Motiv Emot (2009) 33:353–361 DOI 10.1007/s11031-009-9144-2
ORIGINAL PAPER
An increase in stimulus arousal has differential effects on the processing speed of pleasant and unpleasant stimuli Helena M. Purkis Æ Ottmar V. Lipp Æ Mark S. Edwards Æ Rebecca Barnes
Published online: 25 September 2009 Ó Springer Science+Business Media, LLC 2009
Abstract The arousal value of a stimulus influences its salience, whereby higher arousal should lead to faster processing. However, in previous research, participants consistently made faster valence judgments for low arousal, pleasant stimuli than for high arousal, pleasant stimuli. The speed of valence and arousal judgments for pictures and words were investigated in three experiments. Valence judgments were faster for low arousal than for high arousal pleasant pictures and for high arousal than for low arousal unpleasant pictures and words. Moreover, arousal judgments were faster for low arousal than high arousal pleasant and for high arousal than low arousal unpleasant pictures and words. The current research confirms that the impact of valence and arousal on processing speed does not reflect on the labels (valence versus arousal) used when recording speeded judgments. Similarly to valence, stimulus arousal interacts differentially with the evaluation of pleasant and unpleasant stimuli producing a processing advantage for high arousal, unpleasant stimuli but not high arousal, pleasant stimuli. Keywords Emotion Valence Arousal Stimulus evaluation
H. M. Purkis (&) O. V. Lipp (&) M. S. Edwards R. Barnes School of Psychology, The University of Queensland, Brisbane, QLD 4072, Australia e-mail:
[email protected] O. V. Lipp e-mail:
[email protected]
Introduction Emotions are integral to human experience and were the subject of some of the earliest studies in psychology (James 1884). Recent developments include objective methods of emotion induction (Bradley et al. 2001) and assessment (for reviews see Fazio and Olson 2003; Lane and Nadel 2000), and a growing number of theoretical accounts, such as dimensional approaches to the understanding of emotion (for reviews see special section on the structure of emotion in Journal of Personality and Social Psychology, 1999; Lang et al. 1997; Russell 2003). Although there is some disagreement as to how to best represent emotional experiences, it is generally accepted that valence and arousal form two of the primary dimensions of emotion. Motivational theories of emotion (Bradley et al. 2001) consider that positive and negative responses are independent and are driven by two underlying motivations; defensive and appetitive. Whereas the valence of a stimulus determines whether appetitive or defensive motivation is engaged, the arousal of a stimulus determines the extent of that motivation; a highly arousing, unpleasant stimulus activates a defensive motivation more strongly than a less arousing, unpleasant stimulus. This account is supported in psychophysiological studies involving startle eye-blink as an index of stimulus valence. Blink reflexes elicited during picture viewing display the typical affect-startle effect pattern, a linear increase in blink startle magnitude with increasing picture unpleasantness, but only if pictures are rated as high in arousal (Cuthbert et al. 1996). Such findings suggest a gating function for stimulus arousal which determines whether stimulus valence will affect reflex modulation. Robinson (1998) suggests that stimulus valence and arousal are processed in parallel. Valence detection is an ongoing, low level process that directs attention to
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information of potential significance to an individual. Arousal detection encodes the ‘urgency’ of stimuli in the environment and is primarily concerned with directing attention to potential sources of threat. The arousal value of a stimulus, but not its valence, will lead to motor preparation following preattentive processing. Thus arousal is proposed to facilitate the processing of stimuli, such that a faster response can be made to stimuli that are highly arousing. An interaction between stimulus valence and arousal in stimulus processing was suggested in a study by Bradley and Lang (1999a), who requested that participants evaluate a set of 96 pictures as pleasant or unpleasant as quickly as possible in a two choice response task. As could be expected, participants were faster to evaluate high arousal than low arousal unpleasant pictures. However, the study also found that participants were slower to evaluate high arousal than low arousal, pleasant pictures. This pattern of results was investigated further across seven experiments by Robinson et al. (2004) who looked at the interaction of valence and arousal on direct and indirect measures of evaluative processing. When participants were asked to evaluate pictures as pleasant or unpleasant, evaluation latencies were shortest for unpleasant, high arousal, and for pleasant, low arousal stimuli. The findings reported by Bradley and Lang (1999a), Robinson et al. (2004) suggest that whereas arousal may confer a processing advantage for unpleasant stimuli, the reverse is true for pleasant stimuli. Robinson et al. (2004) proposed that this reflects the automatic recruitment of attention by arousing stimuli and that, regardless of valence, highly arousing stimuli will activate a defensive response initially. The model proposed by Robinson et al. (2004) provides an explanation for the slow evaluation of pleasant stimuli that are high in arousal. In early processing, high arousal, pleasant stimuli are tagged as negative by the arousal module, but as positive by the valence module and this conflict slows evaluation of these stimuli. In previous studies where participants were asked to make valence judgments, this conflict could be linked to the requirement to label high arousal, pleasant stimuli as ‘pleasant’. The extent to which the effect relies on or is enhanced by the requirement to process valence explicitly when making a response has not been independently investigated. However, several previous studies that required a response on an incidental task, performing a movement, instead of a valence judgment also observed faster responses to low arousal than high arousal pleasant stimuli (Robinson et al. 2004). Surprisingly, no studies have required explicit judgments of arousal, though this is the factor proposed to conflict with valence in producing the response pattern. The current studies sought to replicate and extend previous work by examining speeded judgments of stimulus arousal; high or low, in addition to the valence judgments
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used in the previous studies. This measure was employed to explicitly test whether a similar response conflict occurred when labeling stimuli for arousal rather than valence. We included pictorial and word stimuli to examine whether the pattern of results would extend to semantic stimuli and is independent of low level perceptual features such as color or level of detail depicted. The first experiment required participants to make judgments of stimulus valence for picture and word stimuli. The second experiment presented the same picture and word stimuli, and required participants to judge stimulus arousal. The third study required both valence and arousal judgments for a new set of pictorial stimuli that was selected to be more distinct on the dimensions of valence and arousal than is the first one. It was predicted that valence judgments would be fastest for low arousal, pleasant and high arousal unpleasant stimuli, in line with previous findings. A similar pattern was expected for arousal judgments if a valence-arousal conflict occurred automatically when processing the stimuli. Alternatively, it was considered that arousal judgments may be fastest for high arousal stimuli regardless of valence, in line with motivational theories.
Experiment 1 Method Participants Seventy-four first year psychology students (56 female, 18 male) who were native speakers of English provided informed consent and volunteered to participate in the study in exchange for course credit. Their mean age was 19.7 years (range 17–45). A further three participants declined to complete the experiment. All experimental procedures had received clearance for the University of Queensland ethics review board. Materials and apparatus The 96 pictures1 used in the valence judgment task were the same as those used in previous research (Bradley and 1
The IAPS numbers of the 96 pictures used in Experiments 1–2 were: low arousal, pleasant: 1540, 1750, 1910, 1920,2070, 2080, 2170, 2360, 2370, 2510, 2530, 2540, 2620, 4100, 4520, 5020, 5250, 5750, 5870, 7280, 7320, 7340, 7550, 8050; high arousal, pleasant: 1560, 1640, 1650, 1710, 4210, 4510, 4660, 4680, 5600, 5830, 7200, 7230, 7260, 7270, 7501,8030, 8040, 8080, 8090, 8130, 8180, 8200, 8490, 8510; low arousal, unpleasant: 1230, 1270, 1280, 2190, 2200, 2230, 2520, 2690, 2700, 2720, 2810, 5970, 6010, 8010, 9000, 9010, 9070, 9080, 9090, 9110, 9180, 9190, 9290, 9440; high arousal, unpleasant: 1120, 1300, 1930, 2800, 3000, 3130, 3150, 3170, 3250, 5940, 6020, 6230, 6800, 6910, 6940, 8230, 8480, 9040, 9050, 9250, 9300, 9400, 9410, 9500.
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Lang 1999a) with one exception; a picture from the original set that showed the New York skyline before the September 11th incident was replaced with a picture depicting a Las Vegas scene that was rated similar to the original on valence and arousal. The pictures were selected from the International Affective Picture System (IAPS, Center for the Study of Emotion and Attention [CSEANIHM] 1999) on the basis of their normative ratings of pleasantness and arousal to form four sets of 24 pictures each. The four sets, low arousal pleasant, high arousal pleasant, low arousal unpleasant, high arousal unpleasant comprised four pictures of animals, eight pictures of objects and twelve pictures of people. The pictures were edited with Paint Shop Pro 6.0 software and stored with a palette of 256 colors (Web save format, error diffusion method) and a size of 300 9 225 pixels. The 96 words2 were chosen on the basis of their normative valence and arousal ratings, as published in the Affective Norms for English Words (Bradley and Lang 1999b). Of the 96 words, half were rated as pleasant and half as unpleasant, and of these, half were considered low and the other half high in arousal. It was ensured that the four categories of 24 words each did not differ significantly in word frequency (F \ 1.2), or word length (F \ 3.2). The words were displayed centered in white capital letters against a black background, written in bold Arial, 28 pts. The experiment was conducted using a 486 compatible PC with a 17’’ color monitor (Dell Trinitron) and controlled by custom written software run under DOS to ensure millisecond timing accuracy. Responses were collected with a button box attached to the games port of the computer. The two buttons were labeled ‘‘pleasant’’ and ‘‘unpleasant’’, had a diameter of 1 cm and were placed 7.5 cm apart (centre to centre). Participants operated the response buttons with the index fingers of the right and left hand. Button labels were constant within participants, but counterbalanced across participants.
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Procedure After entering the laboratory participants were informed that they would be seeing pictures with a wide variety of content, including violence. To demonstrate the nature of the stimuli, participants were shown some examples that resembled the experimental pictures in content, and were presented at the same size, color depth and duration of presentation. The participants were offered the opportunity to withdraw from the experiment if they felt uncomfortable viewing the pictures. Three participants decided not to complete the task as a result. All participants were ensured that they could discontinue the task at any time and signed a consent form. After signing the consent form the participant was seated in a comfortable chair in front of a computer screen at an approximate distance of 70 cm. The experimenter stated that the experiment would consist of several short computer tasks. At the beginning of each valence judgment task, participants were informed that during the task pictures/words would be presented singularly in the centre of the screen. They were asked to evaluate the stimulus’ pleasantness as quickly as possible by pressing one of two buttons, but to avoid mistakes. Prior to the main task, participants were presented with six practice trials. The main task consisted of 96 experimental trials presented without interruption. On each trial, a white fixation cross was presented for 500 ms followed by the presentation of a picture/word. The picture/ word was presented for 5,000 ms or until a button was pressed. The inter-trial interval was two-seconds. The pictures/words were presented in the same pseudo-random order to each participant. The sequence was constrained such that each subset of eight pictures comprised two stimuli from each of the four valence and arousal categories. Allocation of stimuli to subsets was at random as was the stimulus sequence within each subset. Following the completion of the tasks, participants were debriefed, thanked, given credit, and discharged. Data reduction and analyses
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The following 96 ANEW words were used in Experiment 1 and 2: pleasant, low arousal: adorable, angel, carefree, caress, delight, enjoyment, excellence, hug, improve, kindness, luxury, paradise, pillow, rainbow, reward, satisfied, secure, snuggle, spouse, sunrise, sunset, sweetheart, trophy, waterfall; pleasant, high arousal: adventure, aroused, birthday, cash, ecstasy, excitement, fame, fireworks, flirt, graduate, holiday, intimate, joke, kiss, laughter, miracle, orgasm, passion, profit, romantic, sexy, thrill, treasure, triumph; unpleasant, low arousal: corpse, depression, discomfort, funeral, gloom, grief, hardship, headache, illness, infection, lice, lonely, loser, malaria, morgue, paralysis, poverty, rotten, sad, slum, stench, stupid, unhappy, useless; unpleasant, high arousal: abuse, ambulance, assault, betray, bomb, crash, crucify, demon, disloyal, drown, hatred, hostage, killer, mad, murderer, nightmare, rage, rape, roach, slaughter, terrified, thief, tumor, violent.
Following the procedure used by Bradley and Lang (1999a), incorrect evaluations were regarded as errors and evaluation times were set to missing. A participant’s data set was excluded if errors were made on more than 50% of the trials in one of the eight conditions. No data were excluded based on this criterion. The valid evaluation times provided by each participant were individually screened for outliers, defined as values 3 standard deviations above or below the mean. Outliers were classified as errors and replaced with missing values. The evaluation time and error data from Experiment 1 were analyzed in separate 2 9 2 9 2 factorial ANOVAs (Stimulus material 9 Valence 9 Arousal). Multivariate F-values are reported for all main effects and
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Figure 1 (upper panel) shows the time required to evaluate the pictures and words as pleasant or unpleasant. The valence judgment was performed faster for words than for pictures and was affected not only by normative stimulus valence, but also by normative stimulus arousal. As expected, valence judgments were faster for high arousal unpleasant stimuli than for low arousal unpleasant stimuli whereas the inverse was found for pleasant stimuli. The effect of arousal on the speed of valence judgments seemed to be larger for picture than for word stimuli. These impressions were confirmed by the statistical analysis which yielded main effects for Stimulus material, F(1,73) = 76.46, p \ .001, g2P = .51, and Arousal, F(1,73) = 16.52, p \ .001, g2P = .19, as well as Valence 9 Arousal, F(1,73) = 99.05, p \ .001, g2P = .58, and Stimulus material 9 Valence 9 Arousal interactions, F(1,73) = 8.92, p \ .01, g2P = .11. Pair wise comparisons confirmed that low arousal pleasant pictures were evaluated quicker than high arousal pleasant pictures, that high arousal unpleasant pictures were evaluated quicker than low arousal unpleasant pictures, and that high arousal unpleasant words were evaluated quicker than low arousal unpleasant words, smallest t(73) = 5.34, p \ .01. There was no difference in evaluation time between low arousal pleasant and high arousal pleasant words, t(73) \ 2.0, ns. As can be seen in Fig. 1 (lower panel), the pattern of errors closely resembles that of evaluation times with fewer errors made judging low arousal pleasant stimuli than high arousal pleasant stimuli, and in judging high arousal unpleasant stimuli than low arousal unpleasant stimuli. The analysis confirmed these impressions yielding main effects for Stimulus material, F(1,73) = 40.27, p \ .001, g2P = .36, and Arousal, F(1,73) = 11.10, p \ .01, g2P = .13, as well as Stimulus material 9 Arousal, F(1,73) = 5.01, p \ .05, g2P = .06, Valence 9 Arousal, F(1,73) = 94.39, p \ .001, g2P = .56, and Stimulus material 9 Valence 9 Arousal interactions, F(1,73) = 25.06, p \ .001, g2P = .26. Pair wise comparisons revealed that the differences in errors committed between high and low arousal stimuli were significant for pleasant and unpleasant pictures and for pleasant words, smallest t(73) = 3.49, p \ .05. The difference between unpleasant words, t(73) = 2.26, was not significant if corrected t-values were employed. Taken together the pattern of error results indicates that the differences in evaluation times seen in Experiment 1 do not reflect a speed accuracy tradeoff.
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interactions. Pair wise comparisons of means were made with t-tests. The critical values for these t-tests were derived from Sidak’s tables to protect against the accumulation of aerror (Rohlf and Sokal 1981). Level of significance was set at .05 for all statistical analyses.
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Fig. 1 Mean evaluation time (upper panel) and error percentages (lower panel) in the evaluation of stimulus valence as a function of stimulus materials, and normative valence and arousal ratings in Experiment 1 (vertical bars represent standard errors of the means)
Discussion The current results were consistent with the valence and arousal interaction observed in previous studies. Evaluations were faster for high arousal than for low arousal unpleasant stimuli and, for pictures, were faster for low arousal than for high arousal pleasant stimuli. Participants solved the valence judgment tasks with a reasonable accuracy, committing most errors in evaluating high arousal pleasant and low arousal unpleasant pictures, and overall, fewer errors when evaluating words. Participants were faster to evaluate words than pictures by about 90 ms. Failure to find a difference in processing speed for high and low arousal pleasant words may reflect that these stimuli are not as evocative as their pictorial counterparts, or do not adequately capture the high arousal, pleasant dimension. Experiment 2 examined whether the evaluation of stimulus arousal would be faster for low arousal than for high arousal pleasant stimuli. Experiment 2 used the same
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procedure as Experiment 1, but required participants to evaluate the stimuli presented as either arousing or calm.
Experiment 2 Method Participants Forty-eight undergraduate students, 26 female and 22 male, participated in the experiment in exchange for course credits. Participants volunteered and provided informed consent. Participants’ age ranged from 17 to 45 years (mean = 20.4 years). Materials and apparatus, procedure, data reduction and analysis The materials, apparatus, and experimental procedure used for Experiment 2 were the same as for Experiment 1. The main difference to Experiment 1 was that participants were required to judge the extent to which the stimulus materials were arousing or calm, thus the labels attached to the buttons of the button box read ‘Arousing’ and ‘Calm’. Care was taken to instruct participants as to the meaning of the concept and it was emphasized that the arousal dimension applied here was different from sexual arousal or from pleasantness. Data screening performed after collecting data from 20 participants revealed high error rates in the evaluation of stimulus arousal. Thus, the practice task was modified to include the demonstration of example images from each of the four categories. This additional training had no effect on error rates or evaluation times as indicated by a preliminary analysis and the data were combined across the two data sets. The sequence of practice and main tasks was the same as in Experiment 1. Initial inspection of the data revealed that participants made more errors completing the arousal classification task than were made with the valence classification task. In order to limit the loss of data, a set was excluded only if no response times were available for one of the eight cells of the three factorial design. The data from nine participants were rejected based on this criterion. The statistical analyses were conducted as for Experiment 1. Results The response times for judgments of stimulus arousal are presented in the upper panel of Fig. 2. Evaluations of stimulus arousal were faster for high arousal unpleasant stimuli than for low arousal unpleasant stimuli, as expected. Moreover, responses were faster for low arousal pleasant than for high
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arousal pleasant stimuli. The pattern of results was evident for picture and word stimuli. The analysis revealed Stimulus material 9 Arousal, F(1,38) = 12.08, p \ .01, g2P = .24, and Valence 9 Arousal interactions, F(1,38) = 35.49, p \ .001, g2P = .48. The three way interaction was not significant, F(1,38) = 2.24, p = .143, g2P = .06. Pair wise comparisons of the means revealed that high arousal words were evaluated faster than low arousal words, 818 vs. 868 ms, t(38) = 3.06, p \ .05, whereas there was no difference between high and low arousal pictures, 876 vs. 846 ms, t \ 2.0, ns. Low arousal pleasant stimuli were evaluated faster than high arousal pleasant stimuli, 820 vs. 896 ms, t(38) = 3.72, p \ .05, whereas high arousal unpleasant stimuli were evaluated faster than low arousal unpleasant stimuli, 798 vs. 894 ms, t(38) = 4.70, p \ .05. The overall percentage of errors was quite high which attests to the difficulty of the arousal judgment task. As can be seen in Fig. 2 (lower panel), the pattern of results for the errors was similar to that for the response times with fewer errors associated with faster responses. The statistical analysis revealed main effects for Stimulus material, F(1,38) = 19.09, p \ .001, g2P = .33, and Valence, F(1,38) = 6.68, p \ .05, g2P = .15, as well as Stimulus material 9 Valence, F(1,38) = 75.75, p \ .001, g2P = .67, Stimulus material 9 Arousal, F(1,38) = 23.11, p \ .001, g2P = .38, Valence 9 Arousal, F(1,38) = 139.88, p \ .001, g2P = .79, and Stimulus material 9 Valence 9 Arousal interactions, F(1,38) = 15.79, p \ .001, g2P = .29. The three way interaction reflects that more errors were committed in evaluating low and high arousal unpleasant words and low arousal pleasant words than the respective pictures whereas fewer errors were committed in evaluating high arousal pleasant words than high arousal pleasant pictures, smallest t(38) = 2.60, p \ .05. The slower evaluation times in the conditions with the largest error percentages excludes the presence of a speed accuracy tradeoff. Discussion Consistent with the response pattern for evaluations of valence, arousal judgments were faster for high arousal unpleasant and low arousal pleasant stimuli. Participants made more errors evaluating the arousal of high arousal pleasant pictures than high arousal pleasant words, which may confirm the suggestion from Experiment 1 that the high arousal, pleasant words do not capture the high arousal, pleasant dimension as well as do the pictorial stimuli. The evaluation of pictures and words as arousing or calm seemed more difficult, according to the error data, than the valence judgments. However, arousal is proposed to be more survival relevant than valence and to be extracted automatically. It is possible that the stimulus materials were more appropriate for the assessment of
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and 22 males with a mean age of 19.2 years (range 17–41). Data from an additional participant were not included due to incomplete evaluation time in some conditions.
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Fig. 2 Mean evaluation time (upper panel) and error percentages (lower panel) in the evaluation of stimulus arousal as a function of stimulus materials, and normative valence and arousal ratings in Experiment 2 (vertical bars represent standard errors of the means)
stimulus valence than stimulus arousal as this was the original intention when the pictures were selected. A third experiment employed a new picture set, selected to be clearly delineated on the basis of valence and arousal. Experiment 3 was designed to replicate the valence and arousal judgment tasks used in Experiments 1–2 using pictorial stimuli that were more clearly separated on the valence and the arousal dimensions of the IAPS.
Four sets of twelve pictorial stimuli were selected from the IAPS for use in Experiment 3.3 The four sets were selected to be well separated on the dimensions of valence and arousal, and represented a variety of different content. The normative ratings (valence; arousal) were 7.12 (SD = 0.56); 2.90 (0.39) for low arousal pleasant pictures, 7.30 (0.39); 6.61 (0.39) for high arousal pleasant pictures, 3.26 (0.57); 4.02 (0.11) for low arousal unpleasant pictures, and 1.91 (0.42); 7.01 (0.24) for high arousal unpleasant pictures. Pictures were pre processed as in Experiment 1. The general apparatus was the same as in Experiment 1, and Experiment 3 was conducted in the same laboratory as was Experiment 1. The computer used to present the tasks and to record the evaluation times was a Dell OptiPlex GX1 with a 17’’ colour monitor. The timing of stimulus presentations and of stimulus free periods was the same as in Experiment 1. Prior to the start of the arousal judgment task, participants were carefully instructed as to the meaning of the dimension and asked to complete some practice trials. The two evaluation tasks were completed in an order that was counterbalanced across participants. Data reduction and screening for outliers followed the same procedure as employed previously. Error and evaluation time data were analyzed with separate 2 9 2 9 2 (Evaluation 9 Valence 9 Arousal) factorial ANOVAs after initial analyses including the factor Task sequence did not yield results that indicated an effect of this factor on the overall pattern of errors or evaluation times. Results and discussion
Experiment 3
As can be seen in Fig. 3, valence and arousal judgments replicated the pattern of faster evaluations for low arousal pleasant and high arousal unpleasant stimuli. Moreover, the present experiment confirmed that the evaluation of picture valence was faster and easier than the evaluation of picture arousal. The results of the analyses of variance confirmed these impressions. The analysis of the evaluation times yielded main effects for Evaluation, F(1,57) = 14.56,
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Participants Fifty-eight undergraduate students participated and provided informed consent. The sample comprised 36 females
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The following 48 IAPS pictures were used in Experiment 3: low arousal pleasant: low arousal pleasant: 1450, 1610, 2304, 2370, 2501, 5000, 5030, 5720, 5760, 5891, 7470, 7502; high arousal pleasant: 4670, 4687, 5626, 5629, 8030, 8080, 8180, 8185, 8200, 8400, 8470, 8501; high arousal unpleasant: 2730, 3000, 3053, 30713120, 3170, 3400, 6230, 6260, 6350, 9410, 9810; low arousal unpleasant: 2590, 6010, 9000, 9010, 9045, 9090, 9101, 9110, 9220, 9280, 9331, 9390.
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arousal pleasant pictures, 5.5 vs. 20.7%, t(57) = 7.22, p \ .05, and when evaluating high arousal unpleasant than low arousal unpleasant pictures, 8.5 vs. 32.5%, t(57) = 11.38, p \ .05. The purpose of Experiment 3 was to confirm the results obtained in Experiments 1–2 using a stimulus set that was selected to reduce the potential of misclassifications by permitting a better separation on the dimensions of stimulus valence and arousal. Inspection of the error data indicates that the selection of pictures was partially successful in achieving this. The percentage of errors committed when evaluating picture arousal was reduced relative to that seen in Experiment 2 and the error percentage for the evaluation of picture valence was compatible with that observed in Experiment 1. However, the arousal judgment task was still seemingly more difficult than the valence judgment task.
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Fig. 3 Mean evaluation time (upper panel) and error percentages (lower panel) in the evaluation of stimulus valence and arousal as a function of normative valence and arousal ratings in Experiment 3 (vertical bars represent standard errors of the means)
p \ .001, g2P = .20, and Valence, F(1,57) = 70.88, p \ .001, g2P = .54, as well as a Valence 9 Arousal interaction, F(1,57) = 87.09, p \ .001, g2P = .60. The main effect reflects that, on average, evaluations of arousal were slower than evaluations of valence, 887 vs. 780 ms. The Valence 9 Arousal interaction reflects that evaluation of low arousal, pleasant pictures was faster than evaluation of high arousal, pleasant pictures, 740 vs. 857 ms, t(57) = 5.57, p \ .05, whereas evaluation of high arousal unpleasant pictures was faster than evaluation of low arousal, unpleasant pictures, 789 vs. 849 ms, t(57) = 7.63, p \ .05. The analysis of the Error data revealed a similar pattern with main effects for Evaluation, F(1,57) = 9.27, p \ .01, g2P = .14, Valence, F(1,57) = 33.96, p \ .001, g2P = .37, and Arousal, F(1,57) = 11.07, p \ .001, g2P = .16, and a Valence 9 Arousal interaction, F(1,57) = 172.94, 2 p \ .001, gP = .75. Averaged across conditions, participants committed more errors evaluating picture arousal, 14.8 vs. 18.9%. The interaction reflects that fewer errors were committed when evaluating low arousal pleasant than high
The present series of experiments provides clear evidence that evaluations of stimulus valence and stimulus arousal are made faster and more correctly for high arousal unpleasant stimuli than for low arousal unpleasant stimuli, and for low arousal pleasant stimuli than for high arousal pleasant stimuli. This confirms that an increase in stimulus arousal has differential effects on the processing speed of pleasant and unpleasant stimuli, and that this is not a product of the labels (valence versus arousal) used when recording speeded judgments. The pattern emerged for picture and word stimuli. Moreover, participants found it more difficult to perform the arousal judgment than the valence judgment task, which does not support notions that arousal information is extracted prior to valence information. According to motivational theories (Bradley et al. 2001), a high level of arousal more strongly primes responses to pleasant and unpleasant stimuli respectively compared to a low level of arousal. On this basis, it was proposed that faster arousal judgments may be made for high arousal stimuli regardless of valence, but this was not the case. However, the motivational theory allows that responses to pleasant and unpleasant stimuli may be in different directions, and that the difference between them will increase as arousal increases. The gating function of arousal and the dissociation of responses to pleasant and unpleasant stimuli is confirmed in studies of startle modulation, where startle is facilitated during unpleasant and inhibited during pleasant stimuli when stimuli are high in arousal (Cuthbert et al. 1996). Robinson et al. (2004) proposed that high arousal, pleasant stimuli are initially classified as unpleasant and that this results in a slowing of evaluative responses to these stimuli as compared to high arousal, unpleasant
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pictures. The current data confirmed a processing advantage for low arousal pleasant stimuli over high arousal pleasant stimuli. Valence was evaluated more slowly for high arousal, pleasant stimuli as compared to high arousal, unpleasant stimuli, and participants were slower to judge arousal for high arousal, pleasant stimuli compared to low arousal, pleasant stimuli. This suggests that the difference in processing speed occurs regardless of the explicit judgments that are made, and provides some support for the notion that a response-conflict may occur automatically when processing the stimuli. Recent work suggests that some low arousal, pleasant pictures may be processed as quickly as high arousal, unpleasant pictures. Brosch et al. (2008) investigated the time course of attentional modulation to biologically relevant stimuli; fear-inducing (pictures of angry faces) and nurturance-inducing (pictures of baby faces), by recording responses in a dot probe detection task while recording participants’ EEG. The authors reported a similar time course in RTs and EEG modulation to pleasant and unpleasant stimuli (babies versus angry faces). The baby faces were selected by Brosch et al. (2008) as a motivationally relevant stimulus category, with the authors proposing that response preparation for nurturance is highly survival relevant. According to the International Affective Picture System (IAPS, Center for the Study of Emotion and Attention [CSEA-NIHM] 1999), baby faces are low arousal, pleasant stimuli. Likewise, the low arousal, pleasant stimuli used in previous studies, such as babies, children, puppies, and bunnies, may be more nurturance relevant than the high arousal, pleasant stimuli, such as windsurfers, skiers, food, and erotica. Though pictures depicting food and erotica would seem to be quite biologically relevant, perhaps there is a greater potential response cost of not responding immediately to a child, as compared to not responding immediately to food or a reproductive opportunity. It remains to fully investigate whether some pleasant stimuli, such as nurturance relevant stimuli, have similar processing advantages as do threat stimuli. In the current studies, participants made speeded responses for each picture/word. This raises the question as to whether response time is confounded with viewing time. Participants may respond more quickly to remove pictures that they do not wish to view because they are threatening or boring or more slowly to view pleasant content longer. This could provide an alternative explanation for fast responses to high arousal, unpleasant images, and slow responses to high arousal, pleasant images. This explanation is, however, inconsistent with data provided by Lang et al. (1993). In this study, participants viewed IAPS pictures for as long as they wanted, terminating exposure of each picture with a button press. Viewing time correlated with interest and arousal, whereby pictures that were high in arousal were also judged
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to be more interesting and viewed for longer, regardless of valence. The strong correlation between viewing time and rated arousal was unchanged when covarying for rated valence. This suggests that arousal drives preferred viewing time, and thus viewing time cannot account for the current pattern of results. Moreover, differential viewing time seems an unlikely explanation for the pattern of evaluation times observed for word stimuli. Future research could, however, employ a task where viewing time is shorter and independent from response time in order to test this explicitly. It is interesting that participants found it more difficult to perform the arousal judgment than the valence judgment task. Moreover, response times seemed more varied on the arousal judgment task. Arousal was a more difficult dimension to classify on in that participants made more errors, thus the greater variance in arousal judgments may reflect an increased load when performing this task. Arousal judgments may provide an interesting avenue for further investigations of the interaction of valence and arousal. Moreover, a more difficult classification task may reveal a greater distribution of responses. The current pattern of results is consistent with the view that valence and arousal represent interdependent dimensions of emotion, and that both factors contribute to the speed of stimulus processing. An increase in rated arousal has differential effects for the evaluation of pleasant and unpleasant stimuli. Evaluations of valence and arousal were faster and more accurate for low arousal pleasant than for high arousal pleasant stimuli and faster and more accurate for high arousal unpleasant than for low arousal unpleasant stimuli. The differential effect of arousal does not seem to be consistent with a unitary arousal concept as suggested by dimensional concepts of emotion that propose a single dimension of activation. Rather, it suggests that concepts which combine the aspect of intensity with separate dimensions for pleasantness and unpleasantness are better suited to account for the manner in which speeded evaluations of valence and arousal are derived. Moreover, factors such as biological relevance may prove important in further defining the interaction of valence and arousal on processing speed. Acknowledgments This work was supported by Grant DP00770844 from the Australian Research Council. Thanks are due to Susann Wolff for assistance with data collection in Experiment 1 and to Paul Jackson who programmed the tasks.
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