Remembering Pictures: Pleasure and Arousal in Memory

Transcription

1 Journal of Experimental Psychology: Learning, Memory, and Cognition 1992, Vol. 18, No. 2, Copyright 1992 by the American Psychological Association, Inc /92/S3.00 Remembering Pictures: Pleasure and Arousal in Memory Margaret M. Bradley, Mark K. Greenwald, Margaret C. Petry, and Peter J. Lang University of Florida Incidental memory performance for pictures that varied along the affective dimensions of pleasantness and arousal was assessed. For both an immediate and delayed (1 year later) freerecall task, only the arousal dimension had a stable effect on memory performance: Pictures rated as highly arousing were remembered better than low-arousal stimuli. This effect was corroborated in a speeded recognition test, in which high-arousal materials encoded earlier in the experiment produced faster reaction times than their low-arousal counterparts. Pleasantness affected reaction time decisions only for pictures not encoded earlier. These results suggest that whereas both the dimensions of pleasantness and arousal are processed at initial encoding, longterm memory performance is mainly affected by arousal. It has been demonstrated numerous times that emotional stimuli vary along two primary dimensions of affective valence and arousal (e.g., Russell, 1980; Tellegen, 1985; Lang, Bradley, & Cuthbert, 1990). Using the semantic differential, Osgood, Suci, and Tannenbaum (1957) showed that the bipolar factors of pleasantness and intensity accounted for most of the variability in judgments of affective text. These dimensions of emotional language map easily onto the behavioral dimensions of direction (approach or avoidance) and vigor (i.e., mobilization) advocated by a biphasic organization of emotional responses (Hebb, 1949; Konorski, 1967; Lang et al., 1990). Dimensional views of emotion are parsimonious in that, rather than assuming independent, specific emotional states (e.g., fear, anger, and joy), two primary dimensions define the spectrum of emotional behavior. The questions addressed in the current article exploit this organization of emotion to determine the contribution of the valence dimension, the arousal dimension, and their interaction to memory performance. Past research has tended to focus on memory for a particular type of emotional event (e.g., sad, happy, traumatic) rather than using a dimensional analysis. In assessing recall of traumatic events, for example, Christianson and Loftus (1987) found a memory advantage for the occurrence of a traumatic situation, compared to a neutral one. The purported clarity and stability in memory for traumatic events has led some researchers (Brown & Kulik, 1977; Bohannon, 1988) to posit a special "flashbulb memory" mechanism that veridically records moments of trauma. Although receiving somewhat less attention, research on memory for happy events also produces evidence of increased memorability, relative to neutral. The "Pollyanna" effect, This research was supported by National Institute of Mental Health (NIMH) Grants MH37757, MH41950, and MH43975 to Peter J. Lang. We would like to thank Alfons Hamm for his assistance in data collection and interpretation in Experiment 2 and Sven-Ake Christianson and two anonymous reviewers for their helpful comments. Correspondence concerning this article should be addressed to Margaret M. Bradley, Box , Health Science Center, University of Florida, Gainesville, Florida indicating better memory for pleasant materials, is at the heart of a series of experiments that corroborate this phenomenon with verbal stimuli (Matlin & Stang, 1978). Running parallel to reports of improved memory for traumatic or happy stimuli are a number of experiments that have explicitly focused on the arousal dimension (see Craik & Blankstein, 1975, and Eysenck, 1976, for reviews). In these studies, arousal has been variously operationalized as the rated arousal of the stimulus materials, the magnitude of a physiological index of the subject's arousal level (e.g., electrodermal responses), or by the presence of a constant stimulus background (e.g., white noise). The general finding is that verbal items associated with higher arousal at encoding result in better memory performance on a later (especially long-term) memory test. Thus, whereas affective memory has been studied at either extreme of the valence dimension (i.e., pleasant or unpleasant), or along an arousal continuum, a systematic exploration of the contribution of each dimension to memory performance is lacking. A necessary component of such experimentation is the presence of emotionally evocative stimuli that are distributed in the two-dimensional affective space defined by pleasantness and arousal. In the current study, the emotional materials are color photographic slides, drawn from the International Affective Picture System (IAPS; Lang, Ohman, & Vaitl, 1988). This standardized collection of pictures, gathered from a variety of sources, samples contents across a wide range of emotional and semantic categories. Contents include animals, nature scenes, erotica, household objects, expressive human faces, weapons, mutilated bodies, and others. Affective valence and arousal ratings for each slide have been obtained in previous rating studies, allowing precise placement of these stimuli in a two-dimensional affective space, which Figure 1 illustrates (see Bradley, Greenwald, & Hamm, in press; Greenwald, Cook, & Lang, 1989; Lang, Greenwald, Bradley, & Hamm, in press). In addition to assessing judgments of affective experience, a number of studies have convincingly demonstrated that emotional responses to these materials measured by psychophysiological and behavioral reactions reliably covary with the dimensions of valence and arousal (Bradley et al., in press; Greenwald et al., 1989; Lang et al., in press). Facial electro-

2 380 BRADLEY, GREENWALD, PETRY, AND LANG Pleasant Pleasantness Rating Unpleasant Arousal rating Figure 1. The left panel illustrates the distribution of 240 rated slides (currently composing the International Affective Picture System; Lang et al., 1988) in the two-dimensional space formed by mean ratings of pleasantness and arousal; the right top panel illustrates facial corrugator electromyographic responses (defined as the average change over a 6-s slide-viewing interval from a 1 -s baseline immediately preceding slide onset) to slides ranked by pleasantness, whereas the right bottom panel depicts mean skin conductance responses to slides ranked by rated arousal (Greenwald et al., 1989). myographic responses, particularly the corrugator ("frown"; see Figure 1) and zygomatic ("smile") muscles, reliably vary with changes in pleasantness ratings. As the pleasantness of the slide decreases, corrugator activity increases and zygomatic activity decreases, indexing the affective valence dimension. The size of the reflexive eyeblink to a startle probe (e.g., a loud noise burst) presented during viewing of these slide materials is also sensitive to the affective valence dimension: The magnitude of the reflexive eyeblink increases as the unpleasantness of the slide foreground material increases (Langetal., 1990). Whereas these somatic responses are specifically tied to differences in the rated pleasantness of the slides, skin conductance responses reliably covary with changes in rated arousal. As judgments of arousal increase, the magnitude of the skin conductance response to the slide increases as well, as Figure 1 convincingly demonstrates. The arousal dimension also appears to be related to differences in interest or attention, because factor analysis (Lang et al., in press) has indicated that interest ratings and the duration of time a subject chooses to view the slide relate to the same factor as the magnitude of electrodermal reactivity. Importantly, these studies validate the dimensional relation in affective picture processing using a range of dependent measures. In addition, the same patterns in physiological response have been replicated using different subsets of IAPS materials, which implies freedom in sampling from this stimulus collection (Greenwald et al., 1989; Bradley, Cuthbert, & Lang, 1990; Hamm, Stark, & Vaitl, 1990; Lang et al., in press). In the experiments reported here, memory performance for these pictorial materials is assessed using an immediate free-recall task, a delayed free-recall task, or a speededrecognition task. Incidental memory performance is always assessed after an initial encoding phase in which the subject rates each slide on the dimensions of pleasantness and arousal. Predictions for memory performance are motivated by past research and are relatively straightforward. To the extent that the affective valence dimension differentially affects memory performance, a main effect for the pleasantness of the pictorial stimulus should be obtained. If memory processes favor negative events, as some data indicate, free recall as well as recognition speed should be facilitated for these materials. On the other hand, if the Pollyanna hypothesis is correct, better memory performance should be obtained for pleasant stimuli than for unpleasant materials. Because the arousal dimension has been associated with improved memory performance in the past, an effect of arousal is predicted such that increases in arousal will be associated with increased free-recall performance for these materials and faster recognition speed. To the extent that arousal is the only salient dimension underlying memory performance, this effect should occur for both pleasant and unpleasant materials. If, as some data seem to suggest, traumatic (i.e., unpleasant and highly arousing) events are different from all others, an interaction of valence and arousal is expected in both free

3 PLEASURE AND AROUSAL IN MEMORY 381 recall and recognition. In this case, negative, highly arousing events should produce superior recall and faster recognition speed than slides in any other quadrant of the two-dimensional emotional space. Experiment 1 In Experiment 1, a free-recall task was used to assess memory performance. After rating a series of 60 slides on the dimensions of valence and arousal, an incidental free-recall test was conducted both immediately and 1 year after the rating session. This experiment allows an assessment of the effects of valence and arousal on both short-term and longterm memory performance. Method Subjects. Eighty-nine University of Florida undergraduates (48 women, 41 men) participated voluntarily to fulfill a course requirement. Each subject viewed one of four randomized slide orders. The data from 2 male subjects who did not provide clear descriptions of recalled slides in the immediate memory test were excluded from analysis (final n = 87). In the delayed free-recall portion of the experiment, 59 (30 women, 29 men) of the 87 subjects were successfully contacted by phone 1 year later. Two subjects (1 woman and 1 man) remembered participating in the experiment but could not recall any of the slides viewed. The data from an additional 3 males were not used because specific descriptions of slides could not be provided. This filtering resulted in a total of 54 subjects for use in the long-term memory analyses. Materials. Sixty color, photographic slides depicting various contents were selected to sample a wide range in emotional space. Five additional slides were selected to serve as practice slides. All slides were selected from IAPS (Lang et al., 1988). The Appendix contains a complete listing of the pictorial stimuli used in this study, indexed by the IAPS number and accompanied by a brief semantic description. Each slide was rated on the dimensions of valence, arousal, and dominance using a pencil-and-paper version of the Self-Assessment Manikin (SAM) rating system (Lang, 1980; Hodes, Cook, & Lang, 1985). SAM is designed to represent the range of the subject's emotional response to a stimulus, and as Figure 2 illustrates, the valence and arousal dimensions were ordinally scaled withfivefiguresrepresenting each dimension. Ratings were made by placing an X on or between any of the figures, producing a scale that ranged from 1 to 9. The valence dimension depicted a figure that ranged from happy to unhappy. The corresponding SAM figures ranged from smiling with raised eyebrows to frowning with knitted eyebrows. The arousal dimension ranged from excited to calm. The corresponding SAM figures ranged from having an active body and eyes wide open to having an inactive body and closed eyes. A third SAM panel (not pictured in Figure 2) required the subjects to rate their feelings of dominance in reaction to the slide, with the SAMfigureranging from small in size (controlled) to large (in control); this dimension is highly correlated with pleasantness for static pictorial stimuli and is not central to the questions addressed here. Each of the three dimensions was displayed on each page in the ratings booklet; the order of the SAM dimensions on each sheet was 'randomized, whereas the booklets were identical for all subjects. Each slide was rated on all three dimensions. Because relatively fewer ratings occurred at the nongraphic indices of the rating scale (i.e., between the figures), the 9-point scale was transformed postexperimentally into a 5-point scale by using 5 as the neutral rating and collapsing ratings 1 and 2 into a single rating, and similarly, 3 and 4, 6 and 7, and 8 and 9. Rating procedure. The study was conducted in groups that ranged in size from 7 to 14 subjects, roughly balanced for sex. On arrival, subjects were seated approximately 2.2 m to 3.6 m from the projection screen; the size of the slide image projected onto the projection screen - n _J 1 i 1 Figure 2. Paper-and-pencil version of the Self-Assessment Manikin (SAM; Lang, 1980) used to rate affective valence (top row) and arousal (bottom row) of the slide stimuli for Experiment 1.

4 382 BRADLEY, GREENWALD, PETRY, AND LANG was approximately 1.2 m x 1.8 m. After obtaining informed consent, the instructions for the experiment were read. Each trial consisted of a sequence of 3 projected slides. First, the subject viewed a preparation slide for 5 s. This slide was used to instruct the subject to turn to the correct page in the ratings booklet and then to focus attention on the projection screen (actual slide instruction: "Rate the next slide on page "). The second slide in the sequence was the to-be-rated slide and was projected for 6 s. The subject was instructed to view the slide for the entire time that it was on the projection screen. After the offset of this slide, a third slide in this sequence was projected for 15 s, which instructed the subject to "Please rate the slide on all three dimensions." During this rating period, each subject was instructed to rate his or her actual emotional reaction during viewing of the to-be-rated slide. Projection times were electronically controlled by a prerecorded synchronization tape played on a Singer Caramate carousal slide projector. After these instructions, 5 practice slides were presented; then, the 60 experimental slides were presented and rated. No mention of a subsequent memory test was given at this time. Immediate-recall procedure. Following the rating procedure, each subject was given an incidental free-recall test, in which he or she was instructed to write down, in any order, a word or phrase describing each experimental slide that could be remembered (e.g., cow, baby, or couple kissing). The subject was told to provide a clear description so that the recalled slide could be identified; the free-recall period was 5 min. At the conclusion of this recall period, the subject was debriefed, paid (with course credit), and thanked. Delayed-recall procedure. Approximately 1 year after participat ing in the slide rating and immediate-recall experiment, each subject who could be contacted by telephone was given an incidental free - recall test, in which he or she was instructed to again recall, in any order, a word or phrase describing each slide that could be remembered from the slide-viewing procedure the year before. The subject was instructed to continue recalling slides for a 3-min period. At the conclusion of this recall period, the subject was debriefed and thanked again. Scoring. The recall sheets were scored on the basis of the subject's description of the slide; the same criteria applied to scoring immediate- and free-recall data, and the same person scored both sets of data. Correct recall was scored if the description of a particular slide was able to be clearly linked to a slide that had been shown. In almost all instances, the descriptions were completely clear. The main stimuli subject to confusion were (a) two pictures of snakes, one of a cobra (i.e., a black snake) and one of a white and gray snake, and (b) two pictures of guns, one aimed at the viewer, one aimed obliquely. To the extent that the subject recalled both members of the category, there was no problem in assigning a correct recall for both candidates. If only one was remembered (e.g., snake), recall was assigned to the stimulus most frequently mentioned by this sample of subjects. This occurred rarely; in any case, both members of these categories tended to be rated similarly in valence and arousal level, which is the main focus of this investigation. Results Rating information. The Appendix lists mean affective valence and arousal ratings for each slide in this study; the nature of the resulting affective space is highly similar to that obtained in previous studies (see Figure 1). Table 1 lists the mean proportion of slides rates at each of the five positions along the valence and arousal dimensions. To adjust for differences in recall that are produced by the number of slides rated at each position on the scale, the number of slides recalled at each level of valence (or arousal) was divided by the number of slides rated at that level for each subject. These data were subjected to an analysis of variance in which gender (male or female) of the subject was a between-subjects variable, rating level was a within-subject variable, and the proportion of slides recalled at each of the five positions along the scale (i.e., valence or arousal) was the dependent measure. Unless otherwise noted, a significance level of.05 was used. Immediate memory. Memory performance was first assessed as a function of the rated arousal of the slide materials (see Table 1). Level of arousal had a significant effect on immediate-recall performance, F(4, 340) = 20.39, MS e =.036. Newman-Keuls post hoc comparisons indicated that slides rated as highly arousing were remembered better than slides rated in all lower categories. In addition, slides rated as moderately arousing were remembered more often than those in the neutral category. A second analysis showed that the pleasantness of the slide also had a significant effect on the slides recalled, F(4, 340) = 14.39, MS e =.033. A significant quadratic trend indicated Table 1 Proportion of Slides Rated and Recalled at Each of the Five Levels of Valence or Arousal Ratings Proportion rated Immediate recall Delayed recall Highly unarousing Moderately unarousing Arousal dimension Neutral Moderately arousing Highly arousing Valence dimension Proportion rated Immediate recall Delayed recail Mean arousal rating (1-9 scale) Highly unpleasant Moderately unpleasant Neutral Moderately pleasant Highly pleasant

5 PLEASURE AND AROUSAL IN MEMORY 383 that memory for both pleasant and unpleasant slides was greater than that for neutral slides, F(l, 85) = 52.61, MS e =.03 (see Table 1). Newman-Keuls comparisons corroborated this, showing that slides rated as highly pleasant and highly unpleasant were remembered better than those in the middle categories. However, there was no specific advantage in recall for either highly pleasant or highly unpleasant materials, as these did not differ significantly from each other. Table 1 lists the mean arousal ratings for slides in each of the five valence categories. From these data it is clear that the level of rated arousal differs as a function of pleasantness, ^(4, 344) = 62.69, MS C = 1.65, and may be mediating the effect of valence on memory. This hypothesis is supported by a significant quadratic trend for these ratings across levels of affective valence, F(l, 86) = 234, MS t = 1.58, demonstrating that both highly pleasant and highly unpleasant slides produced high-arousal ratings (as well as better memory performance). To determine whether there was any contribution of pleasantness to memory performance, while controlling for intensity, the following data set was constructed. First, the 60 slides were rank ordered by valence ratings for each subject. The ranks for slides given identical valence ratings were decided on the basis of the group valence means. The top 30 slides were assigned to the pleasant category; the bottom 30 slides were assigned to the unpleasant category. Within each valence category, slides were then ranked by their arousal ratings. Ties in this ranking were decided on the basis of the group arousal means. The top 15 slides in each of the two valence categories were designated high-arousal slides; the bottom 15 were designated low-arousal slides. This procedure produced, for each subject, a mean recall score for four categories of slides, defined by the combination of valence (pleasant or unpleasant) and arousal (high or low), with 15 slides in each category. The mean arousal ratings for slides assigned to these four categories were first analyzed to determine if this procedure was successful in controlling arousal across valence categories. As expected, although there was a large difference in mean arousal ratings for slides in the low- and high-arousal categories, F(l, 86) = , MS e =.72, pleasant and unpleasant slides were not generally different in mean level of arousal (F < 1). A significant interaction between slide pleasantness and arousal level, F(\, 86) = 22.20, MS C =.38, however, indicated that although pleasant and unpleasant slides in the higharousal category were not significantly different in rated arousal (M = 5.83 and 6.06, respectively), slides composing the low-arousal category showed a small but significant effect of valence, F{\, 86) = 9.19, MS* =.71. Pleasant slides, however, were rated as only slightly more arousing (M = 4.01) than unpleasant slides (M = 3.6). The mean proportions of slides recalled as a function of these valence and arousal groupings are depicted in Figure 3 (top panel). The arousal level of the slide significantly affected recall, with memory for high-arousal slides better than memory for low-arousal slides, F(l, 85) = 31.97, MS e =.014. There was a marginally significant effect of valence, indicating that pleasant slides were remembered slightly better than unpleasant slides, F(l, 85) = 3.87, p =.053, MS t =.019, which is somewhat consistent with the slight difference in.55 1 O Immediate Recall Delayed recall Low Arousal High PLEASANT UNPLEASANT Figure 3. Mean proportion of slides recalled at immediate (top panel) and delayed (bottom panel) recall as a function of the rated pleasantness and arousal category of the slide stimuli. arousal ratings for these materials. The interaction of valence and arousal was clearly not significant, and no main effects or interaction with the subject's gender approached significance. Long-term memory. The proportion of slides recalled in the long-term memory test are presented in Table 1. The pattern of recall 1 year later is remarkably similar to the effects obtained on the immediate memory test. In the dimensional analyses, the main effect of arousal was again significant, F(4, 208) = 11.69, MS C = 016. Newman-Keuls comparisons indicated that slides rated as highly arousing were still remembered better than slides given lower arousal ratings 1 year later. Similarly, the effect of affective valence was again significant, F(4, 208) = 5.52, MS e =.012, as was the quadratic trend for this dimension, F(l,52) = 14.55, MS C =.015. Once again, to assess any influence of pleasantness without the contributing effect of arousal, a within-subject recall score for the 2 x 2 factorial combination was derived as described above. These data can be seen in Figure 3 (bottom panel). The significant main effect of arousal indicates that highly arousing slides were recalled better than low-arousal slides, even after a retention interval of 1 year, F( 1,52) = 7.44, MS t =.005. The main effect of valence and the interaction of valence and arousal were not close to significance (both Fs < 1), indicating that the slight advantage in memory for pleasant material obtained on an immediate test was lost 1 year later. Memory for specific contents. Slides were grouped, post hoc, into nine semantic categories, and immediate free recall as a function of semantic category was investigated. The categories and the number of slides occurring in each category were as follows: attractive females («= 3), attractive males (n

6 384 BRADLEY, GREENWALD, PETRY, AND LANG = 3), mutilated bodies (n = 4), weapons (n = 5), household objects (n = 5), food (n = 4), animals (n = 7), nature scenes («= 9), and sports activities (n = 5). Table 2 lists the mean proportion of slides recalled in each category for each gender. Not surprisingly, the mean proportion of slides recalled differed as a function of semantic category. For both males and females, attractive members of the opposite sex (i.e., female nudes and male nudes) and mutilated bodies composed the top three categories recalled. More interestingly, it appeared that some categories recalled were related to the subject's gender rather than strictly related to the arousal level of the category. For males, mean recall averaged across the categories of weapons, mutilated bodies, and attractive women was higher than recall of these materials by women, F(l, 85) = 7.98, MS C =.057, even though mutilations were rated significantly higher in arousal by women (M = 7.08) than men (M =6.13), F{\, 85) = 6.19, MS, = On the other hand, the mean arousal ratings for weapons were similar for both males (M = 5.83) and females (M = 5.96). Whereas these differences in memory cannot be attributed to rated arousal, better recall of attractive females is consistent with the significantly higher arousal ratings assigned to these materials by men (M =6.91) in relation to women (M = 3.60), F(l, 85) = 80.21, MS e = For women, mean recall averaged across the categories of household objects, food items, and attractive men was higher than for men, F(\, 85) = 4.61, MS e =.092. The memory bias for household objects and foods was not due to differences in rated arousal, as both men and women produced practically identical arousal ratings for these categories. Again, however, the mean arousal rating for attractive males was higher for women (M = 5.62) than for men (M = 2.99), which satisfactorily accounts for the pattern of recall in this category, F{ 1, 85) = 60.57, MS, = Across the remaining categories of sports, nature scenes, and animals, males and females showed no differences in either proportion recalled or in mean ratings of arousal. Experiment 2 In Experiment 1, the rated arousal level of a slide was clearly related to memory performance at both immediate Table 2 Mean Proportion of Slides Recalled in Each of Nine Semantic Categories for Men, Women, and All Subjects Slide category Mutilated bodies Weapons Female nudes Household objects Food Male nudes Animals Sports events Nature scenes Men Women All subjects and delayed recall. Superior memory performance was consistently obtained for slides rated as highly arousing in comparison with slides that received lower arousal ratings. These data suggest that high arousal facilitates memory performance, at least when memory for the event's occurrence is probed. In Experiment 2, a speeded-recognition task was used to assess memory for these affective materials. To the extent that the arousal effect obtained in Experiment 1 reflects the operation of a general memory mechanism, and not one specific to the free-recall process, similar effects should be obtained in Experiment 2. Based on the facilitatory effects of arousal on free recall performance in Experiment 1, the prediction is that high arousal should aid recognition performance: As rated arousal of the material increases, recognition speed should decrease, indicating faster recognition decisions. In Experiment 1, the affective valence of the slide had a small, marginally significant effect at immediate recall, but no effect on delayed recall. To the extent that the 15-min retention interval used in Experiment 2 is considered a longterm memory test, no effects of affective valence should be seen in the recognition reaction time for slides encoded earlier. The magnitude of the skin conductance response at encoding indexes affective responses, especially arousal, during processing of these pictorial materials. Measurement of these sympathetically mediated responses at encoding allows an additional assessment of emotional engagement during slide processing (i.e., in addition to verbal ratings), and in Experiment 2, these responses were measured during the initial encoding phase. Method Subjects. Sixty-six (33 women and 33 men) University of Florida introductory psychology students participated as part of a class requirement. Subjects were randomly assigned to view one of the two slide series (described below). Gender was balanced across slide series. One female lacking complete data was omitted from thefinalanalyses (final n = 65). Stimulus materials. Two sets of 21 colored photographic slides were chosen from the IAPS (Lang et al., 1988) on the basis of affective (valence and arousal) ratings obtained from independent subject samples. The two stimulus sets were matched for valence and arousal ratings and (as closely as possible) semantic content. Contents of both sets were broadly distributed in the two-dimensional emotional space. One set of 21 slides served as the encoding stimuli for each subject and therefore were the "repeated" slides (correct response = yes) in the recognition phase. The remaining set of 21 slides then served as "new" foils (correct response = no) in the recognition task. The particular set of slides serving as encoding stimuli was counterbalanced across subjects. Apparatus and response measurement. Presentation of stimuli and collection of data were controlled by a Digital Equipment Corporation model 11/23 and an Apple He microcomputer. Slides were projected through a sound-attenuating port into the subject room with a Kodak Ektagraphic HI projector. Slide duration (6 s) was controlled using an electronic shutter (rise/fall time < 5 ms). Valence and arousal ratings were obtained using a computerized version of SAM (Lang, 1980). The SAM display was presented on a video screen located 1.6 m in front of the subject's eyes. Subjects adjusted each of three sequential displays (representing the valence, arousal, and dominance dimensions, respectively) along a 0-29 scale using a potentiometer. The order in which the ratings were displayed was randomly determined on each trial for each subject.

7 PLEASURE AND AROUSAL IN MEMORY 385 Skin conductance was sampled during the encoding phase of the experiment. Skin conductance electrodes were placed adjacently on the hypothenar eminence of the left palmar surface, using Beckman Ag-AgCl standard electrodes filled with K-Y lubricating jelly. The signal was calibrated prior to each session to detect activity in the range from 0 to 40 ps. Electrodermal activity was sampled at 10 Hz for 2 s prior to slide onset, throughout the 6-s slide exposure, and for 2 s following slide offset. A skin conductance response (SCR) was defined as the largest response occurring between 0.9 and 4 s after slide onset; a log transformation of logcx^ + 1) was performed to normalize the data. Detailed description of additional indices of physiological response collected in this experiment are reported in Lang et al. (in press). Encoding procedure. The subject sat in a recliner in a small, sound-attenuated, dimly lit room. The slide image was projected about 1.9 m from the subject's eyes and subtended 20 of visual angle. The subject was told he or she would view a series of slides differing in emotional content. The subject was asked to attend to the slide for the entire exposure time (6 s), to perform SAM ratings after each trial, and to relax during varying interslide intervals (i.e., s). Each subject viewed the slide series in one of three varied orders. After instructing the subject to rest quietly, the experimenter exited the subject room, and the slide sequence was presented. No mention of a memory test was given at this time. Recognition phase. After a 15-min retention interval (during which the subject completed several unrelated questionnaires), instructions for the incidental recognition phase were given. Two game paddles were affixed (using Velcro) on the armrests of the subject's chair, and the subject was instructed that another series of slides would be viewed and memory for each slide assessed. For each recognition trial, a slide was presented on the screen, and the subject was told to decide as quickly as possible whether the slide had been seen earlier in the experiment. If the slide had been seen before (repeated slide), the subject pressed the button associated with the dominant hand. If the subject had not seen the slide previously (new slide), the button associated with the nondominant hand was pressed. Both speed and accuracy were emphasized in the instructions. After making the recognition decision, the subject was allowed to view the slide as long as desired. A simultaneous press on both game paddles terminated the slide presentation. The subject then made an interest rating and an emotion rating (a forced choice selection of one of seven emotions) for each slide (for discussion of these ratings, see Lang et al., in press). Data reduction. Four slides were used in which two identical faces occurred with different facial expressions (e.g., angry face/neutral face). Subsequently, it was determined that these slides were difficult to categorize as new or repeated and resulted in extremely long reaction time responses. These items were therefore excluded from further analyses. Individual subject ratings for affective valence and arousal were obtained for slides seen in the encoding phase only (i.e., repeated slides). To compare new and repeated slides, as well as to compare these data directly to the recall data from Experiment 1, the recognition data were assigned to a 2 x 2 category breakdown formed by factorially combining valence (pleasant or unpleasant) and arousal level (high or low) on the basis of group means. The slides were first rank ordered by the group valence means; the top 9 and bottom 10 slides were designated pleasant and unpleasant slides, respectively. Within each valence category, the slides were ranked by the group arousal means and then divided into high- and low-arousal categories. Results Rating information. The mean arousal ratings for slides assigned to the four affective categories were analyzed to assess the level of arousal across the two valence categories. Because slides were assigned to the low- and high-arousal categories on the basis of these ratings, the significant difference in mean arousal ratings for slides in these categories is expected, F(l, 62) = 315.6, MS t = More important, there were no significant effects involving affective valence in the analysis of arousal ratings, indicating that neither pleasant and unpleasant slides in the low-arousal (M = 11.6 and 10.8, respectively) nor the pleasant and unpleasant high-arousal (M = 20.2 and 19.9, respectively) categories were judged differentially arousing. Reaction time. Only correct trials were used in the analysis of reaction time data. In an overall analysis of variance of these data, gender (male or female) was a between-subjects variable, and slide status (repeated or new), slide affective valence (pleasant or unpleasant), and slide arousal (high or low) were repeated measures. There were no significant effects involving gender in any of these analyses. Figure 4 illustrates the mean reaction time (RT) data for new and repeated slides as a function of the affective valence and arousal level of the slide category. For the RT data, there was a marginally significant main effect of slide status, F(l, 58) = 3.83, p =.06, MS, = 596,132, indicating the mean RT for repeated slides (yes responses) was slightly faster than for new slides (no responses). This effect could arise because these responses were executed with the subject's dominant hand or, more probably, due to savings in encoding time (i.e., these slides were repeated). More important, significant interaction effects indicated that the valence and arousal level of the slide had different effects on RT, depending on whether the slide was repeated or new. A significant interaction between slide status and arousal level, F(l, 58) = 31.03, MS e = 64,958, indicated that slide arousal had opposing effects on recognition speed: For slides that were seen before, RT was facilitated by high arousal, F(l, 58) = 9.72, MS C = 58,215, whereas for new slides, RT was delayed by high arousal, F(\, 58) = 22.01, MS e = 71,625. Similarly, a significant main effect for slide valence, F(\, 58) = 6.01, MS t = 95,550, was modified by a significant Repeated Slides "yes' response AROUSAL Unpleasant Pleasant New Slides no' responses AROUSAL Figure 4. Mean reaction times for recognizing repeated slides (left panel) or new slides (right panel) as a function of slide pleasantness and arousal in a speeded-recognition task.

8 386 BRADLEY, GREENWALD, PETRY, AND LANG interaction wiih slide status, F(\, 58) = 4.00, MS e = 116,158. Unpleasant slides resulted in longer RT than pleasant slides, but only when the slide was new, F(\, 58) = 6.90, MS e = 150,122. If the slide had been previously presented in the experiment (during the encoding phase), the valence of the slide had no effect on retrieval speed, F(l, 58) < 1. Accuracy. The overall error rate in this experiment was low (.04). However, in an analysis of the mean errors made for each of the slide categories, a main effect for gender, F( 1, 58) = 4.08, MS C =.01, indicated that women were slightly more error-prone than men (mean error rates of.05 and.03, respectively). A significant main effect of arousal, F{\, 58) = 5.81, MS; =.009, indicated that slightly, but significantly, more errors were made on slides categorized as low-arousal (M =.05) in relation to high-arousal slides (.03). The arousal variable did not interact with slide status, F(l, 58) < 1, which indicates that high arousal was associated with fewer recognition errors, regardless of slide status. Because high arousal had differential effects on reaction time as a function of whether a slide was new or repeated, the lack of an interaction in error rate suggests that a trade-off of speed with accuracy is unlikely for the effects of arousal obtained here. Encoding and skin conductance responses. Skin conductance responses were measured during the initial presentation of each slide in the encoding phase of the experiment. Figure 5 illustrates the mean skin conductance responses at encoding as a function of the breakdown of the slides into valence and arousal categories. As found in the past, high-arousal slides led to significantly greater magnitude skin conductance responses than did low-arousal slides, F( 1,64) = 80.37, AfS e =.02. In addition, unpleasant slides prompted slightly greater skin conductance responses at encoding than did pleasant slides, F(l, 64) = 3.5, MS, =.02, p =.06. As Figure 5 illustrates, there was no interaction between these variables. Therefore, skin conductance responses at encoding tended to show the same pattern as was obtained for reaction times of new, encoded slides (in the context of the recognition task): Both the affective valence and arousal of the slide were apparent in responses during encoding. General Discussion If emotional events are considered to be organized by the dimensions of affective valence (pleasantness) and arousal, the data obtained here demonstrate that memory retrieval is most sensitive to differences in the arousal level of previously encoded emotional stimuli. In both immediate and delayed free recall, high-arousal materials led to better memory performance than materials rated lower in arousal (Experiment 1). In a recognition task, high arousal facilitated recognition decisions for slides encoded earlier in the experiment (Experiment 2). Thus, although many factors affect memory performance, the dimension of arousal (as indexed by verbal ratings and electrodermal responses) accounts here for substantial variability in remembering emotional stimuli. Arousal and Memory Paradigms using verbal items as the to-be-remembered material have also demonstrated facilitator/ effects of stimu- o.i Skin conductance at encoding Figure 5. Mean skin conductance responses during encoding for slides categorized by pleasantness and arousal level. lus arousal on memory performance (Craik & Blankstein, 1975). For instance, Maltzman, Kantor, and Langdon (1966) found greater recall for high-arousal words, in comparison with low-arousal words, on both an immediate memory test and a delayed test administered 30 min later. Importantly, the retention interval in their study was a between-subjects variable, indicating better long-term memory performance for high-arousal items, in the absence of an earlier memory test. Of the studies reviewed by Craik and Blankstein (1975), however, none covary pleasantness and arousal as in the present set of experiments, and none assess memory for affective pictures. The data obtained here suggest their conclusions extend to these materials and, furthermore, that affective valence is not a controlling factor. A number of studies in the past have only found a positive effect of high arousal on long-term memory tests; on immediate tests, poorer memory performance was obtained (Craik & Blankstein, 1975). In those experiments, arousal level has often been defined by the magnitude of the subject's electrodermal response at encoding, rather than by stimulus arousal. Walker's (1958) action decrement theory accounted for this pattern by suggesting that highly arousing events lead to a longer consolidation process in memory, which, in turn, promotes better long-term memory. However, during consolidation, retrieval is inhibited, which produces poor short-term memory results. In an additional analysis here, we assessed the reaction time data in Experiment 2 as a function of the magnitude of electrodermal reactivity during slide encoding. The same conclusion was supported: High levels of arousal, as measured by skin conductance responses, speeded later recognition decisions. These data support Walker's account, if one characterizes the 15-min retention interval used here as long-term. On the other hand, there is no evidence in any of our data that high arousal impairs memory performance, even on memory tests that occur relatively soon after the encoding episode. More recent studies have also concluded that high arousal can impair, rather than facilitate, memory. The differences are primarily paradigmatic, and a close look suggests past research is not inconsistent with the present data. For instance, Christianson and his associates (Christianson & Nilsson, 1984; Christianson, 1986; Christianson, Nilsson, Mjorndal, Penis, & Tjellden, 1986) utilize a methodology in which slides depicting traumatic or erotic events are accompanied by

9 PLEASURE AND AROUSAL IN MEMORY 387 verbal information related to the pictured event. Memory performance for this peripheral verbal information is typically poorer when the visual context is highly arousing than when it is neutral. Similarly, Loftus and Burns (1982) presented, via a series of slides, a situation designed to be traumatic (i.e., a small boy gets shot) or nontraumatic (i.e., the boy crosses a parking lot unharmed). Recognition memory for a peripheral visual detail (a number on the boy's jersey) produced poorer performance in the traumatic version. Similar data were obtained by Clifford and Hollin (1981) in a test of detailed identification of the perpetrator of a violent crime. Detterman (1975) and his colleagues (Detterman & Ellis, 1972) found poor memory performance for unrelated words that followed or preceded presentation of a single arousing stimulus (e.g., a loud shout or a photograph of a nude).' Across all of these studies, memory for specific and mainly peripheral detail was assessed, and it is in this situation that high arousal produced poor memory performance. When probing memory for the occurrence of the primary event itself, however, Christianson and Loftus (1987) demonstrated that traumatic events are recalled better than neutral ones. In one experiment, subjects were shown a film of a bank robbery in which the ending was violent (i.e., two men were shot) or nonviolent. Six months later, subjects shown the traumatic version were more likely to remember the thematic content of the film. These data are consistent with those presented here, although the effects of an event opposite in affective valence (e.g., pleasant) were not explored in that study. The current finding that pleasantness of the event does not strongly affect memory performance leads us to suggest that the memory advantage found for traumatic events may be primarily due to the high-arousal level, rather than the unpleasantness, of the traumatic episode. More important, one implication from our data is that the better memory found for information central to an arousing event may be related to the physiological reaction engendered during encoding of these types of stimuli. Components of the event that are associated with an emotional reaction (as indexed by skin conductance responses or other measures of responsivity) may be better remembered than features lacking this reactive property. This hypothesis can be tested by measuring physiological and behavioral responses to component elements of the slide stimuli used here. Whatever the outcome of such an investigation, the focus is clearly directed toward intense affective stimuli, regardless of their affective valence. Pleasantness and Memory Interestingly, folklore encourages the view that there is a strong relation between affective valence and memory performance. For instance, the adage that one "views the past through rose-colored glasses" attributes a central role to pleasantness by proposing that memory favors pleasant events. Similarly (although the emphasis was reversed), Sigmund Freud's views regarding repression were singularly tied to degraded memory for events that were negative in affect. Conversely, the lore underlying flashbulb memory pinpoints traumatic events as those that are remembered very precisely and for a long period of time. In the experimental literature, paradigms have tended to assess memory either for traumatic events (e.g., Christianson & Loftus, 1987; Christianson & Nilsson, 1984; Brown & Kulik, 1977; Bohannon, 1988) or for pleasant events (e.g., Christianson, 1986) often compared to neutral but seldom for both types of materials in the same experimental context. This is unfortunate, because differences in memory performance are difficult to interpret when the neutral comparison stimulus differs on both the valence and arousal dimensions. In the present study, the level of arousal was controlled across different levels of affective valence. In this case, the only effect of pleasantness on memory retrieval was at immediate free recall, and this effect was only marginally significant. Pleasantness of the stimuli had no impact either on delayed free recall or in recognizing items encoded earlier in the experiment. A similar null effect of pleasantness on memory performance was recently obtained by Thompson (1985), who found no effect of pleasantness when assessing memory for naturally occurring personal events. In addition, Bradley and Baddeley (1990) found no difference between pleasant and unpleasant words, equated for concreteness and frequency, at immediate or delayed recall. Finally, there was no indication in our data that negative, highly arousing events were remembered differently (i.e., better) than other types of events, as flashbulb theories might predict. For slides never seen before, affective valence did have a significant impact on reaction time: Unpleasant slides resulted in significantly longer decision times than pleasant slides. Similar effects of valence and arousal were obtained for the skin conductance data: Both high-arousal slides and unpleasant slides prompted greater reactions at encoding. However, affective judgments of arousal did not differ for pleasant and unpleasant materials. There are two interesting ramifications of these data. First, they indicate that the dimensions of valence and arousal are both salient at encoding. This is consistent with earlier data indicating dimensional discrimination in reactivity to these slide materials (Greenwald et al, 1989). Second, they suggest that the judged level of arousal is probably not solely determined by electrodermal reactivity at initial encoding. As mentioned earlier, both the amount of free viewing time and ratings of interest also relate to the arousal factor, attesting to other influences underlying this affective dimension. It is likely that judgments of arousal act as a summary statistic, incorporating elements in addition to that engendered by the discrete sympathetic response (measured as skin conductance change) at encoding. The arousal-related effects on memory ' This raises a question concerning the effect of one slide on what followed or preceded it in the current study. The design of this experiment was not intended to specifically assess this issue; in fact, different slide orders were used to counterbalance effects due to slide sequence. To the extent this issue could be assessed in post hoc analyses, however, no additional mnemonic effects due to sequencing were obtained. Concerning physiological responses, other studies (e.g., Greenwald et al., 1989; Lang et al., in press) have demonstrated full recovery within the 10- to 20-s interslide interval used here; in the current study, such recovery is even more likely, given the presence of the postslide SAM rating period.

10 388 BRADLEY, GREENWALD, PETRY, AND LANG performance in our data appear to reflect this broader concept of arousal. Arousal, Pleasantness, and Memory: Some Theoretical Comments As Lang et al. (1990) emphasized, the verbal dimensions of affective valence and arousal can be directly related to the behavioral dimensions of direction and vigor. In this scheme, pleasant events are defined as those that primarily engage approach or appetitive behaviors, whereas unpleasant events are those prompting withdrawal, avoidance, or defensive actions. The arousal dimension is related to the vigor of the behavioral disposition that is currently active and can range from a level of extreme emergency to that of calm expediency. According to this analysis, high intensity signals a motivationally relevant event that can involve either an appetitive or aversive environmental transaction. The data obtained in this experiment (and others as well) suggest that arousal is a complex concept. There have been several attempts to detail, for instance, distinctions between cerebral, behavioral, and sympathetically mediated components (e.g., Lacey, 1967). One can also distinguish between stimulus arousal (as manipulated here) and nonstimulusrelated subject arousal (e.g., by using an injection of adrenalin; Christianson et al., 1986). As discussed above, variables relating to attention, interest, or effort may also be involved. Regardless of its underlying multiplicity, factor analyses consistently confirm the existence of such an organizing dimension in emotion. Furthermore, this dimension seems to be readily apprehended by subjects, who can easily assess the intensity of their affective experience, independent of its valence. The data obtained here clarify its mnemonic effects: Memory for the occurrence of an emotional stimulus associated with high arousal is better than for a stimulus rated low in arousal, regardless of its pleasantness. A memory system sensitive to the arousal level of an event is a broadly functional survival tool. Behaviors that demand high mobilization of resources, regardless of whether these are directed toward approaching a desired object (e.g., a food source when hungry) or fleeing from a feared one (e.g., a predator in one's environment), are good candidates for memory storage. Such intense actions are probably strongly related to survival, in terms of both preservative behaviors (such as eating, drinking, procreating, etc.) and protective ones, and may prove useful in future interactions with the environment. On the other hand, events low in arousal may often be unimportant or redundant with respect to successful environmental maneuvers, resulting in low priority for memory storage. The underlying mechanisms relating arousal to memory are potentially multiple. Neurophysiological factors undoubtedly play a role in facilitating memory performance. For instance, Kety (1972) has proposed that processes associated with encoding emotional stimuli affect the circulation of neurochemical substances that increase the efficiency of mnemonic processes. Gold and McGaugh (1975) and their associates (McGaugh et al ) have explicitly tested the hypothesis that increasing an animal's arousal level by electrical or chemical intervention after training facilitates performance. Enhancement in retention has been obtained both for aversive and appetitive training tasks (Steinberg, Isaacs, Gold, & McGaugh, 1985), supporting the idea that memory is facilitated similarly by arousal, regardless of affective valence. On the basis of such data, Gold and McGaugh (1975) proposed a memory system in which motivationally relevant events (i.e., those that are highly arousing) produce endogenous physiological consequences that can facilitate later memory performance. From a cognitive viewpoint, elaboration may be a sufficient explanatory device. Slides rated as highly arousing (i.e., intensely emotional) produce large skin conductance responses, appropriate facial electromyographic responses, increased cardiovascular activity, and ratings of heightened interest and attention, in relation to low-arousal slides (Greenwald et al., 1989; Bradley et al., 1990). If one attributes a similar informational role to physiological response information as is typically attributed to other types of information (Lang, 1979, 1984), the episodic network encoding of an arousing picture can be considered more elaborate than one representing a nonarousing slide. Assuming this type of network elaboration increases the probability of accessing a stored representation, both the higher free recall and faster recognition performance obtained here could result. If elaboration (involving active responses) is responsible for the advantage found in memory for highly arousing materials, one prediction is that neutral materials exposed to such processing (e.g., by inducing physiological or behavioral responding) could be equated, in terms of memory strength, to emotional stimuli. On the other hand, high emotional arousal engendered by sympathetic activation (aversive or appetitive in orientation) may be singularly different from response elaboration of nonaffective materials. If this were the case, neutral stimuli should tend to be consistently inferior in terms of memory performance. Clearly, these alternative hypotheses can be evaluated. To the extent that the elaborated information represents bodily responses to arousing content, this proposed processing mechanism is also consistent with evidence that concurrently presented, peripheral stimulus information does not appear to be well-represented in episodic memories of arousing events. Activation of emotional responses to arousing stimuli may interfere with deployment of other behaviors necessary for encoding peripheral detail (e.g., in this study, scanning the visual display). If this were the case, high reactivity to an arousing stimulus would produce an elaborated representation (and good memory performance) for elements associated with the arousal response and poor memory for information relying on other types of behavioral operations at encoding. Future investigations that probe memory for either details or central components of the arousing stimuli used here can easily be implemented. The primary conclusion from the current study is an empirical one: When emotion is defined by the dimensions of pleasantness and arousal, the major predictor of future memory performance is not the valence of the specific emotion but its intensity.

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12 IAPS number ! Appendix List of Slides, Affective Ratings, and Free Recall for All Stimuli Used in Experiment 1 Description Attacking snake Black cobra Spider Horse Rabbit Coyote Cow Angry woman Man with baby Neutral face Old man Old woman Mutilated face Burn victim Mutilated body Bloody finger Surgery Romantic couple Couple in bed Mountains River Beach at sunset Man with gun Aimed pistol Bomber aircraft Missile range Rolling pin Hair dryer Trash can Umbrella Light bulb Turkey dinner Strawberry pie Chocolate soda Wine Urban building City landscape Ski jump Sailing Gymnast Tennis player Pole vaulter Cemetery Barbed wire Starving child Plane crash Solemn boy Electric wires Auto exhaust Dirty puddle Animal carcass Soldier Dead seal African woman Erotic Stimuli Woman in swimsuit Nude woman Woman on beach Nude man Man in swimsuit Man in bed Mean Males Mean pleasantness rating pleasantness rating Females Note. IAPS = International Affective Picture System, Lang et al., Males Mean arousal rating Mean arousal rating Females Males Proportion of Ss recalling slide Recall I-emales Received December 3, 1990 Revision received June 28, 1991 Accepted August 23, 1991