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1 This article was downloaded by: [Royal Holloway, University of London] On: 16 May 2013, At: 12:24 Publisher: Routledge Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK The Quarterly Journal of Experimental Psychology Publication details, including instructions for authors and subscription information: An isolation effect in serial memory for spatial information Katherine Guérard a, Robert W. Hughes b & Sébastien Tremblay c a École de Psychologie, Université Laval, Québec, Canada b School of Psychology, Cardiff University, Cardiff, UK c École de Psychologie, Université Laval, Québec, Canada Published online: 21 Apr To cite this article: Katherine Guérard, Robert W. Hughes & Sébastien Tremblay (2008): An isolation effect in serial memory for spatial information, The Quarterly Journal of Experimental Psychology, 61:5, To link to this article: 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, redistribution, reselling, loan, 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 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY 2008, 61 (5), An isolation effect in serial memory for spatial information Katherine Guérard École de Psychologie, Université Laval, Québec, Canada Robert W. Hughes School of Psychology, Cardiff University, Cardiff, UK Sébastien Tremblay École de Psychologie, Université Laval, Québec, Canada This paper reports the first demonstration of an isolation effect or von Restorff effect (von Restorff, 1933) in the context of a spatial-memory task: Short-term serial recall was enhanced for both the location and the serial position of one red dot presented amongst a sequence of otherwise black dots. When the serial position of the isolate was fixed, the spatial isolation effect only emerged when participants received a control block of trials before the block of isolation trials (Experiment 1). However, when the serial position of the isolate was varied across isolation trials, an isolation effect was still produced regardless of condition order (Experiment 2). It is suggested that both temporal grouping strategies and greater item-specific processing may have contributed to the enhanced retention of the isolate. When an item differs in some way from the other items in a verbal to-be-remembered sequence, memory for the distinct item is enhanced. This isolation effect or von Restorff effect as it is also commonly known following its discoverer (von Restorff, 1933) is a robust phenomenon that has been replicated numerous times in many different methodological settings. Although an isolation effect has been found when an item is postcategorically distinct from its neighbours (e.g., a digit amongst syllables; Fabiani & Donchin, 1995), in most demonstrations the isolate has been precategorically (or perceptually) distinct, in terms of, for example, its colour (e.g., Huang & Hynum, 1970; Rosen, Richardson, & Saltz, 1962) or size (e.g., Huang & Hynum, 1970; Kelley & Nairne, 2001). Such effects in which isolation is implemented through precategorical attributes suggest that the phenomenon is unlikely to be related specifically to verbal Correspondence should be addressed to Katherine Guérard, École de psychologie, Université Laval, Québec, G1K 7P4, Canada. katherine.guerard.1@ulaval.ca Thanks are due to Josée Bluteau for assistance in running the experiment. We would also like to thank François Vachon for critical reading of an earlier draft. This research was supported by operating grants from the Natural Sciences and Engineering Research Council of Canada to Sébastien Tremblay. 752 # 2007 The Experimental Psychology Society DOI: /

3 AN ISOLATION EFFECT IN SPATIAL MEMORY memory. Nevertheless, it remains the case that the isolation effect has, hitherto, only been shown in the context of verbal-memory tasks. In this article, we examine whether the isolation effect extends to the spatial domain and, if so, consider the extent to which it can be explained by recourse to constructs derived from verbalmemory studies. Examining whether the isolation effect extends to the spatial-memory domain commands particular interest in light of the wide appeal in the shortterm memory (STM) literature to the construct of item distinctiveness (e.g., Brown, Neath, & Chater, in press; Brown, Preece, & Hulme, 2000; Nairne, 1990; Neath, 1993, 1999). Distinctiveness theories of memory posit generally that the more distinct (temporally, perceptually, or postcategorically) an item is from its neighbours, the better it will be recalled. Such theories are well equipped therefore to provide an explanation of the isolation effect. However, although item distinctiveness refers to a general concept (i.e., one that should apply regardless of the particular content of the to-be-remembered material) such theories as well as most theories of memory have been based overwhelmingly on data from verbal studies (e.g., see Crowder & Neath, 1991; Kelley & Nairne, 2001; Lewandowsky, Nimmo, & Brown, in press; Neath & Crowder, 1990, 1996). The dearth of research on nonverbal serial STM makes it difficult to know whether the concept of item distinctiveness does in fact have such wider applicability. The expectation that an isolation effect should be observed in the spatial STM domain seems a reasonable one on several grounds. First, the degree of distinctiveness between to-be-remembered spatial stimuli influences serial recall. For example, serial recall for matrix patterns is a positive function of the number of cells by which the patterns differ from one another (Avons & Mason, 1999). Second, an effect of isolation has been demonstrated in a task of tachistoscopic perception of linear patterns (Harcum, 1968). In this study, arrays of filled and empty circles were presented, and the participants task was to reproduce the pattern that they had just perceived. It was found that if one of the elements was presented with a line below and above it, that element was perceived correctly more often than the other items. Because distinctiveness has a facilitative effect on recall and perception of spatial information, it seems plausible that short-term recall for spatial information should benefit from the distinctiveness inherent to an isolated item. A more general reason for expecting an isolation effect in the spatial STM domain is that despite the general verbocentrism of memory research many canonical verbal STM phenomena have begun to be replicated with spatial stimuli. For example, similar serial position curves with recency and primacy effects are observed in serial recall of verbal and spatial sequences (Avons, 1998; Jones, Farrand, Stuart, & Morris, 1995). Other phenomena such as the disruptive effect of to-be-ignored stimuli including irrelevant sound stimuli (Jones et al., 1995) and suffixes (Parmentier, Tremblay, & Jones, 2004), the beneficial effect of temporal grouping (Parmentier, Andrés, Elford, & Jones, 2006), and Hebb list repetition (Couture & Tremblay, 2006; Turcotte, Gagnon, & Poirier, 2005) have been demonstrated in spatial as well as verbal serial recall. Such evidence of functional equivalence has led some investigators to posit a single mechanism for verbal and spatial memory (e.g., Jones et al., 1995) and others to propose distinct mechanisms but those that follow similar general principles (Logie, 1995). Thus, establishing a spatial isolation effect would serve to buttress the view that, in the serial STM domain, verbal and spatial materials are subject to processes that are at least highly similar. In addition to seeking to establish whether the isolation effect extends to the spatial STM domain, we also examined whether any spatial isolation effect observed is related to enhanced item memory, order memory, or both. Phenomena of STM can often be distinguished according to whether they are related to the retention of item or order memory and the item order distinction has come to play a key role in theorizing in this area (Healy, 1974; Nairne & Kelley, 2004; Poirier & Saint-Aubin, 1996). For example, THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (5) 753

4 GUÉRARD, HUGHES, TREMBLAY whereas word frequency and lexicality are known to enhance memory for item information, they have no effect on the processing of order (Nairne & Kelley, 2004). To date, however, the available evidence from verbal studies suggests that the isolation effect is one phenomenon to which the distinction does not seem to apply: Enhanced memory for the isolate is observed not only when item memory is emphasized, such as in free recall (e.g., Bellezza & Cheney, 1973; Huang, Ballering, & Nikl, 1974), but also in the context of a serial reconstruction task in which most if not all of the burden of processing falls upon order rather than item retention (Kelley & Nairne, 2001). However, the relative contribution of order and item memory to the enhanced recall of the isolated item has not yet been investigated systematically. There is some evidence that the distinction between order and item memory holds also in the spatial-memory domain: In a comparison between verbal and spatial serialrecall tasks it has been found that the distribution of order errors is bowed shaped whereas item errors increase as a function of serial position (Avons, 1998; Avons & Mason, 1999). Nevertheless, the item order dissociation remains far less well established in the spatial than in the verbal domain. Finally, we also consider the extent to which the particular signature of any spatial isolation effect observed can be explained by the two most influential classes of explanation of the verbal isolation effect. One approach to the phenomenon the grouping account supposes that the presence of the isolate serves to promote a reorganization of the sequence into one that is easier to rehearse (e.g., Cimbalo & Pelonero, 1970; see also Cimbalo, Capria, Neider, & Wilkins, 1977). Another class of explanation centres on the construct of itemspecific processing : Recall of the isolate is enhanced because that particular item enjoys a richer degree of encoding than do the nonisolated items (e.g., Fabiani & Donchin, 1995; Hunt & Lamb, 2001) or because it provides a more effective retrieval cue (e.g., see Kelley & Nairne, 2001; Nairne, 2006). EXPERIMENT 1 Both experiments reported here involved a spatial analogue of the classical verbal serial-recall task in which participants are asked to serially recall sequences of spatial locations (the dots task; Jones et al., 1995). In line with most studies of the isolation effect in the verbal domain, the isolate was always presented in the same serial position (e.g., Lippman, 1983; Rosen et al., 1962). Specifically, in one condition, the fifth out of nine to-be-remembered dots was always presented in a different colour (red) from the remaining dots in the sequence (black). We hypothesized that recall of the isolated item would be better than for the fifth item in control trials consisting of nine black dots. Method Participants A total of 46 students from Université Laval volunteered to take part in the experiment in exchange for a small honorarium. All reported normal or corrected-to-normal vision. Apparatus and materials The stimuli were dots of 1.2 cm in diameter presented at different locations on the screen of a PC, within a cm window. Two sets of 25 sequences were constructed from random orderings of the same nine dot locations. The control block consisted of 25 sequences in which each dot was black. The isolation block was identical to the control block except that the fifth dot was always presented in red. For each trial, the nine dots were presented at a rate of one per second (500 ms on, 500 ms off). Within each block, the sequences were presented in a different random order for each participant. The order of the blocks was counterbalanced across participants. Design The experiment had two repeated measures factors: condition (two levels; control or isolation) and serial position (nine levels; 1 to 9). Condition 754 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (5)

5 AN ISOLATION EFFECT IN SPATIAL MEMORY order (two levels; control isolation or isolation control) constituted a third, between-participants, factor. Procedure Participants were tested individually in a dimly lit room. The participants first read standard instructions informing them that sequences of dots would be presented on the screen and that their task was to recall the locations of the dots in the order in which they were presented. They were informed that in one block, the fifth item would always be presented in red, but they were told not to pay any more attention to it than to the other stimuli. Following the disappearance of the last dot, a thermometer appeared at the right of the window. The participants were required to click in the window, where the dots had appeared, and in their order of appearance. Each time a response was given, the thermometer raised by one degree so that the participant could keep track of how many responses they had made. The experimental session lasted approximately 30 min. Results and discussion The data were scored as follows: An area of acceptance was determined for a response to the location of each dot to be considered as correct. The acceptance area was determined according to the coordinates of the locations of the nine dots. The shortest distance separating two dots was first halved. This value, 49.1 pixels, was then used as the radius for the acceptance area around each dot. Using this measure meant that we had the largest possible acceptance area for each dot without one dot s acceptance area ever overlapping with that for any other dot. For example, any response that was 49.1 pixels or less from the Figure 1. Proportion correct as a function of serial position in the control and isolation conditions, for the control isolation group (upper panel), and for the isolation control group (lower panel) in Experiment 1. Error bars represent 95% confidence intervals. centre of Dot 1 was scored as a correct response for Dot 1 only. This method allowed a comparison of item and order errors using the same type of dependent variable: the proportion of errors. 1 Participants responses were also scored according to a strict serial-recall criterion: Each dot had to not only be located accurately (within 49.1 pixels) but also had to be recalled in the same serial position as that in which it was presented. Figure 1 shows the proportion of dots correctly recalled in the control and isolation conditions for each of the nine serial positions according to the order in which participants received the control 1 Other methods such as the city block procedure (e.g., Farrand & Jones, 1996) and the best-fit solution (e.g., Postma & de Haan, 1996) provide continuous measures in relation to the distance between the response and the location rather than a binary outcome. However, such measures make the distinction between item and order difficult. The increased sensitivity of these procedures is not necessary in the current study as the classical binary scoring method proved to be sensitive to a panoply of effects such as grouping (Parmentier et al., 2006) and interference (Tremblay, Nicholls, Parmentier, & Jones, 2005) and here, to the isolation effect for both item and order criteria. THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (5) 755

6 GUÉRARD, HUGHES, TREMBLAY and isolation blocks. It is immediately apparent that in the control isolation group, a spatial isolation effect was produced: Recall of the fifth dot was enhanced compared to that of the fifth dot in control lists. However, there appears to be no such effect for the isolation control group. This pattern was confirmed by a 2 (condition) 2 (condition order) 9 (serial position) mixed analysis of variance (ANOVA) on the proportion of correct responses. The main effects of condition, F(1, 44) ¼ 9.15, MSE ¼ 0.02, p,.01, and serial position, F(8, 352) ¼ , MSE ¼ 0.02, p,.001, were significant, but the main effect of condition order was not (F, 1). The interactions between condition and condition order, F(1, 44) ¼ 4.08, MSE ¼ 0.02, p,.05, and between condition and serial position, F(8, 352) ¼ 6.31, MSE ¼ 0.01, p,.001, were significant, as was the triple interaction between condition, condition order, and serial position, F(8, 352) ¼ 2.88, MSE ¼ 0.01, p,.05. The interaction between condition order and serial position was not significant, F(8, 352) ¼ 1.30, MSE ¼ 0.02, p ¼.26. Simple effects analyses showed that the interaction between condition and serial position was significant in the control isolation group, F(8, 176) ¼ 7.60, MSE ¼ 0.01, p,.001, but not in the isolation control group, F(8, 176) ¼ 1.58, MSE ¼ 0.01, p ¼.17, again reflecting, at least in part, the fact that an isolation effect was found in the control isolation group but not in the isolation control group. Because an isolation effect was only found for the control isolation group, subsequent analyses focused exclusively on the data from this group. Analysis of item versus order errors To examine a possible dissociation between the action of the isolate in relation to order and item retention, the proportions of item and order errors were assessed for the fifth item in the control and isolation conditions (see Figure 2). An item error was recorded each time the fifth item was not recalled. An order error was recorded each time the fifth item was recalled in the incorrect serial position. The presence of the isolate appears to have reduced the proportion of order Figure 2. Proportions of order and item errors for Item 5 in the control and isolation conditions of the control isolation group in Experiment 1. Error bars represent 95% confidence intervals. errors and, to a lesser extent, the proportion of item errors. In order to confirm this pattern, we conducted a 2 (error type; item or order) 2 (condition; control or isolation) repeated measures ANOVA on the proportion of item and order errors for the control and isolation conditions. The main effects of error type, F(1, 22) ¼ 7.44, MSE ¼ 0.02, p,.05, and condition, F(1, 22) ¼ 51.90, MSE ¼ 0.02, p,.001, were significant. The interaction between error type and condition was also significant, F(1, 22) ¼ 28.35, MSE ¼.01, p,.001, confirming that item errors were reduced to a lesser extent than order errors by the presence of the isolate. Simple main effects analyses revealed that the proportion of order errors, F(1, 22) ¼ 79.52, MSE ¼ 0.01, p,.001, as well as the proportion of item errors, F(1, 22) ¼ 6.51, MSE ¼ 0.01, p,.05, were significantly higher in the control condition than in the isolation condition. Experiment 1 established an isolation effect in the domain of spatial memory: A dot presented in a different colour from the remaining dots in a spatial serial-recall task was better recalled than a corresponding item in a control list. The spatial isolation effect was found to be attributable to an enhancement of memory for order, but also, to a lesser extent, to an enhancement of memory for item information. That the isolate conferred an 756 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (5)

7 AN ISOLATION EFFECT IN SPATIAL MEMORY advantage in terms of both item and order retention is consistent with studies of the isolation effect in the verbal domain (e.g., Bellezza & Cheney, 1973). Another key feature of the results is that, at first glance, an isolation effect was only found when participants encountered the control condition first. However, this pattern may have emerged due to the isolate s promotion of a certain grouping strategy, in line with a grouping account of isolation effects in which participants, in response to the presence of an isolate, spontaneously adopt a strategy of reorganizing the sequence into one that is easier to rehearse (see, e.g., Cimbalo & Pelonero, 1970). To elaborate, the shape of the serial position curve for the control condition of the isolation control group (Figure 1, lower panel) indicates that participants tended to group the list into one group of five and one group of four. This observation suggests a plausible explanation for the condition order effect and the apparent absence of an isolation effect for the isolation control group: Participants in this group, having adopted a four five grouping strategy in response to the isolation trials, may have carried this strategy over to the subsequent control block (despite the fact that none of the lists actually contained an isolate in this block) thereby obscuring any isolation effect. Consistent with this, note that there is still a marked peak in recall for the fifth item in the isolation trials for the isolation control group. That is, it is the alteration in the shape of the curve for the control lists for the isolation control group (relative to that for the control isolation group) that seems to have blocked the peak in performance for the fifth item in isolation trials from showing up as an isolation effect. To confirm whether or not this was the case, we conducted a further 2 (condition) 9 (serial position) mixed ANOVA to examine whether there was an isolation effect in the isolation control group when their data from the isolation block was contrasted with the control data from the control isolation group (which serves as a control condition uncontaminated by any carry-over of a grouping strategy). In line with our contention that the isolation control group did in fact exhibit an isolation effect, this analysis revealed that the crucial condition by serial position interaction was significant, F(8, 352) ¼ 2.05, MSE ¼ 0.01, p,.05. EXPERIMENT 2 To test further the grouping explanation, a second experiment was carried out in which the serial position of the isolated item was varied and was determined randomly for each isolation trial. If the isolation effect obtained in Experiment 1 arose because participants used the isolate as an anchor point around which to temporally group the list, the effect should disappear when the position of the isolate is random and hence far less likely to serve as a useful anchor point. The objective of Experiment 2 is therefore to verify whether the results obtained in Experiment 1 are specific to the type of procedure used, particularly the fact that the isolate was always presented at the fifth serial position. Thus, in this experiment, the same general methodology and procedure as those used in Experiment 1 were adopted but the red item appeared randomly at any one of the nine serial positions. Method Participants A total of 20 students from Université Laval volunteered to take part in the experiment in exchange for a small honorarium. All reported normal or corrected-to-normal vision. Apparatus and materials The materials were the same as those in Experiment 1. A total of 18 sequences in which all items (control items) were presented in black served as control trials. A total of 54 sequences served as isolated trials in which one of the nine items was presented in red (isolated item). All of the other items of the sequence were presented in black (nonisolated items). The isolated item was presented six times at each serial position THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (5) 757

8 GUÉRARD, HUGHES, TREMBLAY and appeared randomly in one of the nine possible spatial locations. Design The experiment had two repeated measures factors, which were item type (three levels; control, isolate or nonisolate) and serial position (nine levels; 1 to 9), and one between-participants factor namely, condition order (two levels; control isolation or isolation control). Within each block, the same sequences were presented in a different random order for each participant. The control trials and isolated trials were blocked and counterbalanced across participants as in Experiment 1. Procedure The procedure was the same as that in Experiment 1. Results and discussion Figure 3 depicts performance for the control items in the control block and for the isolated and nonisolated items from the isolation block for the control isolation (upper panel) and isolation control group (lower panel). Clearly, performance for the isolates is higher than that for the control and nonisolated items regardless of condition order. This was confirmed by a 2 (condition order) 3 (item type) 9 (serial position) mixed ANOVA carried out on the proportion of correct responses. The analysis revealed that the main effects of item type, F(2, 36) ¼ 12.66, MSE ¼ 0.03, p,.01, and serial position, F(8, 144) ¼ 38.83, MSE ¼ 0.03, p,.001, were significant, but not the main effect of condition order, F(1, 18) ¼ 1.83, MSE ¼ 0.22, p ¼.19. The interactions between item type and condition order, F(2, 36) ¼ 1.37, MSE ¼ 0.03, p ¼.27, between serial position and condition order, F, 1, between item type and serial position, F(16, 288) ¼ 1.92, MSE ¼ 0.01, p ¼.09, and between item type, serial position, and condition order, F, 1, were not significant. Pairwise comparisons revealed a significant difference between the recall of control and isolated items (p,.01) and between that of the isolated and nonisolated Figure 3. Proportion correct as a function of serial position in the control and isolation conditions, for the control isolation group (upper panel), and for the isolation control group (lower panel) in Experiment 2. Error bars represent 95% confidence intervals. items ( p,.01), but no difference between the recall of the control and nonisolated items (p ¼ 1). These analyses indicate that enhanced recall is restricted to the isolate. Analysis of item versus order errors Given that there was no effect of condition order, this factor was not included in the following analysis. Moreover, given that performance for the nonisolated items was not enhanced by the presence of the isolate, the analysis of errors was carried out on performance for the isolated and control items only. Inspection of Figure 4 suggests that item and order errors were both increased by the presence of the isolate. A 2 (error type; order or item) 2 (item type; control or isolation) repeated measures ANOVA was carried out on the mean proportion of errors in each condition. The main effect of error type, F(1, 19) ¼ 43.94, 758 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (5)

9 AN ISOLATION EFFECT IN SPATIAL MEMORY consistent with this two-component analysis: Effect size calculations revealed that the isolation effect in Experiment 2 which we suggest might reflect the action of an item-specific encoding mechanism alone was markedly smaller than that found in Experiment 1: The analysis of the magnitude of the isolation effect in Experiment 2 resulted in a Cohen s d of 1.60 whereas that for the isolation effect in Experiment 1 revealed a Cohen s d of Figure 4. Proportions of order and item errors for the control and isolated items in Experiment 2. Error bars represent 95% confidence intervals. MSE ¼ 0.01, p,.001, suggested that there were more order errors than item errors. The effect of item type, F(1, 19) ¼ 14.61, MSE ¼ 0.01, p,.01, was significant, but the interaction between error type and item type was not significant, F, 1. Experiment 2 replicates the isolation effect in the spatial domain and shows that it generalizes to a situation in which the position of the isolate is varied across trials. The increase in performance for the isolate was due to better memory for order as well as item information. Finally, the effect of condition order observed in Experiment 1 was not reproduced in Experiment 2. The survival of the isolation effect with randomly positioned isolates suggests that our grouping explanation of the spatial isolation effect based on the pattern of results obtained in Experiment 1 does not provide a comprehensive account of the phenomenon. One possibility is that the grouping account is not so much incorrect as incomplete. Specifically, when the isolate is fixed, the isolate may indeed serve as a temporal grouping cue but also may enjoy a richer degree of item-specific encoding (e.g., Fabiani & Donchin, 1995; Hunt & Lamb, 2001; see General Discussion). When the position of the isolate is random, the richer item-specific encoding for the isolate is alone responsible for the isolation effect. One strand of evidence from the present data is broadly GENERAL DISCUSSION To summarize the main results of the study, we have shown for the first time that the isolation effect, or von Restorff (1933) effect a phenomenon that had hitherto only been demonstrated using verbal to-be-remembered material extends to the spatial-memory domain. Performance for the isolate was enhanced regardless of whether its position was fixed or randomized, although the magnitude of the effect was larger when the isolate was presented in a fixed serial position (Experiment 1) than when presented in a random serial position (Experiment 2). Also, when the isolate was presented in a fixed serial position, performance was modulated by the order in which the conditions were presented, but not when it was random. Finally, enhanced recall for the isolate was the result of better memory for order information as well as for item information. This result is consistent with previous studies showing that order and item memory are both improved by the presence of an isolate (e.g., Bellezza & Cheney, 1973). However, item and order memory were equally enhanced by the presence of the isolate in Experiment 2, whereas Experiment 1 indicated that memory for item information was affected to a lesser extent than memory for order information. Experiment 1 indicated that when the isolate is fixed, participants seem to exploit its presence in order to group items anchored around the isolate. This grouping strategy appeared to be responsible for the condition order effect: The THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (5) 759

10 GUÉRARD, HUGHES, TREMBLAY participants that started with the isolation condition carried over this strategy in the control condition, which obscured the isolation effect in that group. This pattern of results has been observed previously by Smith and Stearns (1949) who presented two series of words for serial learning: a control list and one containing a centrally isolated red item. Participants who had started with the isolated list showed an isolation effect for the control list that is, enhanced performance for the control item presented in the serial position in which the isolate had been presented. They suggested that participants used the presence of an isolate as an anchor point in the list in order to help recall and carried this strategy over to the following trials. One implication of this condition order effect is that the magnitude of the isolation effect may have often been underestimated insofar as many studies have examined the impact of a centrally isolated item with the isolation and control trials intermixed pseudorandomly within a single block of trials (e.g., Cimbalo & Pelonero, 1970; Kroll, 1972). The abolition of the condition order effect in Experiment 2 further supports the grouping account: Since the participants did not know at which position the isolate would appear, it would have been difficult to use it as an anchor for the remaining items. The grouping strategy used in Experiment 1 could also account for the finding that order memory was enhanced to a greater extent by the presence of the isolate than was item memory. For example, Ryan (1969) reported that temporal grouping gains most of its advantage from an improvement in the recall of the order of the items within a sequence, as opposed to the content alone of the sequence (p. 142). However, since item memory was also enhanced in Experiment 1, and the isolation effect was reduced rather than abolished in Experiment 2, the isolation effect at least in the spatial-memory domain may reflect the action of not only a grouping strategy (when the design is conducive to such a strategy) but also richer item-specific processing. For example, it has been suggested that the isolate captures attention and as a result benefits from selective rehearsal (e.g., Bellezza & Cheney, 1973; Huang et al., 1974). It has also been suggested that when an item is distinct from the background items, the isolate elicits greater item-specific processing that contrasts with the relational processing given to the background items that share similar features (Fabiani & Donchin, 1995). Although it would be plausible to expect greater item-specific processing to confer an advantage in terms of item memory, it is not clear how it could improve memory for order. The finding that both item and order memory were enhanced by the presence of the isolate is more consistent with distinctiveness models of STM. For example, the feature model (Kelley & Nairne, 2001; Nairne, 1990, 2006) suggests that because the isolate is characterized by a unique feature, it provides a more effective cue for item retrieval. It follows that not only should fewer order errors result but the identity of the isolate should also be recalled more often than for a control item. In Scale-Independent Memory, Perception, & Learning (SIMPLE) (Brown et al., in press), items are represented along specific dimensions, such as their time of presentation and their physical properties. Successful recall depends on the items distinctiveness relative to its adjacent neighbours along those dimensions. It follows that because the isolate is endowed with greater relative distinctiveness than are its neighbours on one dimension (e.g., colour), it is more likely to be recalled correctly. In conclusion, establishing an isolation effect in serial-spatial STM, at its broadest, bolsters the view that the processing of spatial information obeys the same, or at least very similar, principles as those of verbal information (e.g., Jones et al., 1995; Logie, 1995). Moreover, a serial-spatial isolation effect suggests that the beneficial impact of item distinctiveness is a general principle of memory that extends to a wide range of stimuli (see also Neath, Brown, McCormack, Chater, & Freeman, 2006). Our results suggest more specifically that distinctiveness may benefit retention in two ways: by promoting grouping or by inducing richer item-specific processing. As such, it seems that both broad classes of explanation for the 760 THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 2008, 61 (5)

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