Is location cueing inherently superior to color cueing? Not if color is presented early enough

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Acta Psychologica 127 (2008) 89–102 www.elsevier.com/locate/actpsy

Is location cueing inherently superior to color cueing? Not if color is presented early enough Ronen Kasten *, David Navon Department of Psychology, The University of Haifa, Haifa 31905, Israel Received 6 August 2006; received in revised form 26 February 2007; accepted 27 February 2007 Available online 8 April 2007

Abstract A possible source for the advantage of location cueing over non-spatial cueing is that orienting attention by a location cue is feasible prior to stimulus onset, whereas that is normally not the case with orienting by a non-spatial cue. To examine how critical that source is for observing an advantage, we eliminated it: In a color-preview condition, subjects were to detect a target presented on the background of one of two differently colored circles (where color-location assignment was random). In a no-preview condition, the circles were both gray, but the target was either red or green (where color assignment was random). Cue type (location vs color) was also manipulated. The color preview in Experiment 1 (in which color onset preceded cue onset) was found helpful: Whereas a substantial disparity in validity effects of the two cue types was obtained with no preview, no significant difference was found when a color preview was introduced. The validity effects of both cue types were found to be about the same also in Experiment 2, in which color onset was exactly synchronized with cue onset, and SOA was manipulated. Furthermore, the absence of an SOA · cue type interaction indicated that the time course of the color cue validity did not lag after the time course of the location cue validity, which seems incompatible with the hypothesis that a color cue cannot affect orienting without first computing a location from it prior to cue onset. Overall, the results suggest that the time course of color cueing is not inherently different from that of location cueing once its main disadvantages are removed.  2007 Elsevier B.V. All rights reserved. PsycINFO: 2346 Keywords: Visual attention; Spatial orienting

1. Introduction The issue addressed here is whether, and in what way, visual selection may differ following different types of cues. Since that issue is quite intimately related with the issue of what does visual selection selects, let us first discuss the latter. A popular view of visual attention likens it to a physical selection apparatus, like a spotlight (e.g., Posner, 1980; Posner, Nissen, & Ogden, 1978; Posner, Snyder, & Davidson, 1980; Shulman, Remington, & McLean, 1979; Treisman & Gelade, 1980), a zoom lens (e.g., Eriksen & *

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0001-6918/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.actpsy.2007.02.002

St. James, 1986; Eriksen & Yeh, 1985), a peephole (e.g., Kosslyn & Koenig, 1992; Navon, 1990), or a filter (e.g., Cave, 1999), applied to continuous regions of space. That space-based view of attention presupposes that preattentive processes1 generate a spatial – retinotopic or spatiotopic – representation and that focal attention selects parts of that representation. A number of authors have challenged that view or proposed various models that relaxed one or more of the

1

This prevalently used term might actually denote processes that do require attention, yet before it has been confined to a certain location. Thus, they might be better captured by the terms prefocal processes (see Navon & Pearl, 1985) or distributed-attention processes (see Rock & Mack, 1994).

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assumptions made by it (e.g., Briand & Klein, 1987; Castiello & Umilta, 1992; Duncan, 1984; Logan & Bundesen, 1996; Van der Heijden, Kurvink, De Lange, De Leeuw, & Van der Geest, 1996; Vecera & Farah, 1994). The most prominent of these is the object-based view of attention (e.g., Bundesen, 1990; Desimone & Duncan, 1995; Duncan, 1984; Kramer & Jacobson, 1991; Vecera & Farah, 1994), which posits that rather than locations, visual attention selects objects. Whereas locations are points or regions within an analog – retinotopic or spatiotopic, parsed or pre-parsed – representation, objects may be defined as bundles of processed attributes where each bundle has a distinct status within a non-spatial representation, e.g., object files (see Kahneman & Treisman, 1984). Recently, it has been proposed that those two modes of selection are neither qualitatively different nor mutually exclusive, or at least that both may take part in selection on different occasions (e.g., Egly, Driver, & Rafal, 1994; Folk, Remington, & Wright, 1994; Goldsmith & Yeari, 2003; Logan, 1996; Macquistan, 1997). Regardless of whether there actually is just one basis for visual attention and what it is, it seems quite evident that space must at least have a fairly distinguished status in visual attention. The reason is quite simple: Visual attention by definition selects constituents of some visual representation (be they regions or objects), each of which corresponds to something that is located at some retinal area that in turn corresponds to something that resides in physical space. It thus seems plausible that their spatial location is both notably available and especially useful for accessing them. Hence, selecting visual input by spatial location must be very natural. That is trivially true for the capturing of attention by exogenous factors such as abrupt onset at (or in the vicinity of) the selected place (e.g., Jonides & Yantis, 1988; Yantis & Egeth, 1999), or the presence of a singleton stimulus (e.g., Theeuwes, 1992; Theeuwes, Atchley, & Kramer, 2000), but it is probably true as well for cases in which attention is controlled by endogenous factors, namely intentions (see, e.g., Bacon & Egeth, 1994; Pashler, 1988; Posner, 1980). In the present paper, we focus on the latter. The feasibility (or effectiveness) of selecting by non-spatial attributes (color, form, etc.) is less self-evident. The space-based view, for one, entails that visual attention can be directly guided only to spatial locations as coded in the visual representation. Non-spatial information might help only indirectly to the extent that it serves to distinguish and locate the to-be-attended region. For example, a detective instructed to attend to a pink Pontiac must do that, according to that view, by first having detected a pink Pontiac in a multitude of other vehicles, then locating it, and then orienting attention to that location. It might be further suggested (see Cave & Bichot, 1999) that such a qualitative difference follows from the reasonable claim that space dominates attributes like color and form in the hierarchy of dimensions proposed by Navon

(1978).2 In a similar vein, it has been argued that stimulus location must differ qualitatively from attributes like shape or color, since space exists independently of any stimulus (Tsal, 1983; cf. also Lamy & Tsal, 2001). Shape or color, on the other hand, are actually dimensions, namely sets of values each of which might qualify any specific stimulus (or background). Another way of putting it is that space operates as a channel – i.e. ‘‘a property of a stimulus that makes information available, but is not an informational aspect of the stimulus’’ (Garner, 1987). Furthermore, it is commonly assumed that stimulus attributes are initially encoded in spatiotopic maps that in turn are interrelated via a master map of locations (e.g., Treisman, 1985, 1988; Treisman & Sato, 1990; Wolfe, Cave, & Franzel, 1989; cf. also Nissen, 1985). Indeed, there is ample behavioral evidence suggesting that in the domain of visual attention location is special in some sense or the other (e.g., Eriksen & Eriksen, 1974; Hoffman & Nelson, 1981; Kahneman & Henik, 1977; Lamy & Tsal, 2001; Posner et al., 1980, Exp. 2, 5; Shulman et al., 1979; Theeuwes, 1989; Tsal, 1983; Tsal & Lavie, 1988, 1993). On the other hand, some of those pieces of evidence have been countered by arguing that they were taskspecific or due to some advantage enjoyed by the spatial attribute within the experimental paradigm (e.g., Castiello & Umilta, 1992; Klein & Hansen, 1990; Kramer & Hahn, 1995; Van der Heijden et al., 1996). We here focus on just one type of evidence – an advantage of cueing visual attention by location cues over cueing it by non-spatial cues. We also limit our concern to one theoretical issue – whether or not such an advantage if found is due to some constraint on, or disposition of, the process that orients visual attention to the destination suggested by the cue. A straightforward prediction from even the weakest form of the space-based view is that location cues, namely stimuli that convey information about the whereabouts of the to-be-attended object or region, must have some advantage over cues that specify some non-spatial information attribute of it. Whether or not that is also predicted, or is easy to accommodate, by other views, is an issue that we will discuss later. Let us now turn to a review of pertinent experimental evidence. 2. A brief review Unlike a location cue, a non-spatial cue is meant to manipulate attention by providing a subject with advance, reasonably valid, information about some attribute of the target stimulus or of its surroundings, aside from location.

2 It should be noted, however, that the conceived hierarchy – being the outcome of some sort of processing – by no means implies that the nature of internal representations mediating between input and output must be analogous to what we conceive the represented world to be.

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For example, the subject might be advised to attend to whatever will appear within a small circular frame to be shortly presented at some uncertain location. In that case the cue does not prime any specific location but rather a specific shape. That information could help to direct attention to the extent that it was informative for distinguishing the target. It would not be particularly necessary, if only one frame – of whichever shape – appeared on every trial. It would not be very helpful, if on each trial many circular shapes were presented. It would, however, be potentially helpful, if the subject was presented on each trial with one circular shape and one rectangular shape. A number of studies (e.g., Humphreys, 1981; Laarni, 1999; Laarni & Hakkinen, 1994; Laarni, Koski, & Nyman, 1996; Navon & Pearl, 1985; Neely, 1977; Posner & Snyder, 1975) have demonstrated that non-spatial cueing satisfying the above conditions does work. However, a question more pertinent to our concern is how effective it is. Several studies compared the effectiveness of location cues and non-spatial cues, typically when both types of cues were given to the subject in any given trial. Some studies (Posner et al., 1980; Theeuwes, 1989) found validity effects for location cues only. Other ones (e.g., Cooper & Juola, 1990; Juola, Bouwhuis, Cooper, & Warner, 1991; Kingstone, 1992; Laarni, 1999; Laarni et al., 1996; Lambert & Hockey, 1986) found validity effects for both cue types, though typically not of the same magnitude. The discrepancy in findings may be due to the extent to which non-spatial cues could serve in the different studies to guide attention. At least from the perspective of the space-based view, a non-spatial cue works by advising the subject to detect a stimulus having that attribute, an operation that has still to be followed by computing its location so that attention could then be oriented to that location. However, as noted above, that cue is informative only when the to-be-attended stimulus appears along with stimuli that do not have that attribute. If no other stimuli were presented, the cue would be gratuitous for locating the stimulus. If, alternatively, other stimuli were presented but all of them had the same attribute, the cue would not uniquely specify a location. Thus, to be effective a non-spatial cue has to convey some guiding information. Posner et al. (1980) compared location cues and form cues on their effects on detection in single-stimulus displays. The location cue was a centrally presented arrow. The form cue was a letter that provided in most trials valid information about the identity of the to-be-attended stimulus (being itself a letter). Since only one stimulus was presented and the subject was asked just to detect it, advance knowledge about letter identity was immaterial, neither for computing stimulus location nor for making the response. Theeuwes (1989) used form cues that provided advance information about the form of a frame in which the imperative stimulus (a line) would be presented. But again, in Theeuwes’ Experiment 1 only one frame was presented, and the task (discriminating line orientation) was unrelated

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with its form. Hence, the form cue was hardly pertinent. It should be noted, though, that in Theeuwes’ Experiment 2 in which there were two frames differing in form, the form cue was still ineffective. Multi-stimulus displays that vary along the cued dimensions were used, e.g., by Juola et al. (1991) who compared size and location cues in a visual search task, by Cooper and Juola (1990) who compared location, color and size cues in a similar search task, and by Laarni et al. (1996) who compared color and location cues in a letter orientation task. In all of these studies, non-spatial cueing conveyed some guiding information, and it was indeed found to be effective. But more important for our concern, in case non-spatial cueing did have an effect, was its effect comparable to the effect of location cueing? A number of studies provided subjects with combined cues and examined whether or not the validities of the two cue types interacted. Some authors obtained additivity and concluded that whereas location cues are used to guide attention during an early stage, non-spatial cues are used later, conceivably to narrow the search (Juola et al., 1991). Other authors obtained interactions, suggesting a joint, concurrent effect of the two cue types, and interpreted that as embarrassing for the space-based view of attention (Cooper & Juola, 1990; Kingstone, 1992). There are, however, some problems with the suitability of the experimental paradigms reviewed above for testing the issue. First, the type of effect seems to vary with the task. Interactions were observed in all the studies in which the subject was asked to perform a choice RT depending on some discrimination, but not when the task was simple detection (Posner, 1980). In a similar vein, interactions between cue validity and stimulus likelihood (an effect termed spotlight failure) were found in discrimination but not in detection (Klein & Hansen, 1990). It is possible that visual attention is exclusively spatial only when the response requires just a very simple datum, such as the presence of a preattentively registered feature, as seems to be the case, for example, in detection of brief flashes. Hence, to rule out the hypothesis that location cues have an advantage under the ex-hypothesi favorite conditions given the space-based view, it seems advisable to use a detection task. We do that in the study reported here. Second, it is quite possible that in some of the experiments cited, non-spatial cues were not used to orient attention but rather to facilitate response selection. Kingstone (1992), Lambert and Hockey (1986) used orientation discrimination tasks in which the form cues were relevant for the discrimination proper. When the orientation of a letter or of any other pattern is to be judged, advance knowledge of its identity is clearly helpful. Thus, the validity effect of non-spatial cues could be due to some process that occurs only after attention has been focused. Whereas spatial cues can direct attention to a specified location, non-spatial cues may operate in a subsequent selection pro-

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cess (see Juola et al., 1991) or in a subsequent stage involving decision about the stimulus identity (see Kingstone, 1992). True, normally that should have produced additivity of cue effects, but the complexity of the double-cue paradigm may have rendered even the computation of location atypically late. The main problem is that subjects probably cannot use both cues concurrently, so they may resort to deliberate strategies concerning order of cue processing that results in a slow process. There is some reason to doubt that their preferences in such a complex situation accurately reflect the differential roles and/or effectiveness of spatial and non-spatial cues in a single-cue paradigm: Using a cue in conjunction with another one may not be the same as using it as the single one. For one, it is well documented that looking for a conjunction is quite different from looking for a single feature (e.g., Treisman & Gelade, 1980; Treisman & Sato, 1990). But suppose cueing is straightforward and the non-spatial cue does carry some guiding information, as it must have been in Theeuwes’ Experiment 2 or in Laarni’s (1999) study, one might still wonder whether or not the observed advantage is due to some inherent difference in the roles of different dimensions for guiding attention. It is possible that the observed advantage is inflated by, or even totally due to, some advantages that just happen to characterize the tasks or designs of the specific experiments. Accordingly, we used the same set of symbolic cues for both spatial and non-spatial cueing. Furthermore, some more conceptual elaboration seems in place to distinguish which differences are inherent and which – subsidiary. 3. What is the theoretical issue? Navon and Pearl (1985) proposed that the process of input selection must actually comprise at least three operations: (a) preprocessing – all stimuli are encoded with respect to the selection dimension(s), e.g., the colors of all stimuli are encoded; (b) singling out – the output of that encoding is used to compute which of the objects (in case there are more than one) or spatial locations actually is the to-be-selected one, e.g., a single red stimulus is marked as the next object of focal attention ; (c) orienting – once singling out is done, an operation has to be initiated to effectuate some change in the state of visual attention, ending in its being focused on the to-be-selected stimulus,3 e.g., the focus of attention is shifted to the red stimulus.

3

To illustrate, consider the analogy of an attempt to inspect an account of a certain American bank in which a deposit of $14 k has been made on the same day by transfer from some Virgin Islands bank. It must start with a search in the American bank database for an account meeting that description. Singling out the account number precedes access to the account. The latter may or may not require, depending on the specific system design, mapping the number onto a spatial address.

This general framework suffices when the selection attribute is known well in advance and serves to specify with perfect validity the to-be-responded stimulus. However, in many experiments in visual attention, the selection cue varies from trial to trial and is provided in close temporal contiguity with the stimuli. In a considerable subset of those, on every trial the subject is given a limited-validity cue specifying a tentative target that just correlates with the ultimate, to-be-responded target. For example, the cue suggests that the latter (e.g., a rectangle whose color is the response dimension) is likely (but not certain) to be located at some specified location to the left of fixation. In those cases, a preliminary pre-selection operation has to first encode the selection instruction/cue, then interpret it. Finally, in invalid trials orienting has to be followed by reorienting. The augmented framework is presented schematically in Fig. 1. The hypothesis we put to test here is that the orienting operation is for some reason not equally affected by interpreted location cues and interpreted non-spatial cues, perhaps because it depends on a spatial address to guide it. That must be readily available once a location cue is provided, but it probably has yet to be computed even after a stimulus is singled out by a non-spatial cue. Thus, a localizing operation is required just in the latter case. On the other hand, there are other conceivable sources for advantage that are of less theoretical import, less general, or sheer artifactual. First, the locus of the advantage may rather be at the earliest stage of processing. Most location cues may be easier than non-spatial cues to interpret as instructions/suggestions to select, shift or re-allocate visual attention, either because in our ecology they are more frequently used as such, or simply because they are easier to encode and/or interpret. Second, spatial and non-spatial attributes may not be equally fast to preprocess. For one, determining spatial location is inherently a sort of relative judgment (since alternative locations are necessarily there), whereas determining color or form may depend on absolute judgment, at least with a single stimulus. The latter is typically harder than the former (see, e.g., Kimchi & Navon, 2000; Navon & Kimchi, 2004). Third, in location cueing the cue-location mapping is consistent on valid trials. In contrast, in non-spatial cueing that mapping is varied, in that the to-be-attended location is determined on each individual valid trial by the combination of the cue and the trial-specific spatial position of the stimuli having the cue attribute. Since varied mapping is known to substantially slow performance (e.g., Schneider & Shiffrin, 1977), the operation of singling out in non-spatial cueing is expected to be more demanding than in spatial cueing, unless the non-spatial attribute could somehow guide the orienting process directly. Finally, perhaps the locus indeed quite often resides at the orienting operation, yet for a very trivial reason: Advance knowledge about location can be employed to

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At the beginning of the block/session Imperative direction: Respond to stimulus with attribute X* (ultimate target) Optional contingent direction: Initially select stimulus/area with attribute y*, where rxy>0, by a pr precue ue (tentati (tentative ta target) g t)

Within a trial Pre-selection Encoding * Cue processing: i : Interpretation t n

Selection * Preprocessing: Encode selection l dimension(s) i of stimuli

Time

* Singling out: Bind target to the actual stimulus/area that fits

* [Localizing]: Find the address of the singled-out item

* Orienting:

Orient attention to the singled-out item

Correction * Reorienting: Change orientation on an invalid trial

Fig. 1. A schematic presentation of the operations comprising input selection in the case that a limited-validity cue is presented. Note: The ‘‘localizing’’ operation is put in brackets, to signify that it is required by hypothesis just for non-spatial cueing.

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4. Rationale for the test This study has been devised to examine whether or not an advantage of location cues over color cues is observed under comparable conditions when those possible concerns are ruled out. First, symbolic endogenous cues (specifically, digits) were used to cue both cueing attributes. That should eliminate the first concern in the last section, namely that less symbolic or non-symbolic location cues (e.g., arrows, abrupt onsets) are easier to preprocess or are used in our ecology more frequently than color cues (e.g., color patches). Second, as suggested above (in the section ‘‘A brief review’’), neither binary discrimination nor search is ideal for comparing cue types on effectiveness of orienting, because they probably call for late processing in which attention might possibly be reoriented. Hence, a simple task of go/no-go brightness change detection of a single target was used. Third, the design enabled color onset to be independent of target onset by making the target achromatic and varying rather the color of the background on which it appeared. In one condition of Experiment 1, for example, possible target locations were colored considerably before cue onset. In Experiment 2 those were colored simultaneously with cue onset. That should eliminate the latter three concerns above (in the section ’’What is the theoretical issue?’’) to the extent that location–color associations were available early enough before target onset, hence afford some direct shortcut to the to-be-attended region even in the color cueing condition: Color selection is done by a two-alternative forced choice just as location selection is done. Mapping color cues to location is still varied, but whichever effect it might have would probably not affect latency, since the operation must be done during the color-cue interval (in Experiment 1) or during the cue-target interval (in Experiment 2). Most important, the paradigm allows color cues to start affecting attention with cue onset, much like spatial cues do.

5. Experiment 1 start the process of orienting as soon as it is available, since space is there independently of any stimulus that might later be presented within it. Hence, when cue onset precedes stimulus onset, orienting can start once the cue has been interpreted, regardless of the timing of stimulus onset. On the other hand, assuming that the operation of orienting results in focusing on a segment of space, the effect of non-spatial cues has to await stimulus onset. When cue onset precedes stimulus onset, a delay in orienting ensues, hence the disadvantage of non-spatial cues. That is presumably also why location cueing enhances the processing of a single stimulus in an otherwise blank field, whereas nonspatial cueing does not (e.g., Posner et al., 1980).

In this experiment, color cueing and location cueing were administered in two conditions. In the color-preview condition, a gray target appeared on the background of either a red circle or a green circle, both presented well before cue onset. In the no-preview condition, the circles were both gray, but the target was either red or green. This design makes it possible to examine the source of the typical advantage of location cueing over color cueing. If that advantage were totally due to the feasibility of starting orienting prior to target onset, it would be expected to vanish in the color preview condition. If it were only partly due to that, the advantage would manifest in that condition as well, yet with reduced magnitude. If, however, that

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advantage were unrelated at all with temporal course, the advantage would remain intact. 1

33 ms

5.1. Method 5.1.1. Apparatus Stimulus presentation and data acquisition were controlled by an O2 SiliconGraphics computer. Stimuli were presented on the screen of a 15 in computer display. The subject sat in front of the monitor, with her/his head positioned on an adjustable chin rest, at a viewing distance of about 57 cm. At this distance, 1 cm of lateral extent in the visual field corresponds almost exactly to 1 of visual angle. Each subject responded by pressing on a designated keyboard key (actually, one of the control keys) with the index finger of his or her dominant hand.

1

533 ms

+

750 ms

e

m

Ti Red Green

5.1.2. Stimuli The fixation mark was a small plus sign at the center of the white screen with black lines having a length of 6 mm each. The symbolic cues were two black digits, 1 or 2, that measured 8 mm in height and about 5 mm (0.5 of visual angle) in width. The display contained two large circles, each with a radius of 19 mm (1.9 of visual angle). The circles were centered 95 mm (9.5 of visual angle) to the left or to the right of the fixation point. At the center of each circle, a smaller white circle was drawn with a radius of 0.35 mm (0.35 of visual angle). The target (having the same radius) was flashed within them. The colors of the large circles depended on the condition. In the color-preview condition, one was a red circle and the other – a green one (RGB values, namely weights of red, green and blue in the additive mixture, were (255, 0, 0) or (0, 255, 0), respectively), and the target was gray (RGB values of (145,145,145)). In the no-preview condition, both circles were gray, and the target itself was colored, in red or green as the case may be. 5.1.3. Design and procedure Subjects were run individually in one experimental session that lasted one hour. The experimental session included 3 breaks for rest of approximately 5 min each. Each experimental trial started by the onset of a fixation mark at which the subject was to focus her/his eyes. Along with the fixation mark, two circles (colored or gray, according to the specific condition) were presented at both sides of it. Then, a cue replaced the fixation mark 750 ms after the onset of the circles and the fixation. In experimental trials, 533 ms after cue onset, a target was flashed in one of the small white circles for 33 ms. The subject had to detect that flash and to respond by pressing on a designated key. A new trial was initiated 1000 ms after the response was given. Fig. 2 presents the sequence of events for each experimental trial in the color-preview condition. In catch trials (used to minimize anticipation responses), no flash was presented, and a new trial was initiated 2000 ms after cue onset.

Fig. 2. The sequence of events in an experimental trial (color-preview condition) of Experiment 1: (a) fixation plus colored circles, (b) cue (plus colored circles), (c) flash (plus cue plus colored circles).

Overall, each experimental session included 728 experimental trials (182 in each of the four blocks) and 144 practice trials (36 in each block). Hundred-and-thirty trials (about 71%) in each block were valid trials, twenty-six trials (about 14%) were invalid, and the same number of trials (26) was catch trials. The location of each color (to the right or to the left of fixation) was varied over trials orthogonally with all other variables and was randomized within blocks. On the other hand, cue coding was not varied, since in a pretest it was not found to have any effect. Digit 1 coded ‘‘right’’ at the location cue condition and ‘‘red’’ in the color cue condition, whereas digit 2 coded ‘‘left’’ and ‘‘green’’, respectively. The experiment was divided into four blocks corresponding to two within-block variables: cue type (location or color) and color preview (yes, no). Block order was counterbalanced. There were four possible orders. The color preview variable was nested within the cue type variable, so that the first two blocks were of the same cue type, and so were the last two blocks. Subjects were instructed to avoid eye or head movements and they were explicitly asked to use the cues. 5.1.4. Subjects Twenty-four undergraduate students at the University of Haifa participated for course credit at the Department of Psychology. All subjects had normal or corrected-tonormal vision. Four other subjects were not included in the analysis due to high rates of errors in catch trials. 6. Results The major analyses were conducted on mean latencies. Trials with latencies shorter than 150 msec or longer than

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Location cue

Color cue

400

95

390

390

386

384

Latency

valid 380 372

invalid

374

370

369 361

360

360 350 340

without

with

without

with

color preview Fig. 3. Mean latency as a function of cue type, validity and color preview in Experiment 1.

700 msec (3%) were excluded from analysis. Data were cast into a within-subject three-way ANOVA, factors of which were validity, cue type and color preview. Fig. 3 presents mean latency as a function of cue type, validity and color preview. A significant main effect was found for the factor validity, F(1, 23) = 17.21, p < 0.001, MSE = 887. No effect of cue type was found, F < 1. The interaction between validity and cue type was also found significant, F(1, 23) = 6.41, p < 0.05, MSE = 670. In addition, a triple interaction cue type · validity · color preview was found significant F(1, 23) = 7.12, p < 0.01, MSE = 114. As can be seen in the figure, validity effects are present in the location cue condition regardless of whether there was a color preview. The main effect of validity with a location cue is highly significant, F(1, 23) = 12.68, p < 0.005, MSE = 1409, whereas the interaction between validity and color preview was not significant at all, F < 1. In contrast, in the color cue condition the interaction between validity and color preview was found significant, F(1, 23) = 10.58, p < 0.005, MSE = 98. In further, simple effect analyses, the effect of validity was found significant only with a color preview, F(1, 23) = 19.41, p < 0.001, MSE = 141, not with no preview, F < 1. Thus, the orienting of attention in the color cue condition depends on the presence of a color preview: Whereas color cueing was not effective at all with no color preview, it did have a substantial validity effect with such a preview. Since the interaction between validity and cue type was not found significant within the color-preview condition, F(1, 23) = 2.11, p = 0.16, MSE = 325, a significant advantage of location cues over color cues has not been demonstrated, although that might be due to insufficient statistical power. Error rates in target-present trials as a function of cue type, validity and color preview are presented in Table 1. The overall error rate was small (3.5%). Analysis of arcsine square root transforms of percentages of those errors did not yield a significant main effect of validity, F < 1, only of color preview, F(1, 23) = 4.4, p < 0.05, MSE = 0.016. On the other hand, the interaction between cue type and validity was found significant, F(1, 23) = 6.18, p < 0.05, MSE = 0.0097, as well as the triple interaction between

Table 1 Error percentages in Experiment 1 as a function of cue type, color preview and validity Cue type

Color Location

Color preview

Validity Invalid

Valid

Without With Without With

4.3 2.9 2.9 2.8

3.5 2.6 8.2 3.8

cue type, validity and color preview, F(1, 23) = 5.62, p < .05, MSE = 0.0054. Analyses of simple effects show that validity affected accuracy in the location cue condition only, primarily with no color preview for some reason. That is clearly irrelevant concerning whether or not the results of the latency analysis might have been due to speed-accuracy tradeoff. Neither are results of the analysis of false alarms (namely, errors in target-absent trials). The overall rate of false alarms was also small (3.9%). Arcsine square root transforms of percentages of those errors were cast into a within-subject two-way ANOVA, factors of which were cue type and color preview (since target-absent trials cannot, of course, be sorted to valid and invalid). No significant effect was found. It, thus, seems that the typical dramatic difference between effects of color cues and location cues is a trivial corollary of the procedural stipulation that only location cues could guide attention prior to target onset. Clearly, in an experiment without a color preview, which is pretty ubiquitous, space enjoys an inherent advantage. Proponents of the space-based view of visual attention might contend that that inherent advantage of space is exactly what makes it special, hence what must make visual attention space-based. However, as argued above, while granting that space is indeed special, we do not see how that entails that the issue we focus on here of whether or not attention can be directed by non-spatial cues effectively enough is to be prejudiced by a paradigm that is biased in favor of spatial cues. First, outside of the labs of experimental psychologists, visual attention is not always pre-cued and is typically used to select within a representation that has been already pro-

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cessed preattentively. Thus, for the sake of testing the two alternative views of visual attention being pitted here, we consider the temporal vantage of space to be a potential confounding factor. Second, ecological considerations aside, the issue we focus on here is whether or not location cues have an advantage in orienting proper. For that matter, any other advantage, general and interesting in itself as it may be, acts as a confounding factor. 7. Experiment 2 Though some sources of advantage of location cues were removed or ruled out in the color preview condition of Experiment 1, an advantage might still surface with other values of cue-target delay. Experiment 2 was designed to study that further, focusing just on the color preview condition yet with variable cue-target delays. Varying cue-target delay enables comparison of the time courses of the effects of the two cue types (cf. Downing & Pinker, 1985; Laarni, 2001; Nakayama & Mackeben, 1989; Umilta, Mucignat, Riggio, Barbieri, & Rizzolatti, 1994, for studies of the time course of visual attention). Any interaction of those delays with cue type, if observed, would be due to a difference in the temporal courses of orienting attention by the cue types. Should a location cue lead to faster orienting of attention than a color cue does, an a-priori plausible conjecture about the source of that putative advantage is that the process that controls orienting needs a spatial (coded spatial, to be accurate) address. Whereas a location cue provides it readily, a color cue provides just a guide for singling-out, following which the address of the singled-out location has to be computed before it is fed to the attention controller. Such a localizing process most probably takes some non-negligible time. Actually, in the no-preview condition of Experiment 1, that localizing process must follow the termination of the process of interpreting the digit cue by the digit-to-color mapping rule, but then await the encoding of target color. In the color preview condition, on the other hand, that operation may possibly be spared by converting the digitto-color mapping rule (e.g., 1 ! red) to a digit-to-location one (e.g., 1 ! right) once the location–color associations (e.g., red is at the right) have been established. If conversion could be completed by cue onset, no location advantage would be manifested, by hypothesis. However, if such a conversion was really done and completed by cue onset, then even in the color cueing condition the cue would actually function for all practical matters as a location cue. One might feel that though color cueing could be quite effective under these conditions, as demonstrated in Experiment 1, it was only thanks to relegating the pre-orienting selection operations that normally follow cue processing (see Fig. 1) to a preparatory stage that precedes cue onset. Thus, though the advantage of location cueing fails to manifest in this case, it does exist nonetheless because orienting requires prior localization.

To examine whether or not localization is necessary at all, the possibility of pre-cue conversion should be obviated by the procedure. For that purpose, in this experiment we introduced another major feature: We modified the color preview so that color onset was simultaneous with cue onset. The onset of the two circles preceded cue onset, as in Experiment 1, but they were gray until cue onset, and differentially colored exactly at cue onset. In that way, if selection by color cueing called for conversion of digit-to-color mapping to digit-to-location one (or in default of such conversion – required color-to-location mapping), then it would take more time to home in the target by color cueing than is needed in location cueing. If, on the other hand, no temporal advantage of location cueing was observed, that would indicate that no conversion was done, namely that subjects probably started encoding and interpreting both cues with cue onset, then used both for singling out and orienting in due course, which suggests in turn that localization is not absolutely necessary for orienting to take place. 7.1. Method 7.1.1. Apparatus and stimuli Both were identical to those of Experiment 1, except that a color preview was employed in all trials. On the other hand, the circles were gray at onset, and were colored (one in green, one in red) only 750 ms afterwards, namely at cue onset (see diagram in Fig. 4). 7.1.2. Design and procedure The procedure was similar to that of Experiment 1. As in Experiment 1, the task was simple detection. Each exper-

1

33 ms

1

50 - 649 ms

+

750 ms

e

m

Ti Red Green Gray

Fig. 4. The sequence of events in an experimental trial (color-preview condition) of Experiment 2: (a) fixation plus gray circles, (b) cue plus colored circles, (c) flash (plus cue plus colored circles).

R. Kasten, D. Navon / Acta Psychologica 127 (2008) 89–102

imental trial started by the onset of a fixation mark at which the subject was to focus her/his eyes. Along with the fixation mark, two gray circles were presented at both sides of it. At cue onset, the two circles were differentiated by their colors. The target was presented after a variable delay. Three variables were manipulated: (a) validity, (b) cue type, (c) SOA (between cue and target). Eleven SOA levels were used. Their rounded values are 50, 116, 183, 249, 316, 383, 449, 516, 583, 649 and 900 ms. The 900 ms level was aimed just to enhance the resort to the cue (based on pretest results), and was not included in the analysis. The SOA range between 50 ms and 649 ms was selected following a pretest showing that the validity effect starts showing up within that range. The interval between SOA values (66.7 ms) is an integer multiple (4) of the display frame time (16.666 ms). Cue type was manipulated between blocks and was counterbalanced between subjects. There were two blocks, one for each cue type. Each block included 630 experimental trials and 30 practice trials. Four-hundred-fifty experimental trials (about 71.4%) were valid, 90 (about 14.3%) were invalid, and another 90 were catch trials. An extremely high SOA value (900 ms) was used in 33.3% of the trials. The other SOA values were used in 6.7% of the trials each. 7.1.3. Subjects Twenty-four undergraduate students at the University of Haifa participated for course credit at the Department of Psychology. None of them participated in Experiment 1. All subjects had normal or corrected-to-normal vision. Two other subjects were not included in the analysis, one due to a high rate of errors that generated an empty cell in the latency means matrix, another one due to flagrant signs of noncooperation. 8. Results The major analyses were conducted on mean latencies. Trials with latencies shorter than 150 msec or longer than 800 msec (4.6%) were excluded from analysis.4 Data were cast into a within-subject three-way ANOVA, factors of which were validity, cue type and SOA (excluding the 900 ms value). Mean latencies are presented in Table 2 as a function of SOA, cue type and validity. In Fig. 5 mean validity scores (mean latency in invalid trials minus mean latency in valid ones) are presented as a function of cue type and SOA. Significant main effects were found for the factors validity, F(1, 24) = 17.91, p < 0.001, MSE = 3802, and SOA, F(9, 207) = 83.46, p < 0.0001, MSE = 2276. No effect of cue type was found, F(1, 23) < 1. One pairwise interaction was found significant: validity · SOA, F(9, 207) = 2.71, 4 A cutoff of 800 ms was used, rather than 700 ms in Experiment 1, because the latter would have led to elimination of too many trials.

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p < 0.005, MSE = 1309. The validity effect appears to start quite early – at an SOA of 50 msec, and as can be seen in the figure, that seems to be true for both cue types. Post hoc analysis showed that the validity levels at SOA levels of 50 – 516 ms were not significantly different (and their average is significantly positive, F(1, 23) = 12.17, p < 0.002, MSE = 2357), and that the only significant differences were between the levels 583 ms and 649 ms and the levels 116 ms, 183 ms, and 249 ms. The validity at the lowest level, 50 msec, is not significantly different from any of those. No other interaction or main effect was found significant or even close to significance. In particular, the overall validity effect of both cue types was quite similar: Although the validity effect was somewhat smaller in the color cue condition (14 ms) than in the location cue condition (20 ms), the interaction between cue type and validity was not significant, F(1, 23) = 1.86, p = 0.19, MSE = 2512. Further analyses yielded significant effects of validity for both location cues, F(1, 23) = 16.21, p < 0.0005, MSE = 2985 and color cues, F(1, 23) = 10.25, p < 0.004, MSE = 2168. The interaction between cue type and SOA was not significant either, F(9, 207) = 1.00, p = 0.45, MSE = 1292. Particularly informative are the nil simple effects of cue type for the two lowest values of SOA, 50 ms and 116 ms, both F(1, 23) < 1. Neither did cue type interact with validity and SOA, F(9, 207) = 1.42, p = 0.18, MSE = 2494. The overall error rate in target-present trials was 10%. Analysis of arcsine square root transforms of error percentages yielded a significant main effect of SOA, F(9, 207) = 13.61, p < 0.0001, MSE = 0.0424. That effect amounts to increase in accuracy with SOA becoming longer. No other main effect or interaction was found significant. Thus, no sign of artifactual effects of speed-accuracy tradeoff was found. The overall rate of false alarms was also small (7.4%). Arcsine square root transforms of percentages of those errors were cast into a within-subject two-way ANOVA, factors of which were cue type and SOA (since target-absent trials cannot, of course, be sorted to valid and invalid). No significant effect was found. Thus, the results do not support any advantage of location cues over color cues when a color preview is available, even one that does not precede cue onset. The lack of any such advantage is glaring at SOA levels below 500 ms. As for levels beyond that, though it appears that such an advantage does exist, we failed to obtain a significant triple interaction due to the fine resolution of SOA that we used in the design. It is quite possible that with greater power, a location advantage would be found at SOA levels beyond 500 ms.5 That, however, would not be pertinent to the hypothesis in question. 5 Actually, had we focused just on the three SOA levels above 500 ms (ignoring the ban on that by customary hypothesis testing procedures), the interaction between cue type and validity would be significant, F(1, 23) = 8.25, p < 0.01, MSE = 1817.

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Table 2 Mean latency in Experiment 2 as a function of SOA, cue type and validity SOA

CUE TYPE

Total

Location

Color Invalid

Total

Valid

Invalid

Total

Valid

Invalid

50 116 183 249 316 383 449 516 583 649

510 456 428 417 401 389 385 369 363 361

532 458 429 424 414 399 398 411 403 414

521 457 429 420 408 394 391 390 383 388

514 450 421 417 397 395 376 379 370 363

541 457 421 421 417 411 391 377 396 387

527 454 421 419 407 403 384 378 383 375

512 453 425 417 399 392 381 374 367 362

536 458 425 423 415 405 394 394 399 400

Total

408

428

418

408

422

415

408

425

Validity (invalid-valid)

Valid

Validity as a function of cue type and SOA

60 55 50 45 40 35 30 25 20 15 10 5 0 -5 -10

Color cue Location cue

50

116

183

249

316

383

449

516

583

649

SOA (ms)

Fig. 5. Mean validity scores as a function of cue type and SOA in Experiment 2.

Were orienting by color cues really mediated by localization, they would be expected to fail to produce a validity effect at some short SOAs at which location cues already do produce a validity effect. Thus, the hypothesis concerns the time course within the low sub-range of SOAs. Clearly, the results do not provide any shred of evidence that the attentional impact of color cues develops more slowly than the impact of location cues. A difference between the validities of the cue types seems to appear first at an SOA of 516 ms. That, however, was not the lowest SOA level where the color cue had an effect: Across the SOA subrange of 50-449 ms, where the main effect of validity was found significant, F(1, 23) = 12.00, p < 0.005, MSE = 1729, the validity levels of both cue types were found to be the same, as indicated by a non-significant effect of validity · cue type interaction, F(1, 23) < 1. The conclusion remains the same even when we suspend the statistical norm to avoid testing for simple effects within a sub-range for which no significant difference was found between them. If localization took some non-negligible time, then the time course of the validity of color cues would lag after the time course of the validity of location cues. That entails

that there has to be some SOA level where the overall validity was significant, and the effect of the validity of location cues was significantly larger than the validity of color cues, while both validites were non-significant at the immediately shorter SOA level. That was found to be trivially false for the shortest SOA level (50 ms): A significant simple effect of validity, F(1, 23) = 8.17, p < 0.01, MSE = 1758, but a non-significant effect of validity · cue type interaction, F(1, 23) < 1. But even if we elect to disregard the finding for that SOA level because the validity-SOA curve appears to show a dip following it, we should note that the same pattern recurs once the curve rises from that apparent dip. At SOA = 316 ms a significant simple effect of validity was found, F(1, 23) = 5.01, p < 0.04, MSE = 1227, but the effect of validity · cue type interaction was still not found significant, F(1, 23) < 1. The same pattern is true also of the next two SOA levels (383 and 449 ms). So the prediction does not bear out. Thus, even though color cues are less effective than location cues at very high levels of SOA, their impact in SOA level shorter than half-a second is most probably not mediated by a localization operation. 9. General discussion In this study, we examined the claim that location cues are faster and more effective in directing visual attention than non-spatial cues are. Our results do not lend support to that notion. They rather demonstrate that once the experimental procedure enables the subject to use any cue as soon as it is perceived, color cues start to be effective as early as location cues are. Since both cue types do not provide any information pertinent to selecting the response (detection of darkening), the results must tell us something about input selection, which must be mediated by visual attention. Since the time courses of validity for the two types were found to be similar up to SOA levels of half a second, no

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support was obtained for the hypothesis, derived from the space-based view of visual attention, that the impact of non-spatial cues is indirect in being mediated by localization, namely by locating the cued attribute and retrieving its spatial address by means of a time consuming computation. Plausible as that might appear, the findings at least leave open the converse hypothesis, namely the claim that non-spatial cues can trigger the orienting process without any mediating computation of a spatial address. 9.1. Implications for the space-based model Since most people believe that location is a sui-generis category, the latter notion may seem odd. Even proponents of the object-based view may find it conceptually hard to figure what it means to attend to an object without selecting its location. Note, for example, that the object-based view may be interpreted as sharing with the space-based view most basic assumptions, differing only in regarding the candidates for selection as patterns of ‘‘pixels’’ within a representation generated by having parsed raw sensory stimulation onto an organized perceptual field (see Logan, 1996). Logan claims that object-based attention, like spacebased attention, assumes that ‘‘selection is spatial because objects necessarily occupy regions in space...’’ (p. 603). In other words, the attentional ‘‘spotlight’’ is directed to regions of a spatial representation, but seldom – and only with extra effort – to regions that disregard object boundaries. This version of the object-based attention approach is actually a variant of the spatial view, though a variant that does not regard empty space as the medium to which visual attention applies, as the space-based variant does. To the extent that object-based attention selects spatial domains of objects in an organized perceptual field, it must also predict that spatial cues would be special. However, a model of object-based visual attention may in theory postulate a set of candidates that are not locations, hence that need not be accessed by a spatial address. For example, if object files (Kahneman & Treisman, 1984) were generated preattentively, visual attention would be able to select any one of them by some encoded non-spatial attribute like color. Yet, even granting that the representation is spacebased, the notion of non-spatial selection does not look odd at all, once it is admitted that location need not be the most, let alone the only, manifest information in a representation (see Navon, 1990, for a distinction between manifest, latent and irrecoverable information in perceptual representations). Even physical objects in real space are not always reached by a process that computes directions by using information about their locations. Consider the following analogy: Imagine a one-armed police robot devised to pick up, using its single arm, a suspected item that is known to be located somewhere on a plate, along or without other items. The arm is clearly an extremely limited resource that can be operative only after reaching an object which requires traveling in physical

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space. The robot can shift its arm to the item’s location once it receives that location, either from outside or by its own detection. If the item is not yet on the plate, the robot could prepare ahead of time provided it had some reasonably valid information of the item’s future location. That seems to be the only reasonable procedure in the case that the item cannot be distinguished from other ones by any single property or that that property cannot attract the arm directly. But now imagine a different case: Suppose, for example, the suspected item is the only one on the plate made of iron. Further suppose that the arm is equipped with a magnetic device that is switched on once it is known that only suspected items are made of iron. In that case, the arm could aim at, and reach at, the iron-made item without having to compute its location first. That should be as fast as, or even faster than, a computed-location-mediated process. Note, however, that the magnet would not be able to help the robot before the item was actually on the plate. Thus, a direct orienting process is conceivable provided that the to-be-oriented object can be singled out by a property that may serve as an attractor. In that case, prior localization of the object would be gratuitous, hence selection by location would not be superior to selection by that attractor property. 9.2. Other grounds for hypothesizing direct selection How plausible is it that non-spatial attributes can serve as attractors in the visual representation generated by preattentive processes in the brain? True, some early stores in the primary visual cortex are known to be spatiotopic (see Cowey, 1979). On the other hand, the orderly spatial arrangement of cortical detectors of different orientations or different colors (e.g., Blasdel, 1992; Blasdel, Obermayer, & Kiorpes, 1995; Bonho¨ffer & Grinvald, 1991; Grinvald, 1992; Hubel & Livingstone, 1990; Livingstone & Hubel, 1984) renders the notion of selection by non-spatial attributes neurophysiologically reasonable as well. Furthermore, although it has been shown that visual attention affects processing which is believed to be cortical (e.g., Balz & Hock, 1997; Maunsell & Hochstein, 1991; Moran & Desimone, 1985; Motter, 1993; Spitzer, Desimone, & Moran, 1988; Yeshurun & Carrasco, 1998, 1999), it may also operate on representations further downstream. It is not clear that those later representations are spatial, let alone that they are not accessible by any non-spatial attribute. Thus, it is no longer straightforward to postulate that visual attention must be selecting regions in space. In principle, attention could be set to pick from some representation accessible to it a site representing a stimulus marked by having a specific color or form just as it could be set to pick a site marked by having a specific location. Were that so, there would be no reason to expect attentional selection to be necessarily mediated by addressing spatial location – neither points or regions in a pre-parsed

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space nor spatial domains of objects in an organized perceptual field. Attention could be guided by various nonspatial cues directly to the desired site (see, e.g., Duncan, 1984). Within such a view, the effectiveness of a dimension for serving as a cue depends on the extent to which it discriminates stimuli in the visual field. Location may be the preferred choice in many cases. In other cases (like in experiments with superimposed stimuli, e.g., Duncan, 1984; Goldsmith, 1998), location (at least in the sense of a closed, convex region) cannot be used or is at least less preferable than other dimensions like color. All that does not entail that location is not special in a way. Clearly, visual attention is distributed over represented space, at least until encoding is over. Clearly, processing of a stimulus is affected by the amount of attention allocated to the segment of space where it is located. Clearly, attention can select locations even in a blank space. Location might be special not in being the only way by which objects can be accessed. It might, for example, be special in being a mandatory or a salient attribute within object files (see Kahneman & Treisman, 1984). In other words, once a file is accessed by any route, information about location is more available in it than other sorts of information, like that about color. Another possibility is that files having similar locations may be more interrelated than sites having similar color. Thus, once a given file is accessed, it is more likely to refer further attention to another file having the same location than to another file having, for example, the same color. That notion helps to accommodate the finding that even after attending to the color of a stimulus, subjects tend to report closer targets more than the ones having the same color (Tsal & Lamy, 2000; Tsal & Lavie, 1993) with the possibility indicated here that the initial access to a file may not be necessarily mediated by stimulus localization. 9.3. Cueing specificity? One might contend that the findings we obtained should be attributed to the kind of symbolic cueing we employed. Mapping digits to locations, however practiced it might be in the experiment, is clearly not as natural or as habitual as the process which uses arrows to orient attention. It might further be argued that whereas any habitual, probably implicit, process that uses cues to guide visual attention must be space-based, that need not be the case when the cue requires for interpreting its reference a controlled, non-habitual process. For example, symbolic cues that are arbitrarily mapped to to-be-attended objects may perhaps guide visual attention in any arbitrary way, so that location may not be special in any qualitative way for that type of cue, but not for others. The difficulty with this dichotomy is that however natural and habitual arrows are as indicators of location, cueing by arrows is considered for various reasons to be an instance of endogenous cueing employing symbolic cues for directing attention at will (e.g., Jonides, 1981).

Although arrows were recently found to elicit some automatic orienting (Hommel, Pratt, Colzato, & Godijn, 2001; Ristic, Friesen, & Kingstone, 2002; Tipples, 2002), they are known to be pretty much under voluntary control. In any event, further tests with less symbolic cues, like arrows, would be quite pertinent. 9.4. Decidability Our conclusions rest on the assumption that the localization operation entailed by the strong version of the space-based model must take non-negligible time, enough to be detectible by our design. One might contend that perhaps such an operation does not take time (at least not within an order of magnitude measurable by presently available means). Though generalizing that logic may lead to questioning the basis of mental chronometry as a tool for studying information processing, it is undeniable that the possibility surmised above might be true. However, even if it was, that would not entail that the strong version of the space-based model was valid. To the extent that the localization operation really did not cost any measurable time, the issue would be undecidable. That, however, should not be held as a pending onus on the shoulders of advocates of direct selection more than as a pending onus on the shoulders of advocates of computed-location-mediated selection, unless the latter was taken for granted on some other ground. Is there any reason to take it for granted? Note that the belief in it results from inference by extension: Since locations in space can surely be selected, even prior to stimuli presentation, it seems sound to suppose that objects are selected via location. That is a reasonable way to formulate a hypothesis, not to adopt it as a preconception, let alone one that rolls away the burden of proof to alternative notions. If that hypothesis proved to be impossible to falsify, that would hardly be a good reason to admit it as a scientific claim. At least Popper would not. With due caution, it seems right to state that though admittedly the issue could be undecidable, presently it is just undecided yet. Testing between the views in question calls for converging operations. The one presented in this paper is just one of them. The results reported here somewhat tip the scale in favor of direct selection by shaking confidence in the belief that non-spatial cueing is inherently inferior to location cueing. Because of the different nature of space and attributes such as color, there may be a real difference between the roles typically played by them in visual attention. However, speed of selection may not be one of them. Acknowledgements This paper represents a shared contribution of both authors. The experiments reported in it followed on ones conducted by the first author for his doctoral thesis under the supervision of the second author. They were supported

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