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Understanding the relationship between repetition priming and mere exposure.

Over the last 35 years a research tradition has developed within psychology dedicated to studying the mere exposure effect. In a classic monograph, Zajonc (1968) described the mere exposure effect as the observation that the mere repeated exposure of the individual to a stimulus is a sufficient condition for the enhancement of his attitude toward it' (p. 1). Following Zajonc's initial observations, numerous demonstrations of the mere exposure effect have been reported (see e.g. Bornstein, 1989; Harrison, 1977 for reviews), with most using one of two procedures for measuring such effects, namely Likert-type ratings (e.g. Bornstein & D'Agostino, 1994; Zajonc, Marcus, & Wilson, 1974) and forced choice preference judgments (e.g. Kunst-Wilson & Zajonc, 1980; Seamon et al., 1995). Surprisingly though, as Seamon, McKenna, and Binder (1998) observe, a widely accepted theoretical interpretation remains elusive. Of the various theories that have been proposed over the years (e.g. Berlyne, 1974; Harrison, 1977) the primacy of affect (Zajonc, 1980, 2000, 2001) and perceptual fluency/attribution (Bornstein & D'Agostino, 1994; Seamon, Brody, & Kauff, 1983) approaches have been most influential.

More recently, a major development in cognitive psychology has been the growth in interest in implicit memory. Implicit memory is defined as the non-intentional, non-conscious retrieval of previously acquired information, and is demonstrated on tasks that do not require conscious, intentional recollection of past experiences (Graf & Schacter, 1985). Although Roediger and McDermott (1993) outline a number of phenomena that could be described as implicit memory, the most widely studied form is repetition priming; that is the facilitation or bias in the processing of a stimulus as a function of a recent encounter with that stimulus. Numerous procedures now exist for studying such priming (see Fleischman & Gabrieli, 1998; Roediger & McDermott, 1993), with commonly used tasks including word identification and word-stem completion. Part of the interest in repetition priming tasks stems from the variety of ways in which they have been shown to dissociate from explicit measures of memory such as standard recall and recognition which require the conscious and intentional recollection of past experiences. Such findings have typically been used in support of either processing (e.g. Roediger, 1990) or system-based (Tulving & Schacter, 1990) accounts of repetition priming.

Historically, the fields of mere exposure and implicit memory have developed largely independently. However, in a number of reviews (e.g. Schacter, 1987; Squire, 1992) and articles (e.g. Bornstein & D'Agostino, 1992; Butler & Berry, 2001b; Jacoby & Kelley, 1987; Seamon et al., 1995) the mere exposure effect has been described as an example of implicit memory. For example, Schacter (1987) and Squire (1992) cite studies in which mere exposure effects have been obtained in the absence of explicit recognition (e.g. Kunst-Wilson & Zajonc, 1980; Mandler, Nakamura, & Van Zandt, 1987). By definition, the fact that mere exposure can be obtained in situations wherein no direct reference is made to the learning episode means that such demonstrations would qualify as examples of implicit memory in its broadest sense (as would implicit learning, classical conditioning, etc). However, tacit in this classification is the assumption that the mere exposure effect belongs to a particular implicit memory subgroup, namely repetition priming (e.g. as measured by word-stem completion or perceptual identification; see Butler & Berry, 2001a, b; Fleischman & Gabrieli, 1998; Seamon et al., 1995). Although intuitively appealing, this assumption has received relatively little attention. This is surprising, as even a cursory analysis is sufficient to reveal significant differences, as well as similarities, between the two phenomena. These are expanded upon in the first part of this paper. Undoubtedly, there is some utility to be gained from integrating the mere exposure effect within a repetition priming account, not least in terms of theoretical clarity. However, having established that the relationship is not as straightforward as might be thought, the second part of the paper examines whether existing mere exposure frameworks can accommodate any of the differences identified between the two phenomena. First, consideration is given to the perceptual fluency/attribution framework which embodies the 'cognitive' approach to mere exposure (e.g. Bornstein & D'Agostino, 1994; Seamon et al., 1983), and can be thought of as an extension to the repetition priming account. Second, attention turns to the primacy of affect framework (Zajonc, 1980, 2000), which draws on neurological and evolutionary concepts and ideas. In short, appealing to the notion of an attributional component appears to offer some success in accounting for the differences between the two phenomena. Notwithstanding this, there is still a series of findings that are problematic even for an extended repetition priming/attributional account. This indicates that further theoretical specification is needed. The final part of the paper provides some initial thoughts as to the direction that such a specification might take, as well as identifying key areas for future research.

Terminology

As mentioned above, while the term implicit memory encompasses a range of phenomena including implicit learning and classical conditioning, the most widely studied form is repetition priming. For this reason, a popular convention in cognitive psychology has been to refer to the procedures for measuring priming as 'implicit memory tasks' (e.g. word-stem completion and word identification). To avoid any unnecessary confusion in the current paper, these procedures are simply described as repetition priming tasks. To reiterate, although the classification of many mere exposure effects (e.g. Bornstein & D'Agostino, 1994; Kunst-Wilson & Zajonc, 1980) as implicit memory phenomenon per se is not in question, it is important to query the assumption that such effects are necessarily demonstrations of repetition priming.

Differences between mere exposure and repetition priming tasks

One of the most striking aspects of the repetition priming literature is the sheer diversity of paradigms, materials, populations and experimental variables that have been explored (for reviews see, e.g. Roediger & McDermott, 1993; Schacter, 1987). Although studies of mere exposure have been more limited in terms of the paradigms employed, here too, the range of materials and variables examined has been impressive (see Bornstein, 1989). Notwithstanding this, it is possible to identify a number of general features of mere exposure paradigms that distinguish them from repetition priming ones (see Table 1). The crucial distinction is that mere exposure tasks involve some form of affective judgment (e.g. liking ratings, preference judgments, pleasantness/attractiveness ratings), whereas repetition priming tasks require participants to make non-affective judgments (e.g. word-stem completion, picture clarification, word identification). It should be noted that a small number of mere exposure studies has also employed tasks involving non-affective judgments (e.g. Mandler et al., 1987). However, as described later, the evidence for non-affective mere exposure effects is mixed (e.g. Seamon et al., 1998).

In addition to the differing processing demands of the two tasks (affective vs. non-affective judgments), a number of further task-based distinctions can be drawn. Thus, whereas mere exposure paradigms involve the presentation of fully intact test stimuli, most common repetition priming tasks comprise partial (word-stem and fragment completion) or data-limited (perceptual identification, picture clarification) test cues. Also, whereas most repetition priming studies employ familiar materials, especially words, the mere exposure effect has been traditionally studied using unfamiliar materials (e.g. Chinese ideographs, Turkish 'words', irregular polygons). In some mere exposure studies, stimuli are also presented very briefly, sometimes for only a few milliseconds during encoding (e.g. Bornstein & D'Agostino, 1992; Kunst-Wilson & Zajonc, 1980). In contrast, presentation times in repetition priming studies tend to be much longer (e.g. Rajaram & Roediger, 1993). Finally, whereas stimuli tend to be presented on a single occasion during encoding in repetition priming studies, repeated study presentations are common in mere exposure paradigms.

So at a task level it is possible to identify a number of broad differences between mere exposure and repetition priming. The crucial question, however, is whether any form of distinction is required at a system level. Specifically, does the mere exposure effect reflect the operation of the same underlying systems and/or processes as are responsible for producing repetition priming? In the next section evidence is presented which directly informs on this question. However, in comparing the two phenomena it is important to note that comparatively few studies have explored mere exposure and repetition tasks directly within the context of a single study (for exceptions see, e.g. Butler, Berry, & Helman, in press; Stone, Ladd, & Gabrieli, 2000). As such, some of the cross-study comparisons of mere exposure and repetition priming reported below differ, not only in terms of the type of judgment performed at test (affective vs. non-affective), but also in other ways (e.g. familiar vs. unfamiliar materials). Such differences, where they occur, are highlighted in the next section. In assessing their likely impact however, it is important to stress that associations between mere exposure and repetition priming have been reported where such material differences exist (e.g. Stone et al., 2000). Equally, dissociations in performance can be obtained when only the type of test performed (perceptual identification vs. liking judgments) is varied (e.g. Butler et al., in press).

Mere exposure and repetition priming

The evidence reviewed in this section takes the form of parallels and differences between mere exposure and repetition priming as a function of various mutually studied variables. The assumption is that if similar processes and/or systems are in operation, parallels in performance should be observed. Although logical, some caution is warranted as differences are occasionally reported even between repetition priming tasks (Gabrieli et al., 1999; Witherspoon & Moscovitch, 1989). However, such findings are substantially outweighed by reports of parallels (e.g. Rajaram & Roediger, 1993; see, e.g. Roediger & McDermott, 1993 for review) across different repetition priming measures.

In making comparisons, one important point to note is that mere exposure and repetition priming have enjoyed largely independent research traditions and have, therefore, been the focus of somewhat distinct research questions, objectives and methodologies. As such, only in a comparatively small set of studies have the effects of a particular variable (e.g. modality, number of exposures) been tested in relation to both phenomena. Notwithstanding this, existing evidence is already sufficient to reveal a complex relationship between mere exposure and repetition priming.

Evidence consistent with a repetition priming framework

The following section outlines evidence in support of the repetition priming position. Collectively, this takes the form of parallels between mere exposure and repetition priming as a function of various variables. In interpreting these findings it should be noted that these parallels can take two forms. First, both phenomena may dissociate from explicit memory performance. This serves to underline the general position of both as examples of implicit memory. For the purposes of this paper, however, the more important form is that both mere exposure and repetition priming are affected in the same way by the variable in question (e.g. retinal size). This is crucial in informing on the relationship between the two phenomena, as it suggests the operation of some shared processes and/or systems.

Size and reflection

Seamon and colleagues (Seamon et al., 1995, 1997) have conducted much of the pioneering work regarding the relationship between mere exposure and repetition priming. They have demonstrated a number of parallels between mere exposure and a particular repetition priming task, namely, the object decision task. Seamon et al. (1997) showed that two variables (retinal size and left--right orientation of objects) produced the same pattern of results in a mere exposure and matched recognition task as previously obtained in object decision and recognition tasks (Cooper, Schacter, Ballesteros, & Moore, 1992). Specifically, mere exposure was unaffected by study-test changes in the size or orientation of objects, whereas performance on a recognition task was affected. It should be noted that this pattern was achieved using the same type of unfamiliar stimuli (line drawings of possible and impossible three-dimensional objects) as previously employed by Cooper et al. (1992).

Rotation

Seamon and Delgado (1999) found that explicit recognition, but not affective preference, was reduced when novel three-dimensional objects were depth rotated by 80 degrees between study and test. This pattern of findings was similar to one previously obtained by Biederman and Gerhardstein (1993), who found that object depth rotations did not affect repetition priming using an object-naming task. However, using the same objects and encoding task as Seamon and Delgado (1999), Srinivas (1995) found that certain depth rotations did reduce repetition priming on a task requiring participants to judge object orientation. Thus, it is difficult to draw clear conclusions regarding the relationship between mere exposure and repetition priming based on the evidence from rotation manipulations.

Modality

Butler and Berry (2001b) examined the effects of study-test changes in modality on mere exposure task performance. In keeping with findings from common perceptual repetition priming tasks (e.g. Craik, Moscovitch, & McDowd, 1994; Rajaram & Roediger, 1993), mere exposure when tested in the visual modality was greater for items studied in the same (i.e. visual) rather than a different (i.e. auditory) modality. Butler and Berry (2001b) also obtained a broadly similar pattern when the test was performed auditorally.

Attention

Seamon et al. (1983) showed that, whereas mere exposure for irregular octagons was unaffected by a division of attention (verbal shadowing task) at study, recognition performance was impaired. The finding is generally consistent with studies using perceptual repetition priming tasks (e.g. Mulligan & Hartman, 1996; Parkin & Russo, 1990). More recently, a study by Stone et al. (2000) directly compared the effects of a selective attention manipulation on a mere exposure task (preference judgment) and two repetition priming tasks (word identification and word-fragment completion). In this case, performance on all three tasks was impaired under conditions where participants were required to name the colour of the font in which study words were presented rather than read the words themselves. Interestingly, this broadly parallel finding was obtained despite the presence of a materials confound. Specifically, the two repetition priming tasks employed familiar real words, whereas the mere exposure task employed unfamiliar non-words.

Evidence inconsistent with a repetition priming framework

The studies reviewed in the previous section lend support to the view that the mere exposure effect represents an example of repetition priming. However, in the next section a number of previously unreported differences between repetition priming and mere exposure are described.

Number of exposures

Typically, increasing the number of times that a stimulus is exposed at study has little effect on performance on perceptual repetition priming tasks (e.g. Jacoby & Dallas, 1981; see Challis & Sidhu, 1993 for review). However, both in a meta-analytical review (Bornstein, 1989) and at an individual experiment level (e.g. Kail & Freeman, 1973; Kruglanski, Freund, & Bar-Tal, 1996; Lee, Sundberg, & Bernstein, 1993; Zajonc, 1968; Zajonc, Crandall, Kail, & Swap, 1974) increasing the number of stimulus exposures at study typically increases the size of the mere exposure effect, at least initially. Although it could be argued that some of these findings may be due to the contaminating influence of explicit memory, Bornstein and D'Agostino (1992) produced a similar effect using an affective rating task under conditions where contamination would be extremely unlikely (i.e. subliminal presentation). One issue that requires clarification, however, is whether this dissociative effect of number of study exposures reflects the tendency to employ unfamiliar vs. familiar materials in mere exposure and repetition priming studies respectively. Specifically, multiple exposures might be most beneficial in the formation of new stimulus representations.

Exposure duration

The influence of exposure duration has not been widely studied in the repetition priming literature. Generally, however, the available evidence suggests that, whereas perceptual repetition priming is relatively unaffected by stimulus duration, recognition improves with longer durations (e.g. Hirshman & Mulligan, 1991; Neill, Beck, Bottalico, & Molloy, 1990). By contrast, at a meta-analytical level (Bornstein, 1989), and individually (e.g. Hamid, 1973; Bornstein and D'Agostino, 1992), short exposures typically produce greater mere exposure effects than longer exposures. One exception to this was reported by Seamon, Marsh, and Brody (1984) who did not obtain greater mere exposure effects at shorter durations. However, they employed a much smaller range of near subliminal durations (from 2 to 48 ms) than some studies (e.g. Bornstein & D'Agostino, 1992). It is possible that an effect would have been demonstrated over a larger range which incorporated some supraliminal durations.

Chance recognition (1)

One of the most exciting features of the mere exposure effect is that it can be obtained when recognition approaches chance (e.g. Kunst-Wilson & Zajonc, 1980; Mandler et al., 1987; Seamon et al., 1983). Indeed, evidence suggests that the effect is, if anything, stronger when recognition is at chance (Bornstein, 1989). In contrast, there is little evidence that repetition priming can be obtained under the same conditions. For example, Moscovitch and Bentin (1993) observed that priming on a lexical decision task was eliminated when recognition was reduced by forgetting. Also, Kamiya, Tajika, and Takahashi (1994) failed to obtain repetition priming using fragment completion and word identification under conditions where recognition was at chance.

Beauregard, Benhamou, Laurent, & Chertkow (1999) provided some evidence that priming could be obtained on a semantic classification task when recognition approached chance. However, this requires replication and it is unclear whether similar effects will be obtained using more common measures of repetition priming (e.g. stem completion, word identification).

Seamon et al. (1995) have provided some particularly compelling evidence. Specifically, they examined the mere exposure effect for a set of possible/impossible objects previously used by Schacter, Cooper, and Delaney (1990) in conjunction with an object decision task. Importantly, Seamon et al. (1995) showed that, in contrast to the two or more seconds that were necessary to produce priming on the object decision task, a mere exposure effect could be obtained for the same stimuli after only a few milliseconds (five exposures of 4 ms) of exposure. These encoding conditions did not support above chance recognition.

Recently, an elegant study by Whittlesea and Price (2001) demonstrated that claims of mere exposure without recognition may not be as clear-cut as suggested in earlier studies. Rather, the findings suggested that these classic dissociations might actually reflect the different strategies adopted to complete the two tasks. We return to this study in a later section. However, it remains the case that under the conditions that typically produce near or at-chance levels of recognition performance (for whatever reason), mere exposure, but rarely repetition priming, is preserved.

Stimulus type

Schacter, Delaney, and Merikle (1990) noted that the vast majority of implicit memory (repetition priming) studies have employed verbal materials, particularly real words. This remains the case today, with repetition priming for real words having been observed across a wide range of tasks. In contrast, a closer look at the corresponding mere exposure literature reveals that many studies purporting to show effects for words are, in fact, based upon correlational data between actual word frequency and liking rather than experimental manipulations of exposure per se (e.g. Harrison, 1968; Martindale, 1972; Matlin, 1970; Sluckin, Colman, & Hargreaves, 1980; Zajonc, 1968). Indeed, only a study by Grush (1976) appears to show any experimental evidence of a positive mere exposure effect for real words, and in this case only a non-significant trend. Given this, Butler et al. (in press) conducted a direct comparison between mere exposure (liking and preference judgment) and repetition priming tasks (perceptual identification). They found that, whereas a set of non-words (e.g. fulflict, timvert) produced strong effects across all three tasks, no mere exposure effects at all were observed for low-frequency real words (e.g. fiscal, headland). However, the same stimuli produced a large priming effect for perceptual identification. It should be noted that this dissociation between mere exposure and repetition priming was obtained under conditions whereby only the specific requirements at test were varied (e.g. identify briefly presented non-words vs. rate them for liking).

Brain imaging data

In the only published functional imaging study of mere exposure, Elliott and Dolan (1998) used positron emission tomography to record activity associated with making preference judgments. Intriguingly, they found that performance on this task was associated with right lateral prefrontal activation. This stands in apparent contrast to the signature reductions of blood flow in occipital regions typically observed when using perceptual repetition priming paradigms (e.g. Schacter, Alpert, Savage, Rauch, & Alpert, 1996; Squire et al., 1992). Although suggestive of a neural level difference between mere exposure and repetition priming, Elliott and Dolan's procedure did vary significantly from that employed in standard repetition priming studies. Specifically, it involved the repeated exposures of briefly presented Japanese ideograms. As such, the general pattern of activity across mere exposure and repetition priming tasks requires replication within a single experiment, using matched procedures.

Summary

Collectively, the evidence presented above demonstrates that the status of mere exposure as an example of repetition priming is by no means conclusive. Specifically, the two phenomena appear to be differentially affected by a range of variables including number of study exposures and stimulus type. However, it would be premature to reject a repetition priming-based account of the mere exposure effect on this basis as there were also clear examples of parallels between the two phenomena (e.g. as a function of retinal size, modality) suggestive of a common underlying mechanism. Given this, a logical next step would be to ask whether a modified repetition priming account could accommodate any of the observed differences between the two phenomena. As such, the next section considers a popular account for the mere exposure effect which proposes the operation of an attributional component on performance (Bornstein & D'Agostino, 1992; Jacoby & Kelley, 1987). As well as the perceptual fluency/attributional framework, which may be thought of as an extension to the repetition priming position, the merits of a second influential framework (Zajonc, 1980, 2000) are also considered for completeness.

Bridging the gap between repetition priming and mere exposure

Perceptual fluency/attribution

The perceptual fluency/attribution framework (Bornstein & D'Agostino, 1992, 1994; Seamon et al., 1983) is largely based on the work of Jacoby and his colleagues (e.g. Jacoby & Dallas, 1981; Jacoby & Kelley, 1987). It proposes that repeated, unreinforced exposure to a stimulus results in increased or enhanced perceptual fluency for that stimulus (Jacoby & Whitehouse, 1989). This is the same fluency as that thought to support performance on repetition priming tasks (see, e.g. Jacoby & Dallas, 1981). Furthermore, the model proposes that when people experience such fluency for a stimulus they do not report that their perceptual encoding processes for that stimulus have been enhanced. Rather, they will generate the most reasonable explanation available to explain the experience of perceptual fluency, given both situational constraints and available contextual cues. In a series of studies, Jacoby and others have shown that people are willing to attribute such fluency to a variety of stimulus properties including duration (Masson & Caldwell, 1998), truth (Begg & Armour, 1991) and loudness (Jacoby, Allan, Colins, & Larwill, 1988). In a similar way, Bornstein and others have suggested that people will also attribute such fluency effects to liking (e.g. Bornstein & D'Agostino, 1992, 1994; Seamon et al., 1983).

Primacy of affect

An alternative characterization of the mere exposure effect is one based on the conjecture that cognition and affect are distinct and separable processes (Zajonc, 1980, 2000). Zajonc proposed that affective reactions (e.g. preference for one stimulus over another) could operate prior to and independently of cognitive processes (e.g. recognition), and could occur quickly and require minimal stimulus elaboration. Early evidence, suggestive of this, was provided by Kunst-Wilson and Zajonc (1980) who, in their classic study, demonstrated a mere exposure effect in the absence of any apparent recognition. Such non-conscious or 'subliminal' mere exposure effects have since been widely replicated (e.g. Bonnano & Stilling, 1986; Seamon et al., 1983, 1998). Zajonc (1980) accounted for this pattern of findings by proposing that affective and recognition judgments are based upon different classes of stimulus features. Specifically, he argued that affective judgments depend on gross, global features called 'preferenda', whereas recognition judgments rely on discriminable components of a stimulus called 'discrimimanda'. It should be noted that some converging evidence for the separability of affective and cognitive processes has been obtained from neurological and neuropsychological studies (e.g. Elliot & Dolan, 1998; LeDoux, 1995 see Zajonc, 2000 for review).

Evaluation

Overall, the largely descriptive primacy of affect framework seems ill-equipped to account for the apparent differences between mere exposure and repetition priming described here or, more generally, for the wide variety of findings in the mere exposure literature (see, e.g. Lazarus, 1984; Seamon et al., 1983 for criticisms). Presumably, repetition priming measures would fall within the remit of 'cognitive processes', and therefore any differences would be explained in terms of the proposed independence of affective reactions. Obviously, however, such a characterization is too coarse to be useful. In contrast, the perceptual fluency/attributional framework does have some explanatory power. Its major success comes from being able to accommodate the observation that mere exposure effects are often larger when stimuli are presented at short rather than longer durations (see Bornstein, 1989). Bornstein and D'Agostino (1992, 1994) suggested that, in the latter case, participants realize that their performance may be affected by their prior exposure to the stimuli and in some sense 'correct' their initial interpretation of the resulting perceptual fluency. Therefore, at longer durations, the fluency should feature less prominently in directing the participant's final liking decision. However, when stimuli are presented quickly, or even subliminally, there is less opportunity to become 'aware' that stimuli have been presented previously and so participants are more likely to (mis)-attribute fluency to liking for the stimuli. As this interpretative stage presumably features less prominently in repetition priming tasks, a similar pattern is not observed.

The framework does not accommodate all findings equally well, however. Thus, it is unclear why increasing the number of exposures actually increases the size of the mere exposure effect, at least until some threshold has been reached (e.g. Kruglanski et al., 1996; Lee et al., 1993; Zajonc, 1968). As the number of exposures increases it might be expected that the likelihood of recognizing study items would also increase. This should have the effect of increasing correct attributions of fluency to past exposure, and thus decrease the size of the mere exposure effect. The fact that, even under supraliminal conditions, this does not appear to occur (e.g. Kruglanski et al., 1996) is not predicted by this model. Interestingly, in a recent study, Lee (2001) observed a correction in attribution as number of study exposures increased, but only when participants made a non-affective (density judgment) discrimination at test. Conversely, when participants made preference judgments, mere exposure showed an expected linear increase as exposures increased. Of course, it should be noted that a greater number of study exposures may sometimes enhance perceptual fluency, as well as the likelihood of attributional accuracy. As such, more work is required to understand how these two processes, one of which may weaken and one strengthen the mere exposure effect, combine and interact.

Furthermore, it is worth noting that the framework does not seem to be able to accommodate well the apparent resilience of mere exposure, but not repetition priming, as recognition approaches chance. Finally, as with the standard repetition priming account, the perceptual fluency/attribution framework is seemingly at odds with the fact that Elliot and Dolan (1998) did not find the signature reductions in blood flow in occipital regions typically observed when using perceptual repetition priming paradigms. Of course, this finding requires replication under the conditions described earlier.

Overall, it is clear that although partially successful in accommodating differences between repetition priming and mere exposure, specification of an attributional component alone is not sufficient to account for all of them. However, it is not only these data which highlight the shortcomings of the perceptual fluency (repetition priming)/attributional framework. In the next section further findings are reviewed which appear problematic for existing theories.

Further difficulties for existing frameworks

Specific or non-specific?

In an oft-cited study, Mandler et al. (1987) showed that the mere exposure effect was not seemingly restricted to judgments of liking or preference. Specifically, they showed that an effect could be obtained for irregular octagons regardless of whether participants made liking (but not disliking), brightness or darkness judgments. This finding is crucial because it is consistent with, and indeed predicted by, the perceptual fluency/attribution framework (Bornstein & D'Agostino, 1994; Jacoby & Kelly, 1987). Moreover, the finding casts doubt on interpretations that view the mere exposure effect as an affective phenomenon (Zajonc, 1980, 2000).

With the exception of Mandler et al.'s (1987) study, however, very few additional reports of non-specific mere exposure effects have appeared (for examples see Bonnano & Stillings, 1986; Lee, 2001; Lee et al., 1993). Furthermore, doubts have been raised over Mandler et al.'s finding, with Seamon et al. (1998) failing to demonstrate the non-specific nature of the mere exposure effect. Specifically, Seamon et al. (1998) obtained mere exposure effects only when participants made affective judgments (i.e. liking and disliking) for pairs of octagons. In contrast, no effects were obtained when participants were asked to make judgments concerning which octagon in each pair appeared darker or which appeared brighter. While requiring replication, this pattern of data is problematic for the perceptual fluency/attribution framework (although it would be consistent with the primacy of affect framework). Specifically, if the mere exposure effect was shown to have an affective basis then it would be seemingly difficult to reconcile it with a simple perceptual fluency mechanism thought to support priming on repetition priming tasks.

Generalised mere exposure effects

In a typical mere exposure study, exposure to a stimulus increases liking or preference for that same stimulus when repeated sometime later. Recently, however, there has been growing evidence that such exposure can lead to similar increases for new (unseen) stimuli. These so-called 'generalized' mere exposure effects have typically been observed for new sets of stimuli which are nonetheless related in some way to exposed items. Thus, for example, Manza, Zizak, and Reber (1998) found that liking increased for non-studied, as well as studied, grammatical letter strings. Using photographs of faces, Rhodes, Halberstadt, and Brajkovich (2001) also found increased liking for both studied faces and non-studied composites of those faces. It is important to note that such effects are not just limited to mere exposure, as there have also been reports of repetition priming effects for perceptually (e.g. Hayes & Hennessey, 1996) and conceptually (Cave, Bost, & Cobb, 1996) related, but non-studied, objects. In each of these cases, however, it is possible to appeal to some degree of overlap between the studied and non-studied stimuli to account for the effect. In contrast, Monahan, Murphy, and Zajonc (2000) have reported evidence that appeared to show a generalized mere exposure effect under conditions where there was little, if any, physical, or even semantic, overlap. Specifically, they found that repeated presentations (shown five times) of Chinese ideographs at study not only led to increased liking for those same stimuli, and for non-studied but similar ideographs, but also produced lesser, but still significant, increases in ratings for a set of non-studied polygon stimuli. This finding suggests that repeated exposure generates some sort of generic positive affect which is not just restricted to the source stimuli that generated it but can be applied to totally unrelated stimuli. It is difficult to see how a perceptual fluency/attribution framework (Bornstein & D'Agostino, 1994) could account for this enhancement in liking for the novel octagon stimuli.

Neuropsychological data

A key prediction of the perceptual fluency/attribution framework is that mere exposure effects should be greater in situations where the likelihood of stimulus recognition at test is minimized, hence reducing the chance that perceptual fluency will be 'correctly' attributed to previous exposure. Some support for this comes from studies using normal participants that have shown greater mere exposure effects following subliminal presentations, compared with supraliminal ones (Bornstein & D'Agostino, 1992). As Winograd, Goldstein, Monarch, Peluso, and Goldman (1999) point out, however, one might also expect to find evidence of this in population groups where recourse to explicit memory strategies is severely impaired or even absent. However, Alzheimer's patients did not show a larger mere exposure effect in their study than aged-matched control subjects, despite much poorer explicit recognition performance, nor was this the case in a study involving Korsakoff patients (Johnson, Kim, & Risse, 1985). Of course, this line of reasoning does presume that these patients have intact perceptual fluency processes, and otherwise normal cognitive and perceptual abilities.

Independence of recognition and affect

Some of the most striking evidence for the mere exposure effect comes from Kunst-Wilson and Zajonc (1980) who demonstrated an effect for simple octagons in the absence of recognition for those same stimuli. This well-replicated finding (e.g. Mandler et al., 1987; Seamon et al., 1983) has also been regarded as providing some of the strongest evidence for unconscious or implicit memory (e.g. Butler & Berry, 2001a; Schacter, 1987). Recently, however, Whittlesea and Price (2001) have produced evidence that casts doubt on these interpretations. They pointed out that it was difficult to understand why the perceptual fluency resulting from previous exposure was used as a basis for affective but not recognition judgments. This is predicated on the notion that there are two bases for making recognition judgments, retrieval of contextual detail and a feeling of familiarity (two factor theory of recognition; e.g. Jacoby & Dallas, 1981; Mandler, 1980). The familiarity component of recognition is assumed to rely on the same fluency as liking in mere exposure tasks (and repetition priming tasks). By this account then, increased fluency might be expected to benefit both preference and recognition judgments. To explain why this did not occur, Whittlesea and Price (2001) proposed that participants might be adopting non-analytic (relying on fluency) and analytic (relying on recall of detail) strategies on preference and recognition tasks respectively, in part, because of the similarity and paucity of encoding of the study stimuli. They explored this in a set of experiments in which the consequences of these different strategies were demonstrated in relation to both preference and recognition tasks. Broadly speaking, recognition performance was shown to improve when a non-analytic strategy was encouraged, and performance on a preference judgment task was reduced when an analytical strategy was employed.

Some researchers have suggested that the familiarity component of recognition may more typically reflect the influence of conceptual, rather than perceptual, fluency (e.g. Wagner & Gabrieli, 1998). Thus, this might provide an alternative explanation for why perceptual fluency resulting from previous exposure is used as a basis for affective but not recognition judgments. However, it is clear from a number of studies that recognition judgments can be shown to benefit from perceptual fluency (e.g. Verfaellie & Treadwell, 1993; Whittlesea, 1993). Moreover, the fluency, which supports recognition in Whittlesea and Price's (2001) study, is unlikely to be conceptual given the encoding conditions employed. Therefore, although requiring further investigation, the results of Whittlesea and Price (2001) suggest that the classic dissociation between mere exposure and recognition tasks may reflect the choice of strategy adopted on each, rather than any fundamental contrast between affective and cognitive systems. This represents a substantial setback for the primacy of affect framework (Zajonc, 1980), built largely as it is on the apparent separability of mere exposure and recognition. However, the findings are consistent with the basic tenets of the fluency/attribution framework. Specifically, when participants are encouraged to use fluency as a basis for recognition, fluency is attributed via familiarity to past experience.

At a broader level, Whittlesea and Price (2001) also interpret their findings as arguing against the need for a distinction between implicit and explicit memory. Rather, their view is that the specification of a single store is sufficient to account for all memorial experiences. Although a full discussion is beyond the scope of this article it should be noted that a number of neuropsychological and neuroimaging studies do seem broadly consistent with the notion of separate implicit and explicit memory systems (see, e.g. Faulkner & Foster, 2002; Schacter, 1997). Moreover, the importance of understanding the relationship between mere exposure and repetition priming is not lessened whether one adopts a classification based upon the implicit/explicit distinction or one founded on the basis of a single store.

Summary

The preceding section described a number of findings that are difficult for existing accounts of the mere exposure effect. Although further studies are urgently needed, collectively they serve to illustrate the shortcomings of existing theories. However, it seems likely that a version of the perceptual fluency/attribution framework still offers the best hope for a comprehensive explanation for the mere exposure effect and its relationship to repetition priming. Thus, in the next section, some initial guidance is provided on ways in which this framework might be extended/modified in light of the evidence presented in previous sections.

The way forward

Positively valenced fluency

In accounting for the affective nature of the mere exposure effect, as revealed by Seamon et al. (1998), one possibility is that perceptual fluency is affectively positive rather than neutral (see Reber, Winkielman, & Schwarz, 1998). This view has received recent support from Harmon-Jones and Allen (2001) who found greater zygomatic muscle region activity for studied over non-studied photographs of women's faces. Such activity is argued to provide a real-time measure of emotional response and it is argued that such activity is increased during positive affective states (e.g. Lang, Bradley, & Cuthbert, 1997). Characterizing fluency in this way would allow Seamon et al.'s (1998) findings to be accommodated within a perceptual fluency/attribution framework (Bornstein & D'Agostino, 1994).

Multiple mechanisms

One intriguing possibility is that there is more than one mechanism contributing to the mere exposure effect. Recently, in a demonstration of a 'subliminal' mere exposure effect, Monahon et al. (2000) showed an increase in liking for a set of novel stimuli completely unrelated to those initially presented. These data are difficult to interpret in terms of a fluency/attribution framework, given the lack of physical or even semantic overlap between the various stimuli. Rather, the data suggest the operation of a 'free floating' form of positive affect that is generated via repeated exposures. Thus, the mere exposure effect might reflect the presence of two distinct mechanisms; one specific (perceptual fluency) and one diffuse (generic positive affect). Although this conceptualization is at an early stage, one might also speculate on the respective influence of these two mechanisms in producing mere exposure effects. One possibility is that both specific and diffuse mechanisms operate in tandem particularly at short study durations. However, as study duration lengthens the relative contribution of fluency is reduced as the likelihood of attributional correction increases.

Clearly, much remains to be determined regarding the operation of two mechanisms in mere exposure. However, this position would undoubtedly strengthen the current perceptual fluency/attribution framework. As described earlier, one especially troublesome set of findings for the framework is the effect of number of study exposures on the size of the mere exposure effect. To recap, as the number of study exposures increases so should the likelihood of later recognizing those items. This should have the effect of reducing the size of the mere exposure effect, rather than increasing it as is often observed. However, it might be argued that the likelihood of correct attribution is offset by the increase in diffuse affect generated as the number of study exposures increases. Furthermore, the contribution of such a mechanism may explain the differential effect of number of exposures on mere exposure and repetition priming. Presumably, an increase in diffuse affect would be of little consequence to performance on standard repetition priming tasks (e.g. perceptual identification).

As a further example, the specification of this additional diffuse affect mechanism may provide an alternative explanation for why non-affective mere exposure effects are rarely obtained. Specifically, whereas affective mere exposure effects may potentially originate from two sources (fluency and generic positive affect), presumably non-affective mere exposure effects, when they occur (e.g. Mandler et al., 1987), can only draw on fluency. This of course, remains to be empirically tested.

Relative sensitivity to fluency

Orientating their explanation around Cooper and Schacter's (1992) proposal of a structural description system, Seamon et al. (1995) suggested 'that the mere exposure effect is the result of implicit memory [repetition priming] for previously viewed stimuli, but it can be based on a different type of implicit memory representation than that which is responsible for object decision priming' (p. 720). This proposal was based on the finding that, in contrast to object decision priming, mere exposure for three-dimensional objects was observed at very short study durations and without the need for a global encoding task. As Whittlesea and Price (2001) also note 'a different type of memory representation' might be interpreted as implying separate memory stores for mere exposure and repetition priming. More likely, however, Seamon et al. (1995) meant that the two phenomena were based on the same memory store, but that a partial or less complete memorial representation was sufficient to support mere exposure. Relating this to a perceptual fluency/attribution framework, both phenomena might be thought of as reflecting the effects of processing fluency but, in the case of mere exposure, fluency generated on the basis of incomplete or partial encoding is sufficient to support performance. The main benefit of this additional specification is that it would provide an explanation as to why mere exposure but not repetition priming appears to be preserved following the sort of impoverished encoding which frequently leads to poor recognition performance.

Of course, these ideas are still very much at an early stage. For example, it remains to be seen whether the relative sensitivity of mere exposure compared with the object decision task also applies to other non-verbal repetition priming tasks (e.g. picture identification/clarification). Equally important is whether this extends to the verbal domain. However, if the mere exposure effect is in some sense more 'sensitive' than standard repetition priming tasks, it would be useful to classify it accordingly.

Summary

The previous section has outlined three preliminary ideas regarding ways in which the existing perceptual fluency/attribution framework might be extended to accommodate the disparate and complex patterns of data described in previous sections. One role of future research will be to assess the extent to which these proposed modifications are helpful. While it seems likely that the available data will necessitate at least some changes to the fluency/attribution framework, researchers must be wary that any resulting theory is not too pliable to be of use.

Future research

In this final section three areas are identified where research should be encouraged. Not only will this help to resolve the theoretical positioning of the mere exposure effect with respect to repetition priming, but it will also permit a much broader understanding of the mere exposure effect itself.

Neuropsychological evidence

The lack of mere exposure studies that have explored the effect with respect to the elderly (Wiggs, 1993) and groups with neurological damage (e.g. Greve & Bauer, 1990; Johnson et al., 1985) stands in stark contrast to repetition priming where a sizeable literature has emerged from studies of the elderly (see, e.g. Fleischman & Gabrieli, 1998) and a range of different patient groups (see, e.g. Squire, 1992 for review). The paucity of mere exposure studies is particularly puzzling given its suitability for use in patient groups (see Halpern & O'Connor, 2000). Increasing the number of mere exposure studies in patient groups would provide useful converging evidence regarding the status of the effect with regard to repetition priming. Overall, there is currently too little evidence to make meaningful comparisons. To date, single studies using schizophrenic patients (Marie et al., 2001), the elderly (Wiggs, 1993), Korsakoff patients (Johnson et al., 1985) and a prosopagnosic patient appear to be consistent with findings using repetition priming tasks, although two studies using Alzheimer's patients (Halpern & O'Connor, 2000; Winograd et al., 1999) have produced conflicting results.

Mere exposure and the perceptual/conceptual distinction

A second important area for future research will be to establish where the effect best fits in relation to the perceptual/conceptual processing framework (Roediger, 1990) which has proven so successful in classifying repetition priming tasks. Broadly speaking, a test engages perceptual processes when performance involves the analysis of stimulus form (e.g. identifying words), and conceptual processes when performance involves the analysis of stimulus meaning (e.g. producing category exemplars). As Fleischman and Gabrieli (1998) point out, the classification has not yet been applied to the mere exposure effect. Recent work suggests that the mere exposure effect might be best classified as a perceptually driven task (e.g. Butler & Berry, 2001b; Seamon et al., 1997). This is perhaps not surprising, as stimuli are typically presented in the same form in study and test phases. However, this does not exclude the possibility that the mere exposure effect might also possess a significant conceptual processing component. In fact, given that participants are required to make affective judgments this seems likely, albeit only in cases where judgments relate to meaningful stimuli (e.g. for faces, paintings, adverts). Future work must assess the extent to which this is the case. One possibility is that mere exposure may conditionally behave more like a perceptual or a conceptual repetition priming task depending on the choice of stimuli (non-meaningful vs. meaningful).

Comparing different mere exposure paradigms

Researchers have typically used two basic paradigms for measuring mere exposure in laboratory studies. The first involves rating singly presented studied and non-studied stimuli (e.g. Bornstein & D'Agostino, 1992), whereas the second involves a forced choice between studied and non-studied items presented in pairs (e.g. Kunst-Wilson & Zajonc, 1980). The assumption inherent in the mere exposure literature is that both paradigms elicit similar performance patterns across a range of different variables and thus tap the same underlying mechanism(s). Similar thinking has also been prevalent in the implicit memory literature where it has typically been assumed that different tasks are affected similarly by particular variables. Recently, however, it has been shown that this assumption may not always be correct. Gabrieli et al. (1999) demonstrated that two apparently perceptual repetition priming tasks (word-stem completion and picture naming) were dissociated in Alzheimer's patients and as a function of attention in normal participants. Gabrieli et al. explained this by appealing to the component parts operating in the two tasks (see Witherspoon & Moscovitch, 1989). They argued that word-stem completion falls into a class of repetition priming tasks that require the production of an item (e.g. category production), whereas picture naming requires identification of an item. As yet there has been no attempt to compare different mere exposure tasks at a component level. This is important because even a cursory analysis of ratings and preference judgment tasks suggests the presence of different components. For example, the preference judgment task involves a comparative component (i.e. between the target and distracter) prior to making an affective decision which is absent in the ratings task. Also, one permits a relatively unrestricted formulation of a response (i.e. using a rating scale), whereas the other involves a more restricted forced-choice judgment. At present there is little actual empirical evidence to suggest differences between the two tasks. Interestingly, however, although Winograd et al. (1999) obtained a mere exposure effect in Alzheimer's patients using a preference judgment task, Halpern and O'Connor (2000) failed to obtain a significant effect when using a ratings task.

Conclusion

The main aim of this paper has been to look more closely at the commonly held assumption that mere exposure is an example of repetition priming. Having demonstrated that the evidence is mixed, it has been shown that existing theories of mere exposure fall some way short of accounting for the differences between the two phenomena. Indeed, even the most successful of these theories, the perceptual fluency/attribution framework, will need to be modified to satisfactorily explain current findings. Overall then, although the same basic mechanism is involved in producing the two phenomena, at present the mere exposure effect might be best conceived of as a special case of repetition priming.
Table 1. Key characteristics of mere exposure and repetition priming
paradigms

Mere exposure              Repetition priming

Affective                  Non-affective
Intact test cues           Data limited cues
Unfamiliar stimuli         Familiar stimuli
Impoverished encoding      Full encoding
Multiple exposures         Single exposures


Acknowledgements

This work was carried out with funding from the Economic & Social Research Council (ESRC), grant number R00233329. The authors would like to thank Wilma Koutstaal for some helpful ideas and suggestions regarding this paper. They would also like to thank Michael Masson and two anonymous referees for their insightful comments on an earlier version of this paper.

Received 3 June 2003; revised version received 21 May 2004

(1) We do not consider evidence form subliminal priming (see, e.g. Marcel, 1980) here as such studies have typically focussed on semantic priming effects, and have employed rather different paradigms (e.g. prime-target lexical decision tasks) to the ones used to measure repetition priming.

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Laurie T. Butler* and Dianne C. Berry

University of Reading, Reading, UK

* Correspondence should be addressed to Laurie T. Butler, School of Psychology, University of Reading, Earley Gate, Whiteknights, Reading RG6 6AL, UK (e-mail: l.t.butler@reading.ac.uk).
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Author:Butler, Laurie T.; Berry, Dianne C.
Publication:British Journal of Psychology
Date:Nov 1, 2004
Words:10463
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