Picture superiority in free recall: Imagery or dual coding?

COGNITIVE PSYCHOLOGY 5, 176-206 ( 1973)

Picture Superiority in Free Recall:

Imagery or Dual Coding?l

ALLAN PAIVIO~

Department of Psychology, University of Western Ontario, London 72, Canada

AND

KALMAN CSAPO

Department of Psychology, London Psychiatric Hospital, London 25, Canada

Free verbal recall is generally higher for items presented as pictures than for items presented as words. Possible interpretations of this effect include differential verbal elaboration, superiority of nonverbal imagery as a memory code, and dual encoding favoring pictures. A series of experiments investigated the relative contributions of imaginal and verbal memory codes using incidental recall tasks in which the orienting task was designed to control the way items are encoded during input. Three experiments required subjects to encode words and pictures verbally, by writing or pronouncing the words or picture labels; or imaginally, by drawing or imaging the picture or object suggested by the word. Two further experiments involved a probability learning task which required no encoding reaction to pictures or words, but an analogue of imaginal and verbal coding was provided in one experiment using picture-picture, picture-word (or word-picture) and word-word repetitions. Recall tests following these manipulations consistently yielded much higher recall for pictures than for words under all conditions except when subjects imaged to words. Strong support was also found for the hypothesis that image and verbal memory codes are independent and additive in their effect on recall. In addition, the contribution of imagery appeared to be substantially higher than that of the verbal code. Thus the usual superiority of pictures in free recall is best explained by dual encoding, or a combination of image superiority and dual coding, both of which are ordinarily favored when items are presented as pictures.

‘This research was supported by grants to the first author from the National Re- search Council of Canada (A6687) and from the University of Western Ontario Research Fund. We thank Ann Anas for her assistance in conducting the experiments and analyzing the data. We are grateful also to Fraser Bleasdale, James Clark, Albert Katz, and Alvin Segal for their suggestions regarding the manuscript.

’ Address: (c/o Mrs. Anas), Department of Psychology, University of Western Ontario, London 72, Canada.

176 Copyright @ 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.

PICTURE SUPERIORITY IN FREE RECALL 177

INTRODUCTION

This paper is concerned with the functions of nonverbal and verbal memory representations in the free verbal recall of pictures and nouns. The facts are simple: free recall is generally higher when the items are presented as pictures of familiar objects than when they are presented as printed concrete nouns, and higher for concrete than for abstract nouns (see Bevan & Steger, 1971; Paivio, 1969, 1971). From the viewpoint of traditional verbal approaches to memory, this ordering of recall performance is puzzling. Why should verbal recall be higher when the items are presented as pictures rather than as printed words? After all, the verbal response that is required during recall is aroused more reliably and quickly by words than by pictures (e.g., Fraisse, 196S), and it seems reasonable to suppose that the printed word would also evoke a more effective memory trace for response retrieval. Moreover, the concrete and abstract words used in these experiments do not differ in such traditional measures of response availability as familiarity or meaningfulness, so why should they differ in recall?

Two general classes of theoretical interpretations have been suggested for such findings. One class asserts that some physical characteristic of pictorial stimuli, such as vividness or multiplicity of cues, might enhance their memorability relative to words (Bousfield, Esterson & Whitmarsh, 1957). Such variables may be relevant to recognition memory but available evidence suggests that they are relatively unimportant in free recall. Thus Paivio, Rogers and Smythe (1968) found that adding color cues to pictures and words did not enhance recall in comparison with black and white versions of the same items. This general class of explanation could undoubtedly be tested in other ways, but for present purposes it will not be considered further (for a general discussion of the topic, see Paivio, 1971, Chapter 7). The other class of hypotheses, which will be the focus of the present paper, emphasizes the encoding processes involved in the recall task. Some of these hypotheses involve assumptions that are part of a more general dual-coding approach to memory and cognition (Paivio, 1971, 1972). Here we will outline those aspects of the theory that are most pertinent to the present problem.

The approach distinguishes between nonverbal imagery and verbal symbolic processes, which are assumed to involve independent but partially interconnected systems for encoding, storage, organization, and retrieval of stimulus information. The imagery system is specialized for dealing with nonlinguistic information stored in the form of images, that is, memory representations corresponding to concrete things. The verbal code refers to stored representations corresponding most directly to linguistic units.

178 PAIVIO AND CSAPO

The twin assumptions of independence and interconnectedness of thy two coding systems are most crucial to the theoretical discussions thai follow. Independence implies that either one of the codes can be avail. able and activated in varying degrees, depending on stimulus attributer and the experimental conditions involved in the task. It also implies thal the two codes can have additive effects on recall. Interconnectedness oi the codes implies that one code can be transformed into the other. In the context of the present problem, the assumption means simply that pictures can be named, words can evoke nonverbal images, and similar transformations can occur entirely at the cognitive level-an object name: covertly aroused, can arouse an image of such an object; conversely, the name of the object presumably can be retrieved from its memory image,

These theoretical assumptions together with relevant empirical data suggest the following interpretation of the way that pictures, concrete nouns, and abstract nouns can be encoded. Pictures of familiar objects readily evoke a nonverbal image and a verbal code, but the availability of the latter is relatively lower because an extra transformation is involved. The verbal code is directly available in the case of concrete and abstract words but the former are more likely to evoke images. Finally, verbal coding of pictures is assumed to be easier than image coding of concrete words. Thus image coding of pictures and verbal coding of printed words have the highest probability; verbal coding of pictures, second; imaging to concrete words, third; and imaging to abstract words, fourth. The summative availability of both codes is accordingly highest for pictures, next for concrete words, and lowest for abstract words. This ranking of code availability is supported by reaction time data on the latency of the arousal of verbal or imaginal codes. Thus, Fraisse (e.g., 1968) and others have shown that it takes longer to name a picture than to read the corresponding printed word, and a number of studies (e.g., Paivio, 1966; Ernest & Paivio, 1971; Simpson & Paivio, 1968) have demonstrated that images occur more quickly to concrete than to abstract words.

Alternative Interpretations of Picture Versus Word Recall

The above assumptions are implicated in the alternative theoretical interpretations of the free recall data, to which we now turn. Four hypotheses are discussed.

Verbal ezaaboration. Despite the lesser availability of a simple verbal label to a picture than to a word, it could be argued that pictures generate relatively more verbal elaboration than do words if there is sufficient time for such elaboration and the subject’s psychological set pre- disposes him to elaborate. For some reason, such verbal elaboration may enhance the memorability of pictorial stimuli. According to this extreme

PICTURJS SUPERIORITY IN FREE RECALL 179

view, nonverbal memory processes need not play any role in the superiority of pictures in recall. Although the theorists themselves would not necessarily subscribe to such an extreme position regarding picture recall, the concept of verbal elaboration as used by Rohwer and his collaborators (e.g., Rohwer, Lynch, Levin & Suzuki, 1967) and the concept of natural language mediation (e.g., Adams, 1967) are representative of this class of explanation.

Image encoding. The above model suggests that images are de- creasingly available from pictures, to concrete nouns, to abstract nouns. Thus it is logically possible that the differential recall observed for the stimuli is due entirely to the differential availability of the image code. This implies that the image code for some reason is superior to the verbal code for the storage or retrieval of the target item. It also means that the codes must be interconnected for, if storage is entirely imaginal, the appropriate verbal response must be retrievable from the image during recall, Note that this hypothesis does not assume that the verbal label must be stored along with the image in order for the verbal response to be retrievable from the latter. This contrasts with Rohwer’s (lW0) sug- gestion that imagery storage facilitates learning only when the image is accompanied by a verbal representation of the same event, and is relevant also to the hypothesis that retrieval cues are effective only if the cues are present at input as well as output (e.g., Tulving & Osler, 1968).

Dual-encoding. This is the most common interpretation of picture superiority in recall (e.g., Bower, 1972; Paivio, Rogers & Smythe, 1968; Sampson, 1970). It states essentially that pictures are remembered better than words because pictures are more likely to be encoded both as images and as verbal traces. The interpretation extends to the superiority of concrete over abstract words as well. Note that this explanation, too, involves the assumptions of independence and interconnectedness. The codes must be at least partially independent, so that encoding could be in terms of one code or the other, or both; if it is both, one code presumably could be forgotten and the verbal response could still be retrieved from the other. Thus the two codes would have additive effects on recall probability.

Some combinations of the above. The three hypotheses are not mutually exclusive. It is possible, for example, that images are superior to verbal memory traces, and yet the two are also independent and additive in their effects.

It should be emphasized that these alternatives are concerned only with the nature of the memory codes involved in the recall effects and not with other general issues regarding the nature of memory. At one level of theorizing, the dual-coding hypothesis (if correct) would pro-

180 PAIVIO AND GAP0

vide a suIhcient explanation of the data in simple probabilistic terms: doubling the available memory codes doubles the probability of correct recall. If, however, the two codes are shown to be differentially effective, it will be necessary to identify the basis of the difference before the explanation is complete. The latter problem will in fact arise and we shall have something to say about it later, but our principal goal in this paper is to identify the effective codes rather than to provide a complete theoretical explanation of their modus operandi in free recall.

Previous Research Evidence

The different hypotheses have not been rigorously compared in research to date, but the dual-coding interpretation tends to be favored by evidence which suggests that the availability of both codes enhances recall of pictures. Thus, procedures designed to encourage verbalization of familiar pictures during their presentation can enhance subsequent verbal recall of pictures (e.g., Bahrick & Boucher, 1968; Kurtz & Hovland, 1953; Wilgosh, 1970). Similarly, instructions to image to words or phrases have been found to facilitate recall of such items (Gupton & Frincke, 1970; Kirkpatrick, 1894).

The most compelling evidence for the dual-coding interpretation, however, was obtained by Paivio and Csapo (1969). Lists of pictures, concrete nouns, or abstract nouns were presented at a very fast rate of 5.3 items per second or a slower rate of 2 items per second. The rate manipulation was designed to prevent verbal coding of pictures at the fast rate. That is, subjects were unable to label the pictures even implicitly during presentation at the faster rate, but apparently were able to do so at the slower rate. Memory was tested in several ways, including free recall. The results showed that free recall was essentially equal for the three kinds of items at the fast rate but pictures exceeded concrete words, which in turn were recalled better than abstract words, at the slower rate. The slow-rate effect replicates the pattern that has been repeatedly demonstrated in free recall studies. The absence of this effect at the fast rate supports a dual-coding interpretation of the slow rate effect. Con- sider the alternative that the effect can be explained entirely in terms of the availability of nonverbal imagery as a memory code. There is no logical basis for choosing between this imagery interpretation and the dual-coding hypothesis on the basis of the slow-rate data alone. However, the imagery hypothesis cannot readily account for the failure to find the usual ordering of recall at the fast rate. That is, images aroused by pictures should be just as much favored at the fast as at the slow rate. The fact that pictures were best only at the slower rate, when the verbal code was available, suggests that the effect was due to the availability of


both memory codes. Similarly, concrete words exceeded abstract words in recall only at the slower rate, presumably because the faster rate did not allow enough time for the arousal of images to concrete words whereas the slower rate did so.

While the dual-coding hypothesis is favored by these data, the alternatives cannot be rejected because it remains possible that the fast rate used by Paivio and Csapo somehow interfered more with imaginal encoding of pictorial information than verbal encoding of linguistic information, or provided insufficient time for organization of pictorial information into higher-order memory units that might be the basis of picture superiority under slower presentation rates, or si.mply interfered with effective verbal elaboration in the case of pictures.

The remainder of this report is concerned with a series of experiments that were designed specifically to provide a more definitive choice from the alternative hypotheses. All of the experiments to be described involved incidental free-recall tasks, where the incidental orienting task was designed either to control or systematically affect the subject’s encoding strategies, that is, to influence the probability that they will encode the items imaginally or verbally. Such control is essentially absent in the standard free recall task, since the subject is free to process items in whatever way his past experience and the present situation dictate. Some degree of control can be added to the task through instructional sets to image or verbalize to items (e.g., Gupton & Frincke, 1970; Mueller & Jablonski, 1970), but evidence from studies of mediated paired-associate learning (Paivio & Yuille, 1969) indicate that subjects will readily switch to strategies other than those suggested by the instructional set if the items seem to demand it. These problems are avoided if the subject’s intended task is to encode the items in the manner suggested by the experimenter, and recall is tested only incidentally. Moreover, it is now generally acknowledged that incidental and intentional learning will not differ provided that the task in each case evokes responses that are favorable to learning (e.g., Postman, 1964; Schneider & Kintz, 1967). Thus, if verbal coding, imaginal coding, and dual-coding differentially favor recall performance, this should be revealed by incidental tasks in which these codes are systematically manipulated.

The first three experiments involved a Type I design (Postman, 1964) in which the incidental orienting task required the subjects to generate either images or verbal codes to pictures and words without any intent to learn the items. Two further experiments involved Type II incidental learning in which subjects were given a probability learning task involving pictures and words, which they were not required to learn as units. The latter task also required no active encoding of items, but suc-


cessive repetitions of items in one experiment provided a passive analogue of imaginal and verbal encoding. Appropriate controls were included in some of the experiments to ensure that results and interpretations could be generalized to the standard free recall situation. The first experiment included pictures, concrete words, and abstract words. The remainder restricted comparisons to pictures and concrete words, thereby simplify- ing the analytic task.

EXPERIMENT 1

Experiment 1 introduced certain methodological features that were followed in the subsequent experiments, such as the use of long lists and an item presentation rate dictated by the incidental orienting task involved in the study. The design was incomplete in that the orienting task required only verbal encoding in the case of both pictures and words, but it established the efficacy of the approach to the problem and per- mitted some predictions to be tested. Since pictures presumably arouse a nonverbal memory code directly and the orienting task also required them to be encoded verbally, it follows from the dual-coding hypothesis that recall for pictures should be as high under incidental as under intentional conditions. On the other hand, attention to the verbal code should interfere with the arousal of images to concrete words more under incidental than under intentional conditions, since the latter presumably would draw the Ss attention to salient aspects of word meaning, such as imagery. To the extent that imagery is crucial to recall, low-imagery abstract words should be less affected than concrete words by the incidental-intentional manipulation. This also implies that the relative superiority of concrete over abstract words should be less under incidental than under intentional conditions.

Method

Experimental design and subjects. The experiment invoIved a 3 X 3 independent-measures factorial design in which the variables were stimulus attribute (pictures, concrete words, abstract words) and leam- ing set (incidental, intentional control, standard free recall)3. One hun- dred and forty-two undergraduate summer school university students served as Ss, nine to sixteen Ss in each cell.

Material. The stimulus words were 72 concrete (C) and 72 abstract

‘The experiment initially included some groups tested after a I-wk retention interval. The results for those groups showed no interaction involving retention interval (see Paivio, 1971, pp. 201-203) so the longer interval was dropped from the overall study.


(A) words selected from the Paivio, Yuille, and Madigan (1968) norms. The two classes of words were approximately equated on meaningfulness (m; 6.85 vs 5.64) an d word frequency ( 46.26 vs 43.25 per million). How- ever, the C items exceeded the A items in mean rated imagery (6.55 vs 3.26) and concreteness (6.87 vs 2.54). All the words were printed in black uppercase block letters. The 72 pictorial (P) items were single-line black line drawings of the objects labeled by the concrete words. White- on-black 35 millimeter slides were made of each item so that all items occupied approximately the same area of the film. The words of each type (C or A) were randomly arranged into a basic list. The order of items in the P list corresponded to the order of their labels in the C list. A second list of each type was created by reversing the order of items in the basic lists. Four additional items of each type served as practice items at the beginning of each list.

The items were presented by a Kodak Carousel slide projector con- nected to a stroboscope-like cardboard disk. The disk was attached to a motor rotating clockwise at a constant rate of one revolution in 5 sec. A small sector of the disk with an arc subtending an angle of 4.5” was cut out, so that the projector’s beam could penetrate the disk for 63 msec (l/is of a set). A microswitch was attached to the disk 150” counter- clockwise to the open sector which triggered the changing mechanism of the projector.

Procedure. The Ss were tested in groups of 10-12, half receiving the items in one order, the others in the reverse order. All groups received the same preliminary instructions, in which they were told that the experiment was concerned with the accuracy of picture (or word) identification when these items are shown for a very brief duration. It was explained that each item would appear only for $ils of a set and during the remaining part of the 5-set inter-item interval, their task would be to record the word or the label of the picture just seen on the screen in a booklet with the numbers 1 to 72 printed on the left-hand side. The Ss in the incidental learning condition were not given any further instructions; those in the intentional control condition were also told that they would be asked to recall the items at a later time. The recording sheets were collected at the completion of the orienting task, new recall sheets were distributed, and Ss were asked to recall as many of the items of the list as they could remember, in any order. About 5 min intervened between completion of the list presentation and subsequent recall. Five minutes were allowed for the written recall. The standard control Ss were simply given standard free recall instructions without any refer- ence to the orienting task, and recall was tested after a comparable interval following list presentation.


Results and Discussion

The labels each S gave during the labeling task were used as the basis for scoring his verbal recall of the pictures. The labels generally corresponded to the words presented to the concrete noun group, and a scoring procedure in which only the latter responses are considered correct did not appreciably alter the picture recall data.

The mean recall scores for each condition are presented in Fig. 1. The data were analyzed by a 3 X 3 analysis of variance for unequal cells using the approximate method of unweighted means (Winer, 1962). The results yielded significant main effects for each factor. Recall increased systematically from abstract words, to concrete words, to pictures (F( 2,134) = 106.65, p < .OOl), strongly confirming the typical results for this variable. Recall was generally highest under standard free recall, intermediate for the intentional control, and lowest for the incidental groups ( F( 2,134) = 9.66, p < .Ol ) . This conforms to the usual pattern of results for incidental-intentional comparisons ( Postman, 1964), indicating that the experimental procedure had achieved its intended purpose.

The predictions from the dual-coding hypothesis also called for an interaction of stimulus attribute and conditions. This turned out to be highly significant (F( 4,134) = 3.68, p < .Ol). It is apparent from Fig. 1 that picture recall was unaffected by the experimental conditions, whereas words clearly decreased from standard to incidental conditions. This differential picture-word pattern can be explained most parsimoniously in terms of ‘the dual-coding hypothesis. The orienting task required the Ss

cl STANDARD INTENTIONAL INCIDENTAL

CONTROL

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8 .30 5 E g .20

E

IO

STIMULUS

FIG. 1. Mean proportions of pictures ( P), concrete nouns ( C ) and abstract nouns (A) that were correctly recalled under incidental, intentional control, and standard free-recall conditions.


explicitly to name the pictures under both incidental and intentional conditions, thereby ensuring arousal of the verbal code and maximizing the probability that the items would be stored both verbally and as nonverbal images. Recall would be enhanced because either memory code could serve as an effective retrieval cue for the appropriate verbal response. The instruction that recall would be tested apparently added nothing to the performance of the intentional groups. In the case of words, however, the incidental orienting task presumably tended to re- strict the Ss’ attention to the verbal code alone, thereby inhibiting image arousal even when the words were concrete. Moreover, the addition of recall instructions did not fully offset the distracting effect of the orienting task for the intentional as compared to standard controls.

The interaction was also expected to reveal a different recall pattern for concrete and abstract words. On the assumption that the orienting task would inhibit image arousal to concrete words more under incidental than under intentional conditions because the latter would draw attention to aspects of word meaning that might aid recall, it was expected that concrete-word recall would be particularly affected by the incidental-intentional manipulation. Abstract words, being relatively devoid of image-arousing value, should be less affected by this variable. The results were consistent with the hypothesis in that the incidental-intentional differences were somewhat greater for concrete than for abstract words. Moreover, the corollary prediction that the superiority of concrete words over abstract would be less under the incidental than under the intentional condition was supported in that concrete words significantly exceeded abstract words in the two control conditions (p < .Oi by Newman-Keuls test), but not in the incidental condition. How- ever, an analysis of variance of the concrete and abstract word groups (with picture groups excluded) showed that the required Concreteness X Conditions interaction was not significant (F = l.ll), so the trends can only be regarded as suggestive.

While the picture-word comparisons were clearly consistent with the dual-coding hypothesis, the results do not rule out alternative interpretations. In the present study, explicit naming occurred under the incidental and intentional control conditions and imagery was manipulated only in terms of item concreteness. The equivalence of the verbal encoding (naming) response to pictures and words weighs against an ex- clusively verbal interpretation of the striking superiority of pictures, but the results do not compel us to choose between the dual-coding and imagery hypotheses. Given the assumption that Ss are able to transform nonverbal memory representation into a verbal form during recall, the concreteness effect itself could be explained entirely in terms of differ-


ential availability of images during encoding and storage. Moreover, even if the dual-coding interpretation is correct, any differential contribution of imaginal and verbal codes could not be detected by the incomplete design.

EXPERIMENT 2

Experiment 1 indicated that the incidental learning task, appropriately extended, might reveal the basis of picture-word differences in recall, but it appeared to be somewhat less sensitive to the encoding differences involved in the recall of concrete and abstract words. Since the inclusion of abstract words also adds considerably to the complexity of the experimental design we decided to simplify the analytic problem in the remainder of the present series of studies by focussing our attention on the problem of picture superiority, doing so in Experiment 2 by com- paring recall of pictures and concrete words under incidental conditions. As in Experiment 1, the orienting task for two groups required the Ss to write the names of flashed pictures or words during the interitem intervals. Added to this were two other groups who were required to generate images during the interitem intervals, i.e., they were required to draw a rough sketch of the presented picture or of the object named by the presented word. The task of drawing to pictures was intended as a single-code imagery condition analogous to the word-write verbal coding condition, and drawing to words was a further dual-coding condition, functionally similar to the picture-write condition. According to the dual-coding hypothesis, recall should be lower in the picture-draw condition than in the two dual-coding conditions, provided that the orienting task successfully reduced implicit labeling in the former. On the other hand, if imagery is qualitatively superior to the verbal code as a medi- ator of recall, performance in the picture-draw condition should exceed the word-write condition and approach the level of the two dual-coding conditions.

Method

Procedure. The 72 pictures and 72 concrete noun labels used in Ex- periment 1 were again presented for $$a set each, at 5-set inter-item intervals. The procedure for the groups required to write the label of the picture was identical to that used in Experiment 1. Differences were introduced only in the instructions and task given to the two drawing groups. They were told that this was an item recognition task and that they were to indicate identification of each item by drawing a rough sketch of the picture that had been flashed (in the case of the picture-


draw group) or of the object named by the word that had been presented (in the case of the word-draw group). It was emphasized that they had only a brief interval in which to draw the sketch and that they should not be concerned about its quality. (Pilot work in fact demonstrated that Ss were able to perform the task successfully.) To reduce misses of items while the Ss were engaged in drawing or writing, all groups were alerted to the click of the slide changer, which preceded the projection of each item by 1.50 sec. The Ss drew the pictures or wrote the labels on recording sheets, each with I2 numbered spaces in two columns of 6.

Following the orienting task the response sheets were collected, the Ss were given another sheet, and were asked to recall as many of the items as possible, in any order, by writing down the words or the names of the pictures that had been presented. Five minutes were allowed for recall. The Ss were tested in groups with Ns ranging from 19 to 21. Two different basic presentation orders of the 72-item lists were used (the two orders for pictures corresponding to the two orders used for their labels). Approximately equal numbers of Ss received each order.

Subjects. The Ss were 81 students (28 males) enrolled in introductory psychology who were fulfilling a course requirement.


Recall was scored so that the responses in the picture conditions were considered correct if they corresponded to labels appearing in the normative lists of responses to those items, or if they were obviously reasonable labels according to the Es and a research assistant, or if the Ss had used the labels to identify the pictures during the orienting task.

The mean proportions of correct recall are plotted in Fig. 2, where it can be seen that recall in the word-write condition is about one-half of that in the other groups. Note that the mean recall proportion of 0.212 for the word-write group compares favorably with the mean of 0.205 obtained for the comparable group in Experiment 1. Moreover, picture- write recall is comparable in the two experiments (.486 and .450 for Experiments 1 and 2, respectively). Thus the results can be regarded as generally reliable.

The recall scores were analyzed by a 2 X 2 analysis of variance for independent groups, with input mode (words or pictures) and encoding mode (written labels or drawings) as factors. The results revealed highly significant effects of input mode (F( 1,77) = 27.6, p < .OOl); encoding mode ( F( 1,77) = 60.2, p < .OOl) ; and the interaction of the two (F( 1,77) = 63.6, p < .OOl).

The. most novel and striking finding is the high performance of the


Picture Word

STIMULUS

FIG. 2. Mean proportion of correct incidental recall for pictures and words encoded by drawing (Dr) the imaged objects or writing (Wr) their verbal labels.

picture-draw and word-draw conditions, both showing more than double the recall of the word-write group. These results are inconsistent with a simple dual-coding hypothesis, inasmuch as there is no evidence of any reduced performance in the case of the picture-draw condition. While the mean recall proportion of 0.446 for that group is suggestively lower than the mean of 0.495 for the word-draw group, the difference is not significant by a Newman-Keuls test. Thus, wherever a picture occurred either as an input item or as an S-produced drawing, recall was high. This could be taken as support for the superiority of imaginal memory over verbal memory rather than for the dual-coding hypothesis.

It is possible, however, that the picture-draw condition did not ade- quately control implicit labeling. That is, the Ss may have labeled the pictures before or while drawing them; indeed, labeling may have been a necessary guide for the drawing response. If that was the case, the results simply provide further support for the dual-coding hypothesis. It could be argued, moreover, that the drawing and writing responses used in Experiments 1 and 2, being explicit, differ from the implicit stimulus coding that is assumed to occur in the standard free recall task. These objections were handled in Experiment 3 using a procedure designed to manipulate internal stimulus coding while holding the overt orienting response constant for all conditions.

EXPERIMENT 3

The conditions in this experiment corresponded precisely to those used in Experiment 2, with the difference that the orienting tasks re-


quired Ss to generate mental imaginal and verbal codes to pictures and words, and to indicate the ease of doing so by a numerical rating.

Method

The items, method of item presentation, and general task instructions were identical to those used in Experiment 2. The orienting task differed in that Ss were instructed either to generate an image to a stimulus item or to pronounce its name to themselves, then to rate the ease of imaging or pronouncing on a 7-point Difficult-Easy scale during the inter-item interval. Thus Ss in the picture-image condition were asked to try to image each picture immediately after its presentation by imagining themselves drawing the picture, and then indicate how easy it was to image the details of the picture by writing a number from 1 (most difficult) to 7 (easiest) in the appropriate space on a recording sheet. The Ss in the word-image condition were asked to try to generate a mental image of the object suggested by each stimulus word (drawing it men- tally) and to indicate the ease of doing so by the numerical rating. The Ss in the picture-pronounce condition were asked to pronounce the name of each picture to themselves, and then to rate the pronunciability of the name using the numerical scale. The Ss in the word-pronounce condition were similarly required to pronounce each printed word to themselves and then rate its pronunciability. To facilitate the numerical ratings, the scale was reproduced at the top of the rating sheet as a linear scale with the end points, 1 and 7, labeled Difficult and Easy, respectively. It can be seen that in order for the ratings to be carried out according to the instructions, the Ss were required to generate implicit imaginal and verbal codes, but the overt indicator of the differential encoding was identical for all Ss.

Following the rating task the response sheets were collected, Ss were given another sheet, and were unexpectedly asked to recall as many of the items as they could remember, in any order, by writing the names on the sheet. They were allowed 5 min for recall, then they were asked to write “yes” or “no” on the back of the recording sheet to indicate whether or not they had expected the recall test at the time of the rating task, Following this, the picture stimulus groups were again shown the stimulus items one at a time, and were asked to write their label for each picture on a new recording sheet. This made it possible to use the Ss’ own labels as the basis for scoring their verbal recall.

The Ss were 88 students (40 males) enrolled in introductory psychology who participated in fulfill ment of a course requirement. They were tested in groups of 7-12 Ss. Two groups were assigned to each experiment condition, each receiving the items in a different order.



Five Ss indicated that they expected the recall test and were excluded from the analysis. The recall scores of the remaining Ss were scored using a scoring procedure in which the responses of the picture stimulus groups were scored using their own post-experimental labels as the criterion in those instances where the labels differed from the normative ones,

The mean recall proportions for each condition are plotted in Fig. 3. It can be seen that the pattern of results is highly similar to that of Ex- periment 2, with recall being lowest for the word-pronounce group (the analogue of the word-write group of Exp. 2) and substantially higher and approximately equal for the remaining groups. A 2 X 2 analysis of variance for independent groups again revealed highly significant effects of stimulus mode (pictures vs words) (F( 1,79) = 19.60, p < .OOl); orienting task (image vs pronounce) (F( 1,79) = 17.93, p < .OOl); and the interaction of the two (F( 1,79) = 24.54, p < 601). It is apparent by inspection that these effects are due entirely to the low recall in the word-pronounce condition.

Comparison of Experiments 2 and 3. To determine the comparability of the effects of explicit and implicit coding, the results of Experiments 2 and 3 were analyzed by an overall analysis of variance involving experiment (2 vs 3) as a third factor. Significant results were of course obtained for stimulus mode, orienting task, and their interaction ( F( 1,156) 2 46.74, p < .OOl). Further analysis of the interaction by Newman-Keuls comparisons revealed that the conditions involving verbal coding (writing or pronouncing) of word stimuli were significantly in-

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Im

STIMULUS

FIG. 3. Mean proportion of correct incidental recall of pictures and words encoded by imaging (Im) to the items or pronouncing (Pr) their labels.

PICI’URE SUPERIORITY IN FREE RECALL 191

ferior to each of the other conditions, p < .OOl. No other comparison was significant in that interaction.

Recall was also higher on the average under the explicit coding involved in Experiment 2 than under the implicit coding in Experiment 3 ( F( 1,156) = 39.49, p < 601). This was qualified, however, by a significant double interaction with orienting task (image vs verbal), and an overriding triple interaction involving experiment, stimulus mode, and orienting task (F( 1,156) = 4.11, p < .05). Further Newman-Keuls comparisons revealed that explicitness of verbal coding (writing vs pronun- ciation) had no effect in the case of word stimuli, but explicit coding (writing or drawing) was superior to implicit coding (pronouncing or imaging) in each of the other conditions, p < .Ol. The interaction suggests that explicit encoding somehow enhances the probability that imagery or dual coding (or both) will occur. The effect merits further study in its own right but it does not suggest any qualification of the major implications of the two experiments, so it will not be discussed further in this report.

The most important conclusions from Experiments 2 and 3 are as follows. First, the overall results provide no support for the view that verbal encoding alone accounts for the superiority of pictures over words, inasmuch as recall of pictures averaged over the two image conditions was almost double the recall of words under the two verbal encoding conditions. Second, dual encoding of pictures or words provided no significant advantage over imaginal coding of pictures, suggesting that the arousal of a concrete image either by instructions or by picture stimuli may be sufficient to account for the superiority of picture stimuli and imaginally encoded words over verbally encoded words. The high recall of word stimuli under the implicit image condition relative to the implicit verbal coding condition in Experiment 3 provides particularly im- pressive support for the potency of mental imagery as a memory code, inasmuch as the encoding in both cases was entirely implicit, with no difference in the overt response required of the Ss. In fact, the usual superiority of pictures over words disappeared under imagery instructions.

EXPERIMENT 4

The results from the first three experiments seem to be consistent either with the image-superiority hypothesis or a combination of image superiority and dual coding. They appear not to be consistent with a simple dual-coding view that does not differentiate the two codes in terms of memory value. However, two problems with these studies hinder any clear interpretation. First, subjects may have verbalized to pictures in the picture-image condition, despite the fact that the tasks appear not


to require verbalization. Second, the conditions in these experiments involved code repetition in the sense that S was first required to identify each picture or word and then to generate from memory a further imaginal or verbal representation of the stimulus. There is no information from any of these experiments on memory for pictures and concrete words without such additional coding. This feature is relevant to the original problem because the dual-coding interpretation of picture- superiority implies a kind of repetition effect and we need to know the relative memorability of pictures and words independent of repetition, as well as what the effect of repetition is both within and between codes. Finally, the data provide no direct evidence relevant to the assumption that the two codes are independent.

These problems were investigated in two further experiments involving an incidental orienting task that did not require active encoding of items to higher levels although it required Ss to attend to them and presumably register the pictures cognitively as unelaborated images and the words as verbal traces. The present experiment was concerned specifically with the recallability of pictures and words presented once under the incidental or a standard control condition. Experiment 5 extended the same design to include item repetitions that produced conditions analogous to those involved in Experiments 2 and 3.

Method

Lists. The lists were constructed from the 72 pictures and 72 concrete words used in the previous experiments. Each list included 36 pictures and 36 different words, i.e., the words in the list were not the labels of the pictures. The pictures and words were randomly distributed throughout the list with the restriction that the same number of pictures and words appeared in the first half of the list as in the second half. To control for the mode of item representation (picture or word) and any biasing serial position effects, four versions of the list were constructed. The pictures and words in one list became words and pictures, respectively, in a second list. Two other lists were constructed by reversing the positions of items in the two halves of the first two lists, so that the items in the first half were moved to the second half and vice versa, with some re- ordering of items within halves.

Procedure. Different groups of Ss were tested under incidental learning and standard free recall conditions. In both conditions, the items were presented to groups of Ss at a 5-set rate, as in the previous experiments. The four different versions of the basic list were used with approximately equal numbers of Ss in each condition. The incidental Ss were told that this was a probability learning experiment in which they would be


presented a series of randomly ordered pictures and words; between items, they were to guess whether the next item was going to be a picture or a word and also record whether their guess for the preceding item was correct or not. They did so on response sheets containing 72 numbered spaces. During each inter-item interval, they checked the cor- rectness of the prior guess, then wrote P (for picture) or W (word) in the following space to indicate their guess about the mode of the next item to be presented. Since the task only required the Ss to recognize whether an item was a picture or a word, it seemed possible that Ss sometimes might not identify particular items presented for only 1/1e sec. This possibility was reduced by increasing the exposure duration to 1/ set per item.

Following presentation of the entire list, Ss were unexpectedly given new recording sheets and were asked to recall as many of the items as they could. Four minutes were allowed for the recall. They were then asked to write “yes” or “no” on the back of their recall sheets to indicate whether or not they had expected to be asked to recall the items. The recall sheets were collected and the Ss were instructed that they would be shown the 72 items again, but this time they were to write the names of each item on a sheet containing 72 numbered spaces. This information was collected so that E could identify the specific names with which Ss labeled the pictures, thereby permitting rigorous scoring of picture recall.

The standard free recall groups were not given the orienting task, but were told simply to learn the 72 items as they were presented, since they would be asked to recall them later. Following the presentation of the items, the Ss were given the same instructions as were the incidental groups at that stage in the session.

Subjects. The Ss were 84 introductory psychology students (38 males) who volunteered for this experiment to fulfill a course requirement. There were 41 Ss ( 15 males) in the incidental learning condition, and 43 (23 males) in the control group. They were tested in groups ranging in size from 8 to 12.


The data of 2 Ss (one male) were eliminated from the incidental condition because they indicated that they had expected the recall test following the probability learning task. The mean proportions of pictures and words recalled were 0.162 and 0.088, respectively, under the incidental learning conditions, and 0.440 and 0.283 for the standard recall group. An analysis of variance of these data revealed that pictures were recalled better than words (F( 1,SO) = 68.31, p < .OOl); and recall was higher under standard than incidental conditions (F( 1,80) = 173.95,

194 PAM0 AND CSAPO

p < ,001). The interaction was also significant (F( 1,SO) = 7.47, p < .Ol), indicating that the picture superiority was somewhat greater under the intentional than the incidental condition. This may reflect a floor effect in the case of incidental recall of words. In any event, the interaction does not qualify the main conclusions from the experiment.

The low recall under the probability learning orienting task indicates that this was truly an incidental learning task in which subjects did little or no elaborative processing of individual items. The fact that incidental recall was twice as high for pictures as for their names is strong support for the superiority of nonverbal visual images over verbal memory representations independent of any higher-order encoding of the input items. This suggests that the two-fold superiority of the picture-image over word-label conditions in the previous experiments indeed reflected the relative mnemonic potency of the two coding systems for recall of individual items. The implication that an image is worth two mental words was explored in yet another way in Experiment 5, along with direct in- vestigation of the possible independence of the two codes.

EXPERIMENT 5

This experiment involved the same probability learning task as Ex- periment 4, but the design was extended to include picture-picture (PP), word-word ( WW), and picture-word or word-picture ( PW) repetitions along with once-presented pictures (P) and words ( W) . The repetition conditions, involving two successive presentations of an item, were intended to be analogous to the imaginal and verbal coding involved in Experiments 1-3, and to the coding that is assumed to take place fre- quently in standard free recall of pictures and concrete words. Since this task had not been used previously to investigate the problem and in order to permit the most general possible conclusions to be drawn from the results, the design also included two types of intentional controls along with the incidental recall condition.

A number of predictions follow from the assumption that the repetition conditions are analogous to imaginal and verbal coding responses to pictures and words. The most general expectation is that the pattern of recall for repetitions should parallel those obtained in Experiments 2 and 3. That is, recall for PP, PW, and WP repetition should not differ significantly, but each should be superior to WW repetition. Given almost a two-fold superiority, on the average, of the imagery or picture conditions over the purely verbal conditions in the preceding experiments, and assuming that the effect of word-word repetition is no greater than additive, it can be predicted that recall of once-presented P items should be approximately equivalent to WW recall. Other predictions are re-


lated to the hypothesis that imaginal and verbal codes are independent. This implies that recall of PW repetitions should approximate what would be expected for independent events. That is, the effects should be an additive combination of P and W, as estimated from the recall scores for once-presented P and W items.

In the context of the present experiment, the concept of code independence must be distinguished from the possible independence of repeated events, as in PP and WW repetitions. The latter has been discussed in relation to the effect of the spacing of repetitions in free recall. It has generally been found that the beneficial effect of word repetition on free recall is greater when other items intervene between the successive occurrences of the repeated item than when the repetitions are massed (e.g., see Melton, 1970). One interpretation of this effect is based on the concept of differential encoding, according to which the longer the interval between repetitions, the greater the likelihood that the encoding of the second presentation will be a new encoding. Such differential encoding is presumed to enhance the probability that the target item will be retrieved from memory (e.g., Madigan, 1969; Melton, 1970). The important point for present purposes, however, is that the differential encoding hypothesis suggests that the two occurrences of the same word would approach independence and manifest additive effects on recall only at longer lags (cf. Glanzer, 1969; Melton, 1970). When no other items intervene between the two occurrences, as in the case of WW items in the present experiment, the effect should be less than additive, To the extent that PP behave like WW repetitions, they also should manifest less than an additive contribution of the individual events to recall, These predictions, too, can be tested using recall scores for once-presented P and W items to estimate predicted values for repetitions based on the independence hypothesis.

These predictions could be qualified, of course, by the incidental-intentional variable. The predictions for PW repetitions should hold under either incidental or intentional conditions, since the independence of codes should not be affected by the intent to learn, Of course, Ss would be expected to verbalize to pictures and image to words under intentional conditions, but such coding should simply add further independent and additive components to both once-presented and repeated items without seriously affecting the overall independence of the memory traces corresponding to pictures and words, unless there is a strong ten- dency for Ss to encode all items in one way (e.g., verbally). There was no reason to expect such a bias in the present experiment. The predictions for WW (and perhaps PP) certainly apply under standard (intentional) free recall, since that is where the relevant interpretations of repeated


events have been worked out, but predictions for these events under incidental conditions hinge on one’s general theoretical assumptions concerning the coding of repetitions in memory: Is a single trace strengthened or are separate traces formed with each repetition (Hintzman & Block, 1971)? And what is the role of active cognitive processing in such effects? We need not speculate on those questions here, although the results will have some bearing on them.

Method

Lists. The pictures and concrete words used in the previous experiments were arranged into 96-item mixed lists that included 12 items in each of the five experimental conditions. That is, 12 pictures were repeated in a given list as pictures, I2 words as words, 12 items were repeated in the other mode (6 pictures as words and vice versa), 12 items were presented once as pictures and I2 once as words. Two basic lists were constructed so that all of the 72 items appeared in at least one of the five conditions. First, the 72 items were randomly divided into two pools of 36 items each. To make up a list, the 36 items from one pool were assigned equally to each of the three repetition conditions. Then 12 pictures and 12 words were randomly chosen from the other pool for the two single-presentation conditions. The second basic list was constructed similarly, except that the pools were reversed before selection was made, i.e., the pool from which the single-presentation items were taken for the first list was divided equally into the three repetition conditions. An additional version of each basic list was made by replacing all pictures in the lists with words, and all words with pictures. The once-presented and repeated items were arranged in the list so that all five conditions (P, W, PW, PP, WW) occurred within each block of 8 items, in a random order. The conditions were therefore distributed equally throughout the list.

Procedure. The lists were presented to separate groups of Ss under an incidental learning procedure or one of two control conditions. All groups were presented the 96 items at a S-set rate and each item was exposed for 1/ sec. As in Experiment 4, the incidental learning groups were told that this was a probability learning experiment in which they would be presented a series of randomly-ordered pictures and words. Following this task, they were unexpectedly given new recording sheets and were asked to recall as many of the items as they could. They were then asked to write “yes” or “no” on the backs of their recall sheets to indicate whether or not they had expected the recall test. The recall sheets were collected and the Ss were told that they would see the 96 items again but this time they were instructed to simply write the names of the items as


they were presented. Booklets were provided with 96 numbered spaces. This information enabled E to score the picture recall protocols using Ss’ labels.

The two groups of Ss in an intentional control condition received the same treatment as the incidental learning groups. They were given the probability learning task and the same instructions but they were told before the items were presented that they would also be asked to recall the items later on. Following the presentation of the items, the Ss were given the same instructions as those given to the incidental groups at that point of the session. They were then given 6 min for recall.

The second (standard free recall) control condition did not involve the orienting task. Subjects were simply asked to learn the items, since they would be asked to recall them later. They were given the same recall instructions as the other groups after the items had been presented, and given 6 min for recall.

Both control conditions also involved the presentation of the items after recall so that the Ss’ labels for the pictures could be obtained for scoring purposes.

Subjects. The Ss were 124 introductory psychology students (53 males) who volunteered for this experiment to fulfill a course requirement. They were tested in groups ranging in size from 8 to 12.


The data of two Ss were excluded from the incidental condition because they indicated that they had expected to be asked to recall the items after they had been presented.

A preliminary analysis showed that recall of PW and WP repetitions did not differ significantly in any of the conditions (t 5 OH), so the two sequences were pooled in the remaining analyses. The mean proportions of items recalled correctly under each experimental condition are presented in Table 1. The recall scores were analyzed by an analysis of variance with experimental condition (incidental, intentional control, standard control) as an independent-groups factor and item type (P, W, PW, PP, WW) as a repeated measures factor. Significant effects were obtained for condition (F(2,119) = 115.59, p < .OOl), item type (F (4,476) = 83.00, p < .OOl). Their interaction was not significant, (F 8,476) = 1.57). The main effect of condition reflects the classical pattern for intentional-incidental comparisons, with recall being consistently highest for all item types under standard free recall, intermediate for the control condition with the orienting task, and lowest for incidental recall. Thus the experimental tasks again achieved their intended purpose. The effect of item type Z&O shows a consistent pattern, recall in each condition


TABLE 1 Obtained Correct Recall Proportions for Once-presented and Repeated Pictures (P)

and Words (W) and Predicted Proportions for Repeated Items

Incidental FR (N = 39) P W PP PW(WP) ww

Obtained Predicted

Intentional Control (N = 43) Obtained Predicted

Standard FR (N = 40) Obtained Predicted

,137 ,068 ,263 ,231 .124 ,243 ,194 ,129

,171 ,112 ,320 .339* ,178 ,296 ,262 203

.433 ,298 .550* 567 385* 649 .595 .486

* p < .OOl, obtained versus predicted.

being significantly higher (Newman-Keuls tests) for P than W in the case of once-presented items, and higher for PP and PW than for WW in the case of repeated items.

The comparisons of once-presented P and W items once again confirm the reliability and generality of picture superiority in recall. More striking is the observation that in no case did recall of P items differ significantly from recall scores for repeated words. Consistent with the two- to-one ratio of code efficiency that could be inferred from the results of the previous experiments, one exposure to a picture is generally worth two to a word! These data appear to provide conclusive support for the image-superiority hypothesis of picture recall.

Comparisons of repeated items indicate equally conclusively, however, that dual encoding enhances recall. In all three experimental conditions, PW significantly exceeded P and WW recall, p < .Ol by Newman- Keuls test. Thus dual encoding increases recall of a target item beyond a level that can be accounted for either by an unelaborated imaginal memory representation (presumably aroused by P) or a strengthened or renewed verbal trace (WW). Moreover, recall scores for PW do not differ significantly from PP in any of the conditions. These results are precisely comparable in pattern to those obtained in Experiments 2 and 3, where picture-image and the two dual-coding conditions did not differ significantly from each other but all exceeded recall of verbally encoded words.

The present results suggest further that the repeated events are either more than additive in the case of PW or less than additive in the case of PP, inasmuch as once-presented pictures generally exceeded once- presented words in recall. This raises the issue of independence of codes and repeated events, to which we now turn.


Independence of codes. The expected values for repetition effects according to the independence hypothesis were estimated using the recall data for once-presented items. Thus, the expected values for PW repetition were calculated for individual subjects by the formula P,, = P, + P, - P,P,. These values were then averaged for each experimental condition, A comparable analysis was done for PP and WW repetitions. The mean expected values are shown in Table 1 along with the obtained values. The differences between obtained and expected values were evaluated statistically using the normal curve approximation to the binomial expansion. Since nine comparisons are involved, alpha was set conservatively at the 0.025 level.

Recall that, under standard free-recall conditions, PW repetition was expected to reflect independence of codes whereas WW and PP repetitions were expected to have less than the additive effect characteristic of independent events. These expectations were precisely confirmed: expected and obtained values did not differ significantly for PW repetition (2 = 1.25), h w ereas obtained values for WW and PP were significantly lower than expected, 2 > 4.43, p < .OOl. Thus, statistical independence and additivity is supported in the case of PW, but not PP or WW repetition.

The results of the comparisons for the incidental and intentional control groups contrasted sharply with the above pattern. The differences between obtained and expected values did not reach significance for PP and WW, Z < 1.41, suggesting independence in the case of each type of repetition. Obtained PW recall was greater than expected, however, as though the repeated events were more than additive in their effect. The reliability of the latter difference is doubtful in the case of the incidental group (2 = 2.02, p < .05) but highly significant in the case of the intentional control (2 = 3.98, p < .OOl). With the exception of the inex- plicable PW observation, these data are consistent with the view that the recall task was indeed treated as incidental and that Ss had therefore given minimal attention to the items during presentation. Each item, whether previously seen or not, functioned essentially as a new event.

To summarize, Experiment 5 systematically confirmed and extended the results of the preceding experiments. Pictures again showed a two- fold superiority over words in comparisons of conditions that presumably did not involve dual coding of pictures. Recall did not differ for PP and PW repetition conditions but each substantially exceeded WW recall, paralleling the superiority of picture-image and dual coding conditions over verbal coding of words in Experiments 2 and 3. Finally, Experiment 5 provided direct evidence that imaginal and verbal memory codes are independent and additive in their effect on recall, at least under standard


recall conditions, whereas within-code repetitions are not independent and additive under such conditions.

GENERAL DISCUSSION

Let us now consider the theoretical conclusions that are suggested by these results, and what issues remain unsolved or arise from them. First we deal with the central problem of picture superiority for recall and then, more briefly, with the implications of the results for other issues in the memory literature.

The most important general conclusion from the series of experiments is that picture superiority under standard free recall conditions can best be explained in terms of an additive contribution of imaginal and verbal memory codes, with the contribution of the former being decidedly greater than that of the latter. The most direct support for this conclusion comes from Experiments 4 and 5, which showed a two-fold superiority of once-presented pictures over words under incidental conditions and an additive effect of picture-word repetition on recall. Picture-picture and word-word repetitions, on the other hand, appear to be less than additive in their effects. Moreover, the similarity of the recall patterns for repetitions in Exp. 5 and the encoding conditions in Experiments 2 and 3 suggests that a common interpretation holds for the memory trace evoked directly by the stimulus and the encoding process evoked indi- rectly by the orienting task. Whether the image was aroused directly by the stimulus and the verbal code by the orienting task or vice versa, dual- encoding apparently resulted in an additive increment to recall, whereas imaging to pictures or verbalizing to words did not. The latter condition must involve less than additive effects because, otherwise, given the superiority of once-presented pictures over words in Experiments 4 and 5, imaging to pictures would have produced higher recall than imaging to words or verbalizing to pictures in the earlier experiments. Since these three conditions instead produced equivalent recall, it is reasonable to assume that imaging to pictures resulted in overlapping traces rather than two independent events in memory. Such an analysis is consistent with Glanzer and Duarte’s (1971) analysis of the effect of massed repetitions on free-recall, as well as other differential encoding interpretations of the phenomenon (e.g., Madigan, 1969; Melton, 1970). We shall return to that issue briefly following further discussion of the central issue.

The facilitative effect of dual encoding is readily understandable in terms of enhanced recall probability, but not so the apparent superiority of images over verbal traces. Indeed, we may ask whether the data necessarily support such a difference in code effectiveness. Perhaps the superiority of once-presented pictures over once-presented words in Experi-


ments 4 and 5 simply reflect uncontrolled dual-encoding effects for some items. That is, despite the set created by the probability learning task, which clearly did not require any verbalization to pictures, subjects may nevertheless have implicitly labeled some pictures but were less likely to image to words. The same argument can be applied to the superiority of repeated pictures over repeated words and of image encoding to pictures over verbal encoding to words. A similar account of the equivalence of once-presented pictures to repeated words is somewhat more strained, since we would have to assume that the uncontrolled dual encoding of pictures occurs with a probability that exactly compensates for the in- cremental effect of explicitly repeating all of the words. Nonetheless, it remains possible that dual encoding and within-code repetitions could account for all of the effects without any assumption of image superiority.

The plausibility of image superiority would be increased if a reasonable theoretical basis could be suggested for differential code effectiveness. There appear to be two general classes of possible interpretations, one based on interrelations among items; the other, on some differential property of the memory trace aroused by individual verbal and nonverbal items.

Interrelations among items traditionally have been viewed in terms of direct or mediated verbal associative connections among items, such associations being potential sources of either facilitation or interference, depending on the nature of the task. In free recall, associative relation- ships are generally related positively to recall (e.g., Deese, 1959), although interference is apparently possible if associative interrelations are so diffuse as to prevent the formation of distinctive associative clusters (Matthews, 1966; for a review of the problem, see Paivio, 1971, Chapter 7). These observations are relevant to the present problem in that verbally based inter-item relations would be expected to play a greater role in the recall of words than of pictures, especially under incidental conditions that prevented or hindered labeling of the latter. Since pictures were consistently recalled much better than words, the effects of such relations were either interfering or overridden by some other positive factor associated with pictures. In either case, differential inter-item organization is implicated as one possibility.

Plausible organizational interpretations of the superiority of imagery over verbal mediation processes have been suggested in the case of paired-associate learning and standard free recall. The theoretical argument is that images are specialized for synchronous organization or parallel processing of information, whereas the verbal code is specialized for sequential organization (Paivio, 1971). Thus, pairs or groups of pictures or imagined items might be better unitized or chunked in memory than


verbally represented items, thereby resulting in less memory load and superior recall in the case of the former (e.g., Begg, 1972; Bower, 1970; Mueller & Jablonski, 1970). This interpretation is difficult to apply to the incidental tasks, however, unless we assume something like autonomous reorganization of memory information rather than the active (intentional?) processing of items that seems to characterize contemporary organizational approaches to memory. Be that as it may, possible differences in organization as a function of encoding conditions could be assessed in the present series of experiments by examining the degree to which items were recalled in clusters based on the conceptual categories (ani- mals, musical instruments, etc.) that the lists happened to contain, or the picture-word dichotomy in the case of mixed lists. This was done for Experiments 2 and 4, since they are representative of the two different incidental recall tasks. The results showed that clustering by conceptual categories was generally quite low in Experiment 2, involving homo- geneous lists of pictures or words, and there were only small differences in clustering for the different experimental conditions. In Experiment 4, subjects did cluster on the basis of the picture-word dichotomy and the degree of clustering was greater for pictures than for words, suggesting that organizational processes may have affected recall. However, this does not help explain the absence of differential category clustering in Experi- ment 2, nor the results of other experiments (e.g., Paivio, Rogers & Smythe, 1968) in which inter-trial organization for pictures and for their labels did not differ, although the pictures were better recalled. Of course, such negative results may simply mean that the empirical measures of inter-item organization used in the studies are not sensitive to the postu- lated distinction between synchronous and sequential organization of units in memory.

The second general theoretical possibility is that pictures and words differ on some property that affects their memorability as individual units. Perhaps pictured objects are simply more available for recall because of higher familiarity or frequency (cf. Asch & Ebenholtz, 1962); or their memory images might somehow take up less ‘storage space” than the traces of words, so that more of the former can be held in memory, much as Begg (1972) f ound in the case of high imagery as compared to low imagery phrases. Such possibilities were investigated in an explora- tory fashion by obtaining data on various attributes of the pictorial and verbal items and correlating these with mean recall scores for the items obtained in Experiments 25. Attributes that can be regarded as measures of the availability of underlying verbal representations included ratings of printed familiarity, rated pronunciability, frequency counts, and latency and consistency of labeling of pictures. Availability of images was


presumably tapped by rated imagery value, and ratings of imageability of pictures and words by subjects in Experiment 3 in this study. Number of letters and syllables constituted measures of the structural complexity of words with possible implications for storage space and recall.

With few exceptions, the above measures did not correlate significantly with recall scores for pictures and words obtained under conditions that should reveal the differential memorability of imaginal and verbal traces with least contamination from dual coding, namely, the incidental recall conditions of Experiment 4. Paradoxically, the exceptions were that incidental recall scores for pictures correlated significantly with the printed familiarity, Thorndike-Lorge frequency, and K&era-Francis frequency of their printed labels (rs = .26, .29, .54, respectively, elf = 70). These correlations suggest that pictures with readily available (high frequency) labels were more likely to be dually encoded during input than ones with less available labels, or that the labels were more retrievable from the memory images of the former during recall. Code interconnectedness rather than differential properties of verbal and imaginal codes is implicated in either case. The indices of imaginal or verbal code availability more often correlated with recall scores under the elaborative encoding conditions of Experiments 2 and 3 and under standard free recall conditions, again suggesting that the attribute measures may be sensitive to interconnections between the two coding systems but not to their individual mnemonic properties. These correlational results accordingly failed to provide any firm theoretical basis for accepting or rejecting the view that images are mnemonically superior to verbal memory traces and it must be left to the future as a major unsolved problem.

Perhaps the most important general implication of the results for the psychology of human memory is the strong support provided by the data for two major systems of memory and cognition, which are independent and yet functionally interconnected so that particular target items can be retrieved from either form of memory representation. This theory is related generally to other multicomponent or attribute theories of memory (e.g., Bower, 1967; Underwood, 1969; Wallach & Averbach, 1955; Wickens, 1970), but differs from them in its special emphasis on a verbal-nonverbal dichotomy and certain functional distinctions related to that division (see Paivio, 1971, Chapter 7). It is especially important to recognize that this is a distinction in symbolic modalities of memory, which are viewed as orthogonal to sensory modalities in the sense that verbal and nonverbal stimuli alike can be visual, or auditory, or haptic, etc. That this is not an empty semantic quibble is apparent from the fact that, in the present series of experiments, sensory modality was constant, all stimuli having been presented visually, yet nonverbal stimuli were

204 PAW10 AND CSAPO

remembered much better than their verbal counterparts. Conversely, in other studies, symbolic modality has been constant ( all items were words), yet differences in sensory modality have been found to affect recall (e.g., Murdock, 1967; Murray & Roberts, 1968).

The data also provide suggestive information concerning the conditions that determine effectiveness of retrieval cues in recall. Tulving and Osler (1968) originally suggested that retrieval cues are effective only if they are stored along with the to-be-remembered items during input and are reinstated during output as well. Subsequently it was found (Thompson & Tulving, 1970; Wood, 1967) that strongly associated cues presented only at output did facilitate free recall. The present results suggest the further qualification that it may be sufficient under incidental recall conditions for pictorial retrieval cues to be stored without their to-be-remembered verbal labels. The latter apparently can be retrieved entirely from the memory images of the former (cf. Bahrick & Bahrick, 1971). In this case, the to-be-recalled item is absent during input and the retrieval cue is present during output only in an implicit sense. If this interpretation is correct, it contrasts also with Rohwer’s (1970) view that the effectiveness of an image depends on simultaneous storage of a verbal tag. The interpretation requires us to assume, however, that dual encoding was absent in at least some instances of picture recall in the present experiments. Until we know the extent of uncontrolled dual encoding under similar conditions, the issue will remain unresolved.

Finally, the results have strong implications for interpretations of the effect of spacing of repetitions on recall. Specifically, the finding that picture-word repetitions are functionally equivalent to independent events but word-word and picture-picture repetitions are not (at least under standard free-recall conditions) favors the differential encoding hypothesis of the lag effect. A direct test of course requires a comparison of the present findings with comparable spaced repetitions. The hypothesis would be confirmed if, under standard recall conditions, spaced picture-picture, word-word, and picture-word repetitions all display independence of the repeated events.4

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(Accepted December 19, 1972)

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Picture superiority in free recall: Imagery or dual coding?