Imagery and Recognition: Dissociable Measures of ...

EUROPEAN JOURNAL OF COGNITIVE PSYCHOLOGY, 1999, 11 (3), 429± 443

Imagery and Recognition: Dissociable Measures of Memory? Lara Pelizzon, Maria A. Brandimonte, and Alessia Favretto University of Trieste, Italy

An assumption of many theories of visual cognition is that imagery tasks and picture recognition tasks tap the same kind of memory processes (i.e. visual), implying that these two types of tests can be used as interchangeable measures of visual memory. In this paper, we investigated whether articulatory suppressionÐ a variable known to improve imagery performanceÐ has a similar eŒect on picture recognition performance. In Experiment 1, subjects performed either an imagery task or a recognition task while engaging or not in articulatory suppression; in Experiment 2, the same subjects performed ® rst the imagery task, and then the recognition task, while engaging or not in articulatory suppression. When the type of task was manipulated between subjects (Experiment 1), imagery performance was signi® cantly improved by articulatory suppression. In contrast, recognition performance was signi® cantly impaired by the introduction of articulatory suppression. In accordance with results of Experiment 1, in Experiment 2 imagery and recognition performance were found to be unrelated. However, when the same subjects performed both tasks, the opposite eŒect of articulatory suppression on imagery vs recognition performance was observed only on the ® rst item. It does appear that when the imagery task and the picture recognition task are performed in isolation, performance is found to be independent, suggesting that the two tasks are mediated by diŒerent mechanisms. However, when a within-subjects design is used, performance in one task can be contaminated by the presence of the other task.

A substantial amount of work in the ® eld of picture recognition has resulted in the conclusion that there are two general ways in which people make recognition decisions: Sometimes the decision is made on the Requests for reprints should be addressed to L. Pelizzon, Department of Psychology, Via dell’UniversitaÁ, 7, 34123 Trieste, Italy. Email: [email protected] This research was partially supported by the grant 60% MURST 98 to Maria A. Brandimonte. We wish to thank Antonella Chiurco for running the experiments. We are especially grateful to Jonathan Schooler for his helpful advice and comments at various stages of the research and to three anonymous reviewers of this journal for their invaluable suggestions on a previous version of the paper.

1999 Psychology Press Ltd

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basis of a speci® c feature in the picture; at other times a decision is made because the picture looks ``familiar’ ’ (Loftus, 1972; Loftus & Bell, 1975; Loftus & Kallman, 1979; Loftus, Nelson, & Kallman, 1983; Mandler, 1980). ``Speci® c feature’ ’ responses and ``general familiarity’ ’ responses are based on diŒerent types of information acquired from the picture at the time of original encoding. The features tend to be unusual with respect to the rest of the picture, they refer to particular details and can be named (i.e. they are verbalisable); on the other hand, general familiarity information (termed ``general visual information’ ’) includes overall, holistic aspects of the picture, preserves physical information such as spatial relationships, and is di cult to verbalise. In the realm of visual imagery, a similar distinction has been proposed between overall images (involving ``skeletal encoding’ ’), which represent the global shape of objects, and propositional encodings, which describe an object through a list of language-like representations. The lists are accessed by name and contain information about the parts of an object, its abstract description, the name of the object’ s superordinate category, and the name of the literal encodings of the object’ s appearance (Kosslyn, 1980, 1981, 1994). Given these similarities, it is not surprising that a more or less explicit assumption of many theoretical approaches is that the same visual memory representations are stored for use in imagery and in object recognition (e.g. Kosslyn, 1994; Intons-Peterson, 1992). As a consequence, imagery tasks and picture recognition tasks have been used as interchangeable tools for investigating visual memory. However, several important ® ndings argue against such a strong conclusion and call for a more detailed analysis of the processes involved in diŒerent types of imagery and recognition tasks. For example, it is well known that if a subject verbally describes a scene while viewing it, subsequent recognition performance is improved as compared to a control condition of normal viewing (Freund, 1971, cited in Loftus & Bell, 1975; Kurtz & Hovland, 1953). Conversely, if verbal encoding is prevented during viewing, subsequent memory performance is reduced, though not to chance (Freund, 1971, cited in Loftus & Bell, 1975; Loftus, 1972). These results were taken as indicating that recognition performance for visual material relies on two main components: a verbal component (in that verbalisation manipulations aŒect recognition performance) and a visual component (in that performance is not reduced to chance). A crucial ® nding refers to the number of eye ® xations necessary to extract the two types of information from pictures. Results indicate that following one single eye ® xation on a picture at the time of initial encoding, performance based on holistic information is superior to performance based on speci® c features, whereas the reverse is true following su cient study time for multiple ® xations (Loftus et al., 1983, p. 187).

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Exposure durations longer than 250msec facilitate speci® c features (i.e. verbalisable information) to be extracted from the pictures. Given that under normal viewing conditions subjects are given su cient study time for multiple ® xations, it can be concluded that most picture recognition tasks are ``verbally based’ ’, in that performance depends on the extraction of speci® c details that can be named. On the other hand, evidence from imagery experiments indicates that the more images preserve spatial relations and geometrical properties of the original stimulus (i.e. rely on global rather than on featural representations), the higher the performance. Conversely, the more images rely on verbal representations of the original stimulus, the lower the performance. For example, recent studies on mental image transformations demonstrated that impeding verbal recoding of visual stimuli at the time of learning facilitates imagery performance by preventing the establishment of a competing verbal representation. It has been shown that asking subjects to engage in articulatory suppression during initial learning improved subsequent imagery performance when stimuli were easy to name, but not with hard-to-name stimuli (Brandimonte, Hitch, & Bishop, 1992a, c; Brandimonte, Schooler, & Gabbino, 1997). Moreover, supplying verbal labels for pictures impaired imagery performance only when stimuli were hard to name (Brandimonte et al., 1992c). These results provide a mirror image of those reported in the domain of picture recognition: Imagery performance is improved rather than reduced when verbalisation is prevented. So far, however, to our knowledge, a direct demonstration of a dissociation between imagery and picture recognition tasks in the direction outlined earlier has not been documented. The present study includes two experiments aimed at looking for dissociations between imagery and picture recognition tasks as evidence of discrete memory processes or representations. We used articulatory suppression as an experimental variable that might produce opposite results on an image transformation task and on a picture recognition task. In Experiment 1, two diŒerent groups of subjects performed either an image subtraction task or a recognition task. For each group, half of the subjects engaged in articulatory suppression (repeating the irrelevant sound la-la) while learning the stimuli, whereas half studied the stimuli silently. In Experiment 2, the same subjects performed both the image subtraction and the recognition task, with or without articulatory suppression at the time of learning.

EXPERIMENT 1 The main assumption of Experiment 1 was that easy-to-name stimuli elicit more labelling than hard-to-name stimuli. Such an assumption is

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not new in the memory literature (see e.g. Paivio, 1986), and it has been substantiated recently by evidence from Brandimonte et al.’ s (1992c) observation that labelling the stimuli during encoding impairs subsequent imagery performance with hard-to-name stimuli, implying that naming spontaneously occurs with easy-to-name stimuli. For this reason, in the present research, only easy-to-name stimuli were used (Fig. 1). Typically, when viewing an easily nameable picture to be memorised, people spontaneously tend to name and describe it. This process of verbal recoding typically impairs imagery performance (Brandimonte et al., 1992a, c; Brandimonte & Gerbino, 1993, 1996), in that it interferes with the use of non-verbal memories. However, engaging in articulatory suppression at the time of learning prevents the interference coming from verbal recoding. Therefore, as in previous research, we predicted a bene® cial eŒect of articulatory suppression on imagery performance. On the other hand, if picture recognition performance relies on verbal encoding of speci® c details, then the introduction of articulatory suppression at the time of learning should impair recognition performance. In Experiment 1, two diŒerent groups of subjects performed either an imagery task or a picture recognition task. The imagery task consisted of an image subtraction task, already used in previous research (Brandimonte et al., 1992a; Brandimonte, Hitch, & Bishop, 1992b), in which subjects were required ® rst to learn a series of pictures, then form an image of each of them and subtract a previously seen part from the image in order to

FIG. 1. Examples of stimuli used in the present experiments. Copyright American Psychological Association. Adapted with permission.

(1992) by the


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identify in the remaining part another object. The recognition version of the task required subjects to learn the pictures, look at the part to be subtracted from the original stimulus, and then recognise the remaining part among four pictures. In this condition, subjects were not asked to mentally perform the subtraction task. Rather, the experimenter gave them orally the solution immediately after each stimulus presentation. While learning the original stimuli, in each group, half the subjects engaged in articulatory suppression, while half remained silent.

Method Subjects. Sixty undergraduates participated in this experiment as volunteers. They were randomly assigned to four experimental conditions (imagery task with articulatory suppression, imagery task without articulatory suppression, recognition task with articulatory suppression, recognition task without articulatory suppression). Materials. A set of pictures described and used in the experiments reported by Brandimonte et al. (1992a) was used. It consisted of six pairs of pictures that were easy to name. A pair of ® gures on transparencies was also prepared to be used in the training. The ® rst member of each pair showed a composite picture, whereas the other showed a part of the composite. Subtraction of the part from the whole revealed the other part of the composite that, when viewed in isolation, suggested a novel construal. The composite and the solution to the subtraction were unambiguous and easily nameable, as established in a pilot study in which 10 subjects were asked simply to name each item. An additional set of ® gures was prepared for the recognition task. It consisted of a picture representing the solution to the subtraction task and three distractors. The distractors diŒered from the target in only one characteristic (see Fig. 2). Procedure. The procedure was modelled after Brandimonte et al.’s (1992a) study. Subjects were asked to memorise a series of six pictures that were presented by hand at a rate of 5sec per picture, for a total of

FIG. 2. An example of the stimuli used in the recognition task. Copyright American Psychological Association. Adapted with permission.

(1992) by the

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three presentations. Pilot work with an independent group of subjects showed that 90sec was su cient time for subjects to remember all six pictures with 100% accuracy (see Brandimonte et al., 1992a, c). Pictures were displayed face down in a row on a table in front of the subject. The experimenter sat on the opposite side. Of importance, at this stage subjects were not forewarned about the task they would be requested to perform thereafter, so as to make unlikely that they would search for the other ® gure while learning the stimuli. In each task condition, half of the subjects were asked to perform articulatory suppression by repeating irrelevant sounds (la-la) during presentations of each item. Care was taken to maintain subjects’ articulation rate at about two or three `la’ s’ per second (Baddeley, 1986). Immediately after learning, participants were asked to check, in their mind, whether they could remember the members of the series exactly in the order in which they had learned them. All subjects reported that they could do so. A practice trial in the subtraction task (with the transparencies) followed the memorisation phase. After practice, participants were requested to form a mental image of the ® rst picture of the series, to subtract a speci® c part from it and to name the new pattern. The group of subjects assigned to the recognition condition took part in a four AFC (alternative-forced-choice) recognition task. The learning phase was the same as for the imagery group. The only diŒerence was that, after memorisation, the experimenter provided the subject with the (oral) solution to the subtraction task. Immediately after listening to the solution, the subject viewed a series of four pictures (the target plus three distractors). The four alternatives were presented together and the subject was required to choose the picture that represented the solution to the subtraction task. This procedure was repeated for each item. The distractors were chosen so as to diŒer from the target only in one visual characteristic (see Fig. 2). The position of the target among the distractors was random, as well as the position of the distractors in the row. The order of the six stimuli was counterbalanced across subjects.

Results Imagery. Scoring re¯ ected the number of ® gures correctly identi® ed in mental imagery (0 to 6). Figure 3(a) shows the results in the imagery task, expressed as the number of ® gures correctly identi® ed in mental imagery. A one-way ANOVA showed a signi® cant bene® cial eŒect of articulatory suppression [F(1,28) = 7.89, P < .009, MSE = 1.52], in that performance in the imagery task was better with articulatory suppression than without articulatory suppression.


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(a)

(b) FIG. 3. Number of figures correctly identified, with or without articulatory suppression (AS) in Experiment 1, (a) in the mental imagery task; (b) in the recognition test.

Recognition. A one-way ANOVA on the number of correct answers in the recognition test showed a negative eŒect of articulatory suppression [F(1,28) = 6.78, P < .015, MSE = 1.41]. Recognition performance was superior in absence than in presence of articulatory suppression (Fig. 3b).

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Discussion The results of Experiment 1 replicated those obtained in previous experiments in which the presence of articulatory suppression improved performance in the image subtraction task (Brandimonte et al., 1992a, c). These results provide additional support for the view that preventing verbal recoding during initial learning impedes reliance on verbal processes, hence facilitating the recollection of the original visual memories at the time of test (see Brandimonte et al., 1992a, c, 1997; Brandimonte & Gerbino, 1993; Schooler, Fiore, & Brandimonte, 1997). However, performance in the recognition task was signi® cantly impaired by articulatory suppression, implying that the verbal component of memory representations is of crucial importance for optimal picture recognition performance. The double functional dissociation observed in Experiment 1 suggests that at least some diŒerent cognitive mechanisms are at work when people make mental discoveries in their mental images and when they recognise an object. However, functional dissociation does not imply contingent dissociation (Cabeza & Ohta, 1993; Hayman & Tulving, 1989), and hence these two methods may provide diŒerent information. Contingent dissociations are based on comparisons of performance on two successive tests for the same set of items. Typically, if performance on the items of one test does not predict performance on the same items on the other test, the two tasks are said to be stochastically independent (e.g. Jacoby & Witherspoon, 1982; but see Hintzman & Hartry, 1990 and RatcliŒ& McKoon, 1995 for criticisms of this assumption). Thus, contingent dissociations may complement functional dissociations.

EXPERIMENT 2 Testing for stochastic dependence between psychological tasks is one of the most controversial methodological issues. According to Tulving (1985), evidence provided by stochastic independence is more compelling than evidence provided by functional dissociation: If performance for a given subject and a given item on one test is unrelated to performance of the same subject and item on another test, there is a strong presumption that these tests involve diŒerent processes (see Poldrack, 1996 for an examination of this issue). However, others have expressed serious concerns and described many shortcomings of contingency analyses (Hintzman, 1980, 1991; see also Poldrack, 1996). Despite the limits of this method, it is commonly accepted that stochastic dependence testing may represent a useful adjunct to other


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techniques for establishing dissociations between mental processes (Poldrack, 1996, p. 447). In Experiment 2, the same subjects performed both the image subtraction and the recognition task, while engaging or not in articulatory suppression during initial encoding of the stimuli, the prediction being that a double dissociation would be observed, with articulatory suppression having an opposite eŒect on performance in the two tasks. In addition, we expected imagery and recognition performance to be unrelated.

Method Subjects. Thirty undergraduates from the University of Trieste took part in this experiment as volunteers. They performed ® rst the image subtraction task and then the recognition task. Half the subjects performed the two tasks while engaging in articulatory suppression (AS) and half remained silent. They were tested individually. Materials and Procedure. The procedure was the same as in Experiment 1, with the exception that, for each stimulus, subjects performed ® rst the imagery task and then the recognition task. That is, for each stimulus, after memorisation, subjects were shown the part to be subtracted. Then, they were asked to identify and name the ® gure resulting from image subtraction. Immediately after responding, subjects took part in a four AFC (alternative-forced-choice) recognition task in which they had to recognise the ® gure corresponding to the solution to the imagery task among three distractors. Then, subjects were asked to generate an image of the next stimulus, to identify the resulting ® gure to recognise it, and so on. Participants were given as much time as they needed. The order of the six stimuli was counterbalanced across subjects. The order of imagery and recognition tests could not be counterbalanced across subjects, because performing ® rst the recognition test would have arti® cially aŒected imagery performance (given that the target for recognition was the solution to the image subtraction test).

Results Imagery. Scoring re¯ ected the number of ® gures correctly identi® ed in mental imagery (0 to 6). A one-way between-subjects analysis of variance (ANOVA) revealed that articulatory suppression had no eŒect on overall imagery performance (F < 1) (Fig. 4). However, additional analyses on responses to the ® rst item showed the typical bene® cial eŒect of suppression [w 2(1) = 6.0, P < .015]. In addition, given that the order of presen-

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FIG. 4. Number of figures correctly identified in mental imagery, with or without articulatory suppression (AS) in Experiment 2.

tation of the six ® gures was counterbalanced across subjects, it was possible to compare the eŒect of articulatory suppression on the same item when it was in the ® rst or in the last position. As is clearly illustrated in Fig. 5, the proportion of subjects who performed correctly on item 1 is higher with than without articulatory suppression, whereas there is no difference between conditions when the same item is in the last position. Recognition. Scoring re¯ ected the number of ® gures correctly identi® ed in the four AFC recognition task (0 to 6) (Fig. 6). A one-way ANOVA on the number of correct answers in the recognition test revealed a signi® cant eŒect of articulatory suppression [F(1,28) = 5.64, P < .03, MSE = 1.32], with performance in the articulatory suppression condition lower than performance in the no articulatory suppression condition. Correlational analyses showed that the two tasks were unrelated (r = .21). There was a very low proportion of variance in the recognition task explained by the imagery task (r2 = .047).

Discussion The outcomes of Experiment 2 add some important re® nements to the picture emerging from the previous experiment. First of all, once again a dissociation was found between imagery and picture recognition.

FIG. 5. Number of subjects who performed correctly in the first occurrence vs last occurrence of the imagery task as a function of presence of articulatory suppression (AS) in Experiment 2.

FIG. 6. Number of correct answers in the recognition test, with or without articulatory suppression (AS) in Experiment 2.

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Although this was a single dissociation, with articulatory suppression having no eŒect on overall imagery performance and a negative eŒect on recognition performance, additional analyses of the ® rst occurrence showed that the lack of an eŒect of suppression on overall imagery performance may be an artifact due to the experimental design. The bene® cial eŒect of suppression on the ® rst occurrence cannot be attributed to the speci® c item for two reasons: First, because, given to counterbalancing, item 1 was diŒerent across subjects; second, because for the same item we observed a lack of eŒect when it appeared as the last occurrence. Thus, it seems plausible that when people perform, for each item, ® rst the image task and then the recognition task, the nature of the ® nal task (i.e. picture recognition) encourages the application of verbal processes to the imagery task as well. The retrieval of the verbalisable aspects of the representation eliminates any bene® cial eŒect of articulatory suppression, though imagery performance is not reduced to chance.

GENERAL DISCUSSION The results that emerged from the two experiments reported in the present paper converge on the conclusion that imagery and picture recognition tasks involve diŒerent proportions of visual and verbal processing: Whereas imagery tests emphasise global, perceptual processing, picture recognition tests emphasise featural, verbally based processing. These conclusions ® t well with the ® nding that articulatory suppression manipulations negatively aŒect recognition performance but have a bene® cial eŒect on imagery performance (Experiment 1). They also ® t with the results of Experiment 2, which showed that when a within-subjects design is used, articulatory suppression maintains its negative eŒect on recognition performance, while having a positive eŒect on the ® rst response to the imagery task. The lack of a bene® cial eŒect of suppression on overall imagery performance may be plausibly attributed to the nature of the experimental design. When subjects perform ® rst an imagery task and then a recognition task, the tendency to rely on verbally based processes is much stronger, in that the ® nal task may encourage the application of verbal processes, hence eliminating any bene® cial eŒect of articulatory suppression. Given that subjects were not forewarned about the task they would be requested to do after memorisation, responses to the ® rst item were not contaminated by the presence of the recognition task. On the other hand, when the image subtraction task was performed in isolation (Experiment 1), imagery performance was greatly improved by articulatory suppression, plausibly because the ``strength of activation’ ’ of the verbal aspects of the representation was


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much less substantial. This interpretation rests on the assumption that, like recognition, imagery tasks are not ``pure’ ’, in that they involve both global and featural aspects of the object’ s shape. As Kosslyn (1994) pointed out, imagery tasks rely predominantly on the representation of the object’ s global shape. However, this does not imply that representations of overall shape have no internal structure. Rather, a global shape includes the representations of parts and distinctive characteristics and, hence, such a ``compressed-image representation’ ’ has its internal organisation. For example, it has been shown that if an image is degraded so that the overall pattern cannot be matched with the global shape representation, then subjects will match the input to representations of individual parts (Cave & Kosslyn, 1993). Therefore, under particular circumstances (e.g. easy nameability of the stimuli), people tend to rely on the verbalisable aspects of their images. Reliance on verbal processes fosters a kind of transfer inappropriate retrieval (see Schooler et al., 1997), which reduces imagery performance. However, subjects are still able to perform the imagery task. It seems plausible that the eŒect of articulatory suppression is to impede such a transfer-inappropriate retrieval, hence improving performance. This explanation is in accordance with the results from the analyses on the eŒect of articulatory suppression on responses to the ® rst occurrence. It also complements previous arguments on the nature of the eŒect of articulatory suppression on imagery performance (Brandimonte & Gerbino, 1996; Cornoldi & Logie, 1996). Indeed, the present results indicate that, if the task involves continuous processing shift, engaging in articulatory suppression at learning does not guarantee that at retrieval only the non-verbalisable aspects of the representation will be recollected. It should be also noticed that the present results are not at odds with previous demonstrations in the domain of face recognition that describing the appearance of a previously seen face signi® cantly impairs subjects’ subsequent ability to discriminate the target face from verbally similar distractors (Schooler & Engstler-Schooler, 1990). Indeed, face recognition diŒers from picture recognition in at least one important aspect: A face is a kind of stimulus that is very di cult to put into words and it typically gives rise to an holistic, global representation. As such, it represents a good example of ``general familiarity’ ’ information that, similarly to imagery, suŒers from featural, verbally based processing (see Dodson, Johnson, & Schooler, 1997). To summarise, the present results argue against the common interpretation that imagery and recognition tasks both rely on non-verbal processes and point to the conclusion that whereas imagery is predominantly ``visual’ ’, picture recognition is predominantly ``verbal’ ’. However, neither task can be considered purely visual or verbal. The amount of overlap

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between them depends on the nature of the recognition task and on the nature of the experimental design. Manuscript received 21 May 1998 Revised manuscript received 7 January 1999

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