2 years (SD3 years, range: 83 years). Ten participants were female and eight were male. All reported having normal or corrected-to-normal visual acuity. All participants

10 CONTROLLING SPATIAL ATTENTION 45 Task ms (Tone Task) ms SOA 5 ms (, 2, or 9 ms) demonstrated normal colour vision using the Ishihara test for colour deficiency. Apparatus and stimuli. Stimuli were presented on an IBM-compatible microcomputer connected to a 9-inch ViewSonic monitor and a customized response box with buttons labelled 8 from left to right. The average viewing distance was about 55 cm. Within each trial, participants first viewed the fixation display, consisting of five boxes: A centre box surrounded by four peripheral boxes placed at the corners of an imaginary square (topleft, bottom-left, top-right, and bottom-right). Each peripheral box was equidistant from the centre box (7.668, centre to centre) and from adjacent peripheral boxes (.88, centre to centre). Each box was , drawn with thin (.8) white lines. T L Task 2 (Letter Task) 75 ms L T Red Green Mask White # # # # ms Red Green White Until respond Figure 3. An example event sequence in Experiment. In the real experiment, the stimuli were coloured. In this example target display, the top-left letter T would be red, the bottom-left letter L would be white, the top-right letter L would be green, and the bottom-right letter T would be white. The colour of the masks in the mask display matched the colour of the letters in each peripheral box. SOA: Stimulusonset asynchrony. Time

11 46 LIEN, CROSWAITE, RUTHRUFF The Task stimulus was a low-pitched (2 Hz) or high-pitched tone (2 Hz). Task 2 stimuli were letters (.48 width.358 height.38 thick in Arial font) inside each of the four peripheral boxes. Each hemifield (left vs. right visual field) contained one T and one L. One of the letters was red, one was green, and two were white, with the restriction that the red and green items were always located in different hemifields. The mask display consisted of the fixation display with the addition of a mask # (.48 width.468 height.38 thick) inside each of the four peripheral boxes. The colour of the masks matched the colour of the letters in each peripheral box. Design and procedure. As shown in Figure 3, each trial started with the presentation of the fixation display for ms. Then, as a warning signal, the centre box turned off for ms and back on. The Task tone then sounded for 5 ms. After one of three SOAs (, 2, or 9 ms), the Task 2 visual letters appeared for 75 ms. The letters were then replaced by the mask display for ms, which was then replaced by the fixation display. The fixation display remained on the screen centre until participants had responded to both Task and Task 2. Participants were asked to fixate their eyes on the centre box throughout the trial. For Task, participants were asked to press the button labelled with their left-middle finger for low-pitched tones and press the button labelled 2 with their left-index finger for high-pitched tones. For Task 2, half of the participants were instructed to respond to the identity of the red letter and the other half were instructed to respond to the identity of the green letter. They were asked to press the button labelled L with their right-index finger if the Task 2 target letter was L and press the button labelled T with their right-middle finger if it was T. They were asked to respond to Task quickly and accurately and to Task 2 accurately. Also, they were asked to respond to Task before Task 2. Immediately after a response was recorded, the next trial began with the ms fixation display. Participants performed one practice block of 32 trials, followed by 2 experimental blocks of 96 trials each (a total of 52 experimental trials). After each block, participants received a summary of their mean RT and accuracy, and were encouraged to take a break. EEG recording and analyses. The electroencephalographic (EEG) activity was recorded using Q-cap AgCl electrodes from F3, F4, C3, C4, T7, T8, P3, P4, P5, P6, PO5, PO6, O, and O2. These sites and the right mastoid were recorded in relation to a reference electrode at the left mastoid. The ERP waveforms were then re-referenced offline to the average of the left and right mastoids (see Luck, 25). The horizontal electrooculogram (HEOG) was recorded bipolarly from electrodes at the outer canthi of both eyes, and

12 CONTROLLING SPATIAL ATTENTION 47 vertical electrooculogram (VEOG) was recorded from electrodes above and below the midpoint of the left eye. Electrode impedance was kept below 5 kv. EEG, HEOG, and VEOG were amplified using Synamps2 (Neuroscan) with a gain of 2 and a bandpass of.5 Hz. The amplified signals were digitized at 5 Hz. Trials with possible ocular artifacts were identified in two steps (see also Lien et al., 28, 2). First, trials with ocular artifacts were rejected automatically using a threshold of975mv for a 4 ms epoch beginning 2 ms before Task 2 stimulus onset to 2 ms after Task 2 stimulus onset. Next, each of these candidate artifact trials was inspected manually. To determine whether individual participants systematically moved their eyes in response to the Task 2 target stimulus, we computed for each participant average HEOG waveforms for left-target and right-target trials, separately, during the period 828 ms after the Task 2 target onset. Following Woodman and Luck (23), we included in the data analyses only participants whose average HEOG activity was less than 93mV during this time window. To quantify the overall magnitude of the N2pc effect, we focused on the time window identified in previous studies as showing the largest effects: 828 ms after target onset (see also Brisson & Jolicœur, 27c). Specifically, the N2pc effect was measured as the mean amplitude during this time window for contralateral electrode sites to the target location minus the mean amplitude for ipsilateral electrode sites, at the P5/P6, O/O2, and PO5/ PO6 electrode sites, relative to the mean amplitude during a 2 ms baseline period prior to Task 2 stimulus onset. Results In addition to trials with ocular artifacts, we excluded trials from the final analyses of behavioural data (RT and proportion of errors; PE) and EEG data if RT or RT2 was less than ms or if RT was greater than 3 ms (.4% of trials). Rejection of trials with ocular artifacts in the EEG data led to the further elimination of % of trials, but no more than 25% for any individual participant. Trials were also excluded from RT and EEG analyses if either response was incorrect. 2 Analysis of variance (ANOVA) was used for all statistical analyses, with an alpha level of.5 to ascertain statistical 2 It is logically possible that the exclusion of trials where Task 2 was incorrect served to eliminate the very trials where spatial attention was blocked during Task processing. This seems unlikely, a priori, because there generally was no substantial increase in errors at short SOAs. Nevertheless, to investigate this possibility we conducted a follow-up N2pc analysis including trials where Task 2 was incorrect. As in the original data analyses, the N2pc effect was similar across SOAs, FB., and pairwise comparisons revealed no significant differences between any two SOAs, FsB..

13 48 LIEN, CROSWAITE, RUTHRUFF significance. The p-values were adjusted using the Greenhouse-Geisser epsilon correction for nonsphericity, when appropriate. Although our key predictions concern only the N2pc effect, we also report the behavioural data for the sake of completeness. Behavioural data analyses. Table shows mean RT and PE for Task and mean PE for Task 2 at each SOA. ANOVAs on RT and PE for Task and PE for Task 2 were conducted as a function of SOA (, 2, and 9 ms). For Task, PE increased as SOA decreased, F(2, 34).37, pb., h p 2.4; PE was.58,.5, and.4 at the, 2, and 9 ms SOAs, respectively. No other effects were significant. ERP analyses. The N2pc data analyses focused on the time window in which the Task 2 stimulus should produce an N2pc effect (828 ms after Task 2 stimulus onset). The N2pc data were analysed as a function of electrode site (P5/P6, O/O2, vs. PO5/PO6) and SOA (, 2, vs. 9 ms). Each subcondition contained a total of 384 trials per participant before rejecting trials that were incorrect, fell outside our RT cutoff, or showed ocular artifacts. Figure 4 shows the N2pc effects for the P5/P6, O/O2, and PO5/PO6 electrode sites, as well as the pooled data from these electrode sites. The N2pc effect was similar across SOAs, FB.; the effect was.467 mv,.433 mv, and.52 mv at the, 2, and 9 ms SOAs, respectively. Pairwise comparisons revealed no significant differences between any two SOAs, FsB.. The N2pc effect was similar for all three electrode sites, FB.; the effect was.462 mv,.454 mv, and.55 mv at the P5/P6, O/O2, and PO5/PO6 electrode sites, respectively. The interaction between SOA and electrode site was not statistically significant, F(4, 68).97, p.83, h 2 p.. TABLE Mean response time (RT in milliseconds) for Task and proportion of errors (PE) for Tasks and 2 as a function of stimulusonset asynchrony (, 2, and 9 ms) in Experiment Stimulusonset asynchrony ms 2 ms 9 ms Task RT 822 (69) 86 (7) 87 (92) PE.58 (.4).5 (.5).4 (.3) Task 2 PE.83 (.4).8 (.5).74 (.3) The standard error of the mean is shown in parentheses.

14 CONTROLLING SPATIAL ATTENTION 49 P5/P O/O2 P5/P6 Pooled ms SOA 2 ms SOA 9 ms SOA Figure 4. Grand-average N2pc difference waveforms for Task 2 at the P5/P6, O/O2, and PO5/PO6 electrode sites as a function of SOA (, 2, and 9 ms) in Experiment. In addition, pooled data were obtained by averaging the N2pc difference waveforms across all three electrode pairs. The N2pc difference waveforms were calculated by subtracting the ipsilateral potentials from contralateral potentials (with respect to Task 2 target location). The baseline period was the 2 ms prior to Task 2 stimulus onset. Negative is plotted upwards and time zero represents Task 2 stimulus onset. SOA: Stimulus-onset asynchrony. Discussion Experiment used a PRP paradigm to examine whether the shift of spatial attention to the Task 2 visual letter occurs (as indicated by the N2pc effect)

15 5 LIEN, CROSWAITE, RUTHRUFF while central resources are occupied by the auditory Task. There was a substantial overall N2pc effect, which did not depend on SOA. This result, contrary to Brisson and Jolicœur (27a, 27b, 27c), suggests that the shift of spatial attention to the Task 2 letter occurred even when central resources were occupied by the auditory Task. In Figure 4 (the pooled data), there seems to be a very slight trend for the N2pc effect to begin earlier in time at the 9 ms SOA than at the ms SOA. To see whether this trend is genuine, we estimated onset latencies with the pooled data from the P5/P6, O/O2, and PO5/PO6 electrode sites for the and 9 ms SOAs. We used the jackknife method to estimate the onset latencies, defined as when the N2pc effect reached a threshold of.3 mv (see e.g., Kiesel, Miller, Jolicœur, & Brisson, 28; Miller, Patterson, & Ulrich, 998). 3 We found no significant difference between the N2pc onset latencies at the ms SOA (28 ms) and at the 9 ms SOA (26 ms). Even if the 2 ms effect were real, it is trivial compared to the PRP effect (3 ms) typically found in most PRP studies. Clearly, shifts of spatial attention were not subject to the entire central bottleneck. Nor is there any evidence that the shifts of spatial attention were even modulated by ongoing central operations. EXPERIMENT 2 Experiment revealed no evidence that central operations of an auditory Task completely block the shift of spatial attention to visual Task 2. Experiment 2 examined whether this finding of independence would generalize to a visual Task. Intuitively, it seems unlikely. When Task central operations operate on a visual stimulus, it makes sense for spatial attention to remain locked on that visual stimulus; shifting spatial attention to another visual stimulus could impair Task performance. Findings from attentional blink studies are generally consistent with this claim (e.g., Jolicœur et al., 26a, 26b). In Jolicœur et al. (26b), for instance, participants viewed a sequence of rapidly presented stimuli at fixation. Target was a white digit (amongst distractor letters), and Target 2 was a digit in a specified colour (red or a green), presented to the left or right of fixation. Participants were required to report both Target and Target 2 in one block (the report Target condition) but only Target 2 in the other block 3 Following Miller et al. s (998) jackknife method, we calculated n grand-average waveforms for each subset of n participants (i.e., with a different participant being removed each time). The onset latency was determined as the time at which the N2pc effect reached the threshold of.3 mv. The values from the ms SOA and 9 ms SOAs were submitted to a one-tailed t-test. Using this method, we found no statistically significant difference in the N2pc onset latency between these two SOAs in any of the present experiments (Experiments 6).

16 CONTROLLING SPATIAL ATTENTION 5 (the ignore Target condition). The critical manipulation was the SOA between Target and Target 2 (i.e., the lag). They found that N2pc effects to Target 2 were smaller for the report Target condition than the ignore condition. Furthermore, the reduction of the N2pc effect was much more substantial at the short SOA (Lag 2) than at the long SOA (Lag 8). Thus, based on these earlier studies, we expected that, when Task is visual, the shift of spatial attention to the visual Task 2 letter would be delayed at short SOAs. Method Participants. There were 23 participants, drawn from the same participant pool as in Experiment. None had participated in the previous experiment. Data from three participants were excluded from the final data analyses due to low accuracy on Task 2 (B 6%; one participant) or excessive eye movement artifacts in the electroencephalographic data (two participants). Therefore, data from 2 participants were included in the final data analyses. Their mean age was 2 years (SD2 years, range: 823 years). Eleven participants were female and nine were male. As in Experiment, half of the participants were instructed to respond to the red letters and the other half were instructed to respond to the green letters. All reported having normal or corrected-to-normal acuity. As in Experiment, all participants demonstrated normal colour vision using the Ishihara colour test. Apparatus, stimuli, and procedure. The tasks, stimuli, and equipment were the same as in Experiment, except for Task stimulus. Instead of presenting a tone as the Task stimulus, a white digit (digits 9, except 5) appeared in the centre box for 5 ms. The digit was.48 in width.358 in height. Participants were asked to perform the magnitude task on the digit for Task *press the button labelled with their left-middle finger if the digit was smaller than 5 and the button labelled 2 with their left-middle finger if the digit was larger than 5. Results The data analysis was similar to that of Experiment. Application of the RT cutoffs eliminated approximately.6% of trials. Rejection of trials with ocular artifacts in the EEG data led to the further elimination of 8% of trials, but no more than 22% for any individual participant. Behavioural data analyses. Table 2 shows mean RT and PE for Task and mean PE for Task 2 at each SOA. For Task, the main effect of SOA

17 52 LIEN, CROSWAITE, RUTHRUFF TABLE 2 Mean response time (RT in milliseconds) for Task and proportion of errors (PE) for Tasks and 2 as a function of stimulusonset asynchrony (, 2, and 9 ms) in Experiment 2 Stimulusonset asynchrony ms 2 ms 9 ms Task RT 683 (42) 678 (44) 695 (79) PE.5 (.9).43 (.8).44 (.8) Task 2 PE.68 (.24).47 (.25).23 (.22) The standard error of the mean is shown in parentheses. was not significant on either RT, FB., or PE, F(2, 38)2.3, p.227, h p 2.. For Task 2, PE increased as SOA decreased, F(2, 38)2.72, pb., h p 2.52; the PE2 was.68,.47, and.23 at the, 2, and 9 ms SOAs, respectively. ERP analyses. As in Experiment, the N2pc data were analysed as a function of electrode site (P5/P6, O/O2, vs. PO5/PO6) and SOA (, 2, vs. 9 ms). Figure 5 shows the N2pc effects for the individual paired electrode sites, as well as the pooled data from these electrode sites. As in Experiment, there was no main effect of SOA on the N2pc effect, F(2, 38).23, p.39, h 2 p.6; the effect was.454 mv,.66 mv, and.498 mv at the, 2, and 9 ms SOAs, respectively. The pairwise comparisons revealed no significant differences between any two SOAs, Fs(2, 38) 5.83, ps ].99, h 2 p s 5.9. The N2pc effect was larger for the P5/P6 and PO5/PO6 electrode sites (.589 mv vs..577 mv, respectively) than for the O/O2 electrode sites (.43 mv), F(2, 38) 3.25, pb.5, h 2 p.5. The interaction between SOA and electrode site was not statistically significant, FB.. Discussion In Experiment 2, a visual Task was used*participants performed the magnitude task on a digit appearing briefly inside the centre box. This method is roughly analogous to Jolicœur et al. s (26b) attentional blink design. We reasoned that it was much more likely that central operations on a visual Task would block the shift of spatial attention to a visual Task 2, since they both compete for visual-spatial attention. Surprisingly, this was not the case. Even with the visual Task, we found very little reduction in

18 CONTROLLING SPATIAL ATTENTION 53 P5/P O/O2 P5/P6 Pooled ms SOA 2 ms SOA 9 ms SOA Figure 5. Grand-average N2pc difference waveforms for Task 2 at the P5/P6, O/O2, and PO5/PO6 electrode sites as a function of SOA (, 2, and 9 ms) in Experiment 2. In addition, pooled data were obtained by averaging the N2pc difference waveforms across all three electrode pairs. The N2pc difference waveforms were calculated by subtracting the ipsilateral potentials from contralateral potentials (with respect to Task 2 target location). The baseline period was the 2 ms prior to Task 2 stimulus onset. Negative is plotted upwards and time zero represents Task 2 stimulus onset. SOA: Stimulus-onset asynchrony. the N2pc effect at the short SOA compared to the long SOA, essentially replicating Experiment. This finding suggests that the shift of spatial attention to the visual Task 2 letter can occur even during Task central

19 54 LIEN, CROSWAITE, RUTHRUFF operations, regardless of its modality. PE2 did increase slightly as SOA decreased, which could be taken as evidence that shifts of spatial attention were delayed at short SOAs. However, the effect could just as easily be due to a general impairment in letter identification at short SOAs, akin to an attentional blink (see Ruthruff et al., 28). There was a trend towards a slight attenuation of N2pc effect at the shortest SOA comparing to at the longest SOA at the early time window. To determine whether this effect is real, we conducted additional ERP data analyses with 5 ms time windows of 823 ms and 2328 ms after Task 2 stimulus onset. We analysed the pooled N2pc effect (averaged across the electrode sites), comparing only the two extreme SOAs ( ms and 9 ms SOAs). The N2pc effect was still not significantly different between these two SOAs at either of the time windows (823 ms and 2328 ms after Task 2 stimulus onset), Fs(, 9) 5.4, ps ].298, h p 2 s 5.6. Thus, even this finer-grained analysis provided no evidence that central operations of a visual Task block spatial attention from being directed to the visual Task 2. It remains possible that there is a small attenuation of the N2pc effect at short SOAs for some participants. However, any such attenuation appears to be small and it failed to reach statistical significance in any of our tests, despite a relatively large sample of participants (N2). The N2pc effect might be extended over time (see Figure 5), but it is not strongly attenuated. EXPERIMENT 3 Both Experiments and 2 revealed similar N2pc effects across SOAs, suggesting that the shift of spatial attention to Task 2 stimulus occurred even when central attention was allocated to Task. Brisson and Jolicœur (27a, 27b, 27c), however, showed strong attenuation of the N2pc effect at short SOAs, suggesting the opposite conclusion. One notable difference is that a 2S2R Task was used in our Experiments and 2, whereas a 4S4R Task was used in the Brisson and Jolicœur studies. Perhaps control over spatial attention is depressed only during an especially difficult central operation (e.g., with more than just two stimuli and two response choices). Experiment 3 examined this issue. Experiment 3 replicated Experiment 2 with a 4S4R visual Task instead of a 2S2R visual Task. It is well-established in the literature that the number of SR alternatives for a task primarily affects central operations (e.g., Pashler, 989). There are many ways in which the greater difficulty of central operations could block the shift of spatial attention to the Task 2 visual stimulus. First, if central resources are needed for shifts of spatial attention, then the shift to the Task 2 stimulus will be prevented or delayed.

20 CONTROLLING SPATIAL ATTENTION 55 Second, increasing the number of SR from two (as in Experiments and 2) to four (as in Experiment 3) should increase the mental load imposed by the central operations of Task (e.g., keeping the four mapping rules in working memory instead of two), leaving less working memory resources for Task 2. One logical prediction is that this increase would make it more difficult to maintain the attentional settings for Task 2 in working memory and, hence, lead to less shift of spatial attention to stimuli matching this weakened attentional setting. For our 4S4R visual Task, we used a variant of the box-size discrimination task from Johnston and McCann (26) and Lien et al. (26, Exp. 2). In these earlier experiments, one of the four different box sizes was presented in each trial and participants decided whether the box was one of the two narrower boxes or one of the two wider boxes. The present Experiment 3 used the same four box sizes. However, instead of tworesponse task (narrow vs. wide) as in previous studies, participants made one of four responses (i.e., 4S mapped to 4R). They decided whether the presented box was the narrowest, second narrowest, second widest, or widest size. Thus, we expected this 4S4R visual Task to produce relatively long RTs, reflecting the increased difficulty of central operations. This was, in fact, what we found (see later for details). If central operations on a difficult Task prevent the shift of spatial attention, then the N2pc effect elicited by the visual Task 2 letter should be attenuated at short SOAs relative to at long SOAs. On the other hand, if shifts of spatial attention can occur without central attentional resources, regardless of Task difficulty, then the N2pc effect should be similar across SOAs, as in Experiments and 2. Method Participants. There were 23 new participants, drawn from the same participant pool as in the previous experiments. Data from five participants were excluded from the final data analyses due to low accuracy on Task 2 (B 6%; two participants) or excessive eye movement artifacts in the ERP data (three participants). Therefore, data from 8 participants were included in the final data analyses. Their mean age was 2 years (SD2 years, range: 824 years). Eleven participants were female and seven were male. As in the previous experiments, half of the participants responded to the red letters and the other half responded to the green letters. All reported having normal or corrected-to-normal acuity and demonstrated normal colour vision using the Ishihara colour test. Apparatus, stimuli, and procedure. The tasks, stimuli, and equipment were the same as in Experiment 2, except for a change in Task. Instead of

21 56 LIEN, CROSWAITE, RUTHRUFF a2s2r digit task, Task was a 4S4R box-width task. There were four different boxes varying in width, intermixed within blocks. Each box had a height of.28. The width was.78,.298,.448, and.668 for the narrowest, second narrowest, second widest, and widest box, respectively. The Task box appeared in the centre of the screen ms after the offset of the centre box. Participants pressed the button labelled for the narrowest box, 2 for the second narrowest box, 3 for the second widest box, and 4 for the widest box, using the small, ring, middle, and index fingers of their left hand, respectively. Results The data analysis was similar to that of Experiment 2. Application of the RT cutoffs eliminated approximately.7% of trials. Rejection of trials with ocular artifacts in the EEG data led to the further elimination of 3% of trials, but no more than 24% for any individual participant. Behavioural data analyses. ANOVAs on RT and PE for Task and PE for Task 2 were conducted as a function of SOA (, 2, vs. 9 ms). Table 3 shows the means for each SOA condition. Although the main effect of SOA was not significant on RT, F(2, 34).4, p.26, h 2 p.8, mean RT was shorter at the two shortest SOAs than at the longest SOA (RT was 798, 79, and 856 ms at the, 2, and 9 ms SOAs, respectively). PE increased as SOA decreased, F(2, 34)8.5, pb., h 2 p.33; PE was.35,.9, and. at the, 2, and 9 ms SOAs, respectively. PE2 also increased as SOA decreased, F(2, 34)7.7, pb., h 2 p.29; PE2 was.223,.93, and.73 at the, 2, and 9 ms SOAs, respectively. TABLE 3 Mean response time (RT in milliseconds) for Task and proportion of errors (PE) for Tasks and 2 as a function of stimulusonset asynchrony (, 2, and 9 ms) in Experiment 3 Stimulusonset asynchrony ms 2 ms 9 ms Task RT 798 (4) 79 (4) 856 (88) PE.35 (.7).9 (.4). (.) Task 2 PE.223 (.28).93 (.27).73 (.25) The standard error of the mean is shown in parentheses.

22 CONTROLLING SPATIAL ATTENTION 57 ERP analyses. As in the previous experiments, the N2pc data were analysed as a function of electrode site (P5/P6, O/O2, vs. PO5/PO6) and SOA (, 2, vs. 9 ms). Figure 6 shows the N2pc effects for the individual paired electrode sites, as well as the pooled data from these electrode sites. Unlike Experiments and 2, the main effect of SOA was significant, F(2, 34)8.7, pb., h 2 p.32; the N2pc effect was smaller at the two shortest SOAs than at the longest SOA (the effect was.285 mv,.277 mv, and.696 mv at the, 2, and 9 ms SOAs, respectively). The pairwise comparisons revealed that the N2pc effect was significantly different between the ms and 9 ms SOAs, F(, 7)8.62, pb., h 2 p.34, and between the 2 ms and 9 ms SOAs, F(, 7).2, pb., h 2 p.4. The comparison between the two shortest SOAs ( ms vs. 2 ms) was not significant, FB.. No other effects were statistically significant. Discussion Experiment 3 examined whether control over spatial attention to a visual Task 2 can occur in parallel with a demanding central operations of a 4S4R visual Task. By switching to a 4S4R mapping visual task, overall mean RT increased from 685 ms in Experiment 2 to 85 ms in Experiment 3, confirming that we were successful in increasing task difficulty and lengthening central operations. The mean amplitude of the N2pc effect was significantly reduced (by about 59%) from the longest SOA (.696 mv) to the shortest SOA (.285 mv). This reduction was significantly larger than the nonsignificant 9% reduction in Experiment 2 (in which the N2pc effects were.454 mv and.498 mv at the ms and 9 ms SOAs, respectively), F(, 36)4.3, pb.5, h p 2.. These findings suggest that only an especially difficult central operation can block the allocation of spatial attention to a visual Task 2. EXPERIMENT 4 In Experiment 3, we found a substantial reduction in the N2pc effect at short SOAs (about 59%) with a Task that was relatively difficult (because there were four SR alternatives) and operated on a visual stimulus. This finding by itself does not necessarily indicate that Task central operation difficulty alone is sufficient to attenuate the N2pc effect at short SOAs. It is plausible that the attenuation occurs only when difficulty is combined with use of a visual stimulus; that is, spatial attention may not be free to move once an especially demanding central operation is allocated to a visual