Memory Enhancing Effects of Emotional Arousal are Automatic: A Divided Attention Study

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1 Memory Enhancing Effects of Emotional Arousal are Automatic: A Divided Attention Study By Robert Manhas Abstract Many studies have found memory enhancing effects of both emotional arousal and emotional valence. It was hypothesized that the arousal effect is automatic and independent of attentional resources, while the valence effect is a product of categorization, and thus dependent on conscious control and attentional resources. It was further hypothesized that these memory benefits can come via an encoding process, as opposed to solely by a postencoding consolidation process. Twenty-one participants viewed a series of ten word lists. Each word list contained 8 target words and 8 filler words and had a different type of target word associated with it (taboo, categorized negative, uncategorized negative, categorized neutral, or uncategorized neutral). Filler words were always uncategorized neutral words. For each list type, one list was presented in a full attention (FA) condition while the other list was presented in a divided attention (DA) condition. After viewing each list, participants engaged in free recall. Memory enhancing effects were found for arousal and categorization in the FA condition, but only the effect from arousal persisted in the DA condition, consistent with predictions. This study importantly controlled for semantic relatedness between word lists, as well as ensured time between encoding and recall was brief enough that any observed memory effects were due to an encoding process. Introduction Psychologists analyze emotions along two dimensions: valence, the degree of attraction (positive valence) or aversion (negative valence) one feels towards a stimulus, and arousal, the degree to which one is excited or agitated by a stimulus. Phenomenologically, it seems as though emotionally salient stimuli are better remembered than nonemotional stimuli. This makes evolutionary sense, as emotionally salient material (especially when threatening or otherwise fear-inducing) is often relevant for survival. This idea that emotional memory is better than ordinary memory has been supported by a number of studies (for an overview, see Kensinger 2009). This study is particularly concerned with memory enhancement for arousing material. Activation of the amygdala seems to be at the heart of the memory-enhancing effects of arousing stimuli (for an overview, see McGaugh 2004), but amygdala activity itself is only 1

2 involved in emotional responding and processing, not in causing the subjective experience of emotion (Anderson and Phelps 2001; Critchely et al., 2000). This memory enhancement is still crucially related to emotion, however, because the amygdala subserves emotional responses, and its activation is dependent on the perceived stimulus being emotionally salient. Many studies, this one included, exploit this fact by inducing emotional memory enhancement via presentation of arousing stimuli (such as taboo words like cunt that have negative valence and high arousal) which trigger activation of the amygdala. Additionally, it has been found that stimuli such as taboo words and negative words (negative valence, low arousal words like grief ) increase sympathetic nervous system activity and produce other physiological effects consistent with experiencing an emotion, such as increased skin conductance (LaBar and Phelps, 1998). As mentioned earlier, emotional arousal and the amygdala are intimately tied together. When working with rats, for example, creating an arousal response will improve performance on declarative, hippocampal-dependent memory tasks, and furthermore, this improvement is nonexistent in rats whose amygdala is lesioned (Roozendaal et al. 1999). The task itself does not even have to be emotional in nature. Teather et al., (1998) administered drugs that mimicked an arousal response to rats shortly after they learned a hidden platform version of the Morris water maze. Retention on the task was much better for these rats compared to rats that underwent the same training without the arousal response. These findings are consistent with studies involving humans: numerous studies have found that a mild arousal response can enhance declarative memory for emotional events (for an overview, see Christianson, 1992). In one study, two groups of participants were told a story of a young boy who was walking with his mother to see his father s workplace. One group was told a neutral, mundane version of this 2

3 story, while the other group was a told an arousing version of this story (in which the boy suffered severe injury that resulted from a terrible traffic accident and which included graphic pictures of the aftermath). Afterwards, both groups took a multiple-choice recognition memory test, and those who heard the arousing version performed much better than those who heard the emotionally neutral version (Cahill et al., 1994). As was the case with rats, this effect is blocked in humans with amygdala damage (Cahill et al., 1995). Also, unlike normal control subjects who show less forgetting over time for arousing compared to nonarousing stimuli, patients with amygdala lesions forget arousing and nonarousing stimuli at the same rate (LaBar and Phelps, 1998). That the amygdala underlies the improved memory effect for arousing stimuli suggests that it is automatic. This is precisely the hypothesis posited by MacKay et al., (2004), who presented participants with a Stroop task that contained both neutral and taboo words as basewords and then had a surprise memory test for the font color of certain target words. The response time (RT) for font color naming took longer for taboo as opposed to neutral words during the Stroop task, but there was an improved memory effect for the color of taboo words compared to neutral words during the surprise memory task. MacKay and his colleagues posited binding theory to explain these results, which asserts that attentional resources are allocated to arousing stimuli (in this case, taboo words) via an involuntary, automatic process. Taboo words were bound strongly to memory because of this process, allowing participants to better recognize contextual details (in this case, font color) after encoding. Although it improved performance in this manner, the priority processing via binding mechanisms delayed the specific process of color naming for taboo words. 3

4 Notice that the other studies cited thus far have often been chalked up as evidence that arousal and the amygdala activation associated with it improves memory for high arousal material by way of improving memory consolidation, the post-encoding process by which the memories become more permanent and less resistant to loss. This is why, for example, the drugs that mimicked arousal in the rats after they learned the task led to an improvement in memory for the task. MacKay and colleagues, on the other hand, have provided evidence that this improved memory effect can also result from an encoding process. This is because, in that study, font color naming was simultaneous with encoding of the words. If a post-encoding process were responsible for the memory effect, one would not expect to see slower font color naming RTs for taboo words, contrary to what was observed. Further evidence that memory enhancement from arousal is automatic from Kensinger and Corkin (2004), who found distinct neural substrates for the encoding of negative, high arousal words versus negative, low/medium arousal words. High arousal words were found to activate the amygdalar-hippocampal network, while all other types of words were found to activate the PFC-hippocampal network. The amount of activation in these areas was correlated with performance on a subsequent memory task. This suggests that high arousal stimuli are automatically treated differently from low/medium arousal stimuli in the brain during encoding, providing a neural framework for the cognitive process posited by binding theory. Furthermore, the fact that the amygdala is involved during encoding suggests an automaticity to this process, which is in line with many proposals (for an overview, see Dolan and Vuilleumier, 2003; also Pessoa, Kastner, and Underleider, 2003 and MacKay et. al, 2004) and is supported by the behavioral component of the study. Participants were asked to study words with full attention on the task or with divided attention (attention was split between studying the words and 4

5 discriminating between different rhythms played simultaneously), and were told that their memory would be tested after encoding. Following the initial presentation, the target words and distracters were presented one at a time. Participants indicated whether they vividly remembered the word among those presented at encoding (i.e., remembered something specific about the item's presentation, such as a thought they had when viewing the word), knew the word was among those presented at encoding (i.e., sensed that the word was familiar and thus thought it had been presented at study but did not remember any details about its prior presentation), or believed the word has not been presented at encoding. Participants were found to have the best memory for negative, high arousal items, then negative, low arousal items, and finally neutral, low arousal items. These findings are consistent with those of a similarly designed previous study, Kensinger and Corkin (2003), who found that the vividness with which the arousing words were remembered was better than that for negative, nonarousing and neutral, nonarousing words. Importantly, however, Kensinger and Corkin (2004) also showed that the improved memory effect present for negative, arousing items in the full attention trials carried over to the divided attention trials, but this didn t hold for the improved memory effect for negative, nonarousing items found in the full attention trials. This is consistent with the idea that memory for arousing material is automatic, i.e., independent of attentional resources. The vividness of memory for nonarousing words decreased with divided attention compared to full attention trials, suggesting that memory for nonarousing stimuli is dependent on attention-demanding controlled and elaborative processing, consistent with the finding that such stimuli activated the PFC. 5

6 It is worth reiterating that although the negative, low arousal items were better remembered than then neutral items in the full attention task, this memory enhancement disappeared in the divided attention trials. If valence does enhance memory (and, looking at the full attention trials, it does seem to), this boost is a separate effect from the memory enhancement for arousing items (which does not disappear in the divided attention trails). This suggests that different mechanisms (both neural and cognitive) underlie the encoding of high arousal versus low arousal stimuli. The present experiment was designed to investigate the hypothesis that arousalinduced memory enhancement is an automatic process independent of available attentional resources. Unfortunately, Kensinger and Corkin s (2004) results may be seen as problematic because they did not control for the semantic relatedness between the different types of words tested. Talmi and Moscovitch (2004) argue that enhanced memory for emotional stimuli might be the result of a higher semantic relatedness among the high arousal words compared to neutral words. They found that categorized neutral words were recalled just as well as emotional (negative valence, high arousal words that were not taboo) words. To deal with this objection and further explore the relations between valence, arousal, categorization, and attention, this experiment employed five different word list categories: taboo (negative valence, high arousal), categorized negative (negative valence, low arousal), uncategorized negative (negative valence, low arousal), categorized neutral (neutral valence, low arousal), and uncategorized neutral (neutral valence, low arousal). Encoding for these word lists were done under both full and divided attention. All words were matched in terms of number of letters, number of syllables, and written word frequency. Furthermore, all categorized word lists were matched in terms of semantic relatedness, which was established by extensive pilot testing. 6

7 Controlling for all these variables allowed for the effects of valence, arousal, categorization, and attention to be clearly analyzed. As noted earlier, both Kensinger and Corkin (2003) and (2004) presented participants with words after encoding and asked them to report whether they remembered if the words were in the encoding phase, knew if the words were in the encoding phase, or knew if the words weren t in the encoding phase. In contrast to this recognition test, after the encoding of each list, this experiment had participants engage in free recall. Also, the word lists in the Kensinger and Corkin studies were quite long (upwards of 70 words for each list) and Kensinger and Corkin (2004) used a delay of 10 minutes between encoding and recognition tasks, leaving open the possibility that consolidation caused memory enhancement rather than an encoding process. This experiment, on the other hand, avoided this possibility because word lists were quite short (16 words per list, 8 targets and 8 fillers each) and recall immediately followed presentation of each list. It was hypothesized that there would be an enhanced memory effect for the taboo word lists (i.e., those lists with high arousal words) and categorized word lists (i.e., those lists with a high semantic relatedness between words) in the full attention conditions. The benefit from valence that has been seen in many studies was hypothesized to collapse into the benefit of categorization. In line with the thought that such memory enhancements can result from an encoding, as opposed to consolidation, process, these boosts were predicted even though recall proceeded immediately after encoding. Since the benefit of categorization comes from attentional mechanisms, it was predicted in the divided attention condition, when attention is taxed, performance on memory for these types of word lists would decline dramatically. The benefit from arousal, however, comes from automatic mechanisms independent of attentional 7

8 resources, and thus it was predicted that performance for the taboo word lists would remain the same across the full and divided attention conditions. Method Participants The participants comprised 21 young adults (10 men, 11 women; mean age = 18.86). All participants were native English speaking undergraduates and they were paid or given class credit for participation. All participants gave informed consent. Apparatus and Materials The experiment was designed and run on PsyScope X on a Macintosh (imac) computer with speakers. The word stimuli were taboo words (words with high emotional arousal and negative emotional valence; e.g., bastard, fuck ), categorized negative words (words with low emotional arousal and negative emotional valence that which associated with the overarching semantic category of either death or pain ; e.g., mortuary, tomb ), uncategorized negative words (words with low emotional arousal and negative emotional valence that were associated with no overarching semantic category; e.g., loneliness, cancer ), categorized neutral words (words neutral in emotional arousal and valence that were associated the semantic category of types of containers ; e.g., trunk, box ), and uncategorized neutral words (words neutral in emotional arousal and valence that were associated with no semantic category; e.g., casino, hawk ). There were two lists of each type, or 10 word lists total, so that word list type could appear in both full attention and divided 8

9 attention trials for each participant. The target words for each list were matched as closely as possible on frequency of occurrence, number of syllables, number of letters, and semantic relatedness, as shown in Table 1. Semantic relatedness ratings were gathered during pilot testing. 30 participants (15 younger adults and 15 older adults) saw all possible word pairings within each category. They then provided a rating, from one (not at all related) to seven (very related), based on how closely they thought the word words were related. The word frequency is in terms of the Kucera-Francis written frequency for each word (from the MRC Psycholinguistic Database). Word Frequency # of Syllables # of letters Semantic Relatedness Taboo Taboo Cat. Negative 1 Cat. Negative 2 Uncat. Negative 1 Uncat. Negative Cat. Neutral Cat. Neutral Uncat. Neutral 1 Uncat. Neutral Table 1. Word Averages (Targets) Each word list contained 8 target words and 8 filler words. The targets were either taboo, categorized negative, uncategorized negative, categorized neutral words, or uncategorized neutral words depending on the list type, while the fillers were all uncategorized 9

10 neutral words (thus the taboo word lists had 8 target, taboo words and 8 filler, uncategorized neutral words; the categorized negative word lists had 8 target, categorized negative words and 8 filler, uncategorized neutral words; the uncategorized negative word lists had 8 target, uncategorized negative words and 8 filler, uncategorized neutral words; the categorized neutral word lists had 8 target, categorized neutral words and 8 filler, uncategorized neutral words; and the uncategorized neutral word lists had 8 target, uncategorized neutral words and 8 filler, uncategorized neutral words). The stimuli for the secondary task were different tones with frequencies of 270, 300, 330, 360, 400, 420, 430, 440, and 450 Hz. The tones were stimuli in the calibration part of the experiment as well as the later divided attention section. A button box was used to record responses to the tones and small booklets were used for recall of the word lists. Calibration Task The pitch of a comparison tone was calibrated for each subject so as to equalize across subjects the difficulty of the tone discrimination task. The tones played in any given trial were of just two frequencies; the 450 Hz tone plus a second tone differing in pitch. The first trial always consisted of the 450 Hz and 400 Hz tones. The participants pressed a button whenever the tone deviated from the 450 Hz tone. If they performed perfectly (ie., no misses or false alarms) in any given trial (all trials contained 10 tone changes), the next trial moved to a higher level of difficulty in terms of tone intervals by presenting tones with a smaller difference in frequency. For example, if a person correctly identified all changes in the Hz interval trial, the next trial would be Hz. If a participant failed to perform perfectly, the trial would move to a lower level of difficulty by presenting tones with a greater difference in 10

11 frequency. The sound interval on which a participant performed 100% correct yet was unable to achieve a flawless performance on the interval of the next highest difficultly was chosen as the interval for the divided attention task present later in the experiment. This technique allowed us to make the divided attention section equally difficult for participants with different capacities for auditory discrimination. Memory Task Participants viewed a series of ten word lists. Each word list contained 8 target words and 8 filler words. Each word list had a different type of target word associated with it (taboo, categorized negative, uncategorized negative, categorized neutral, or uncategorized neutral) and the filler words were always uncategorized neutral words. For each list type, one list was presented in a full attention condition while the other list was presented in a divided attention condition. The order of the divided attention or full attention trials, as well as the individual word lists contained within, were counterbalanced among four versions. On full attention trials, the participants looked at the monitor while words were serially presented. At the end of the every list, the participant would write down as many words as they could remember (order was irrelevant) in their booklet. After this, the experimenter moved to the next word list. The divided attention trials were essentially the same, except that tones were issued from the speakers while the words were being presented on the screen. The participants pressed the button box when the tone deviated from its usual 450 Hz beep. Participants were instructed to devote equal attention to both tasks and perform both as best as they could (instead of sacrificing performance on one to do better on the other). Responses to the tones were recorded by the experimenter, but no minimum performance was required to 11

12 move on to the next word list (a minimum performance, however, was required for the data to be used in the analysis; this guaranteed the attention was being divided). After recalling each list, participants turned the page in their answer booklet and they were not allowed to refer back to previous recall attempts. Results The taboo word lists, unlike the negative and neutral word lists, were not split into categorized and uncategorized lists. In order to deal with this asymmetry, a 2x2x2 ANOVA (categorized versus uncategorized, full attention versus divided attention, neutral versus negative) on number of target words recalled was run first with the taboo word lists excluded. This analysis revealed main effects of both categorization [F(1, 18) = 4.59, p =.046] and attention [F(1, 18) = 25.93, p <.001]. Recall of target words was better on categorized word lists (M = 5.29, SE =.31) versus uncategorized word lists (M = 4.70, SE =.27) and full attention word lists (M = 5.53, SE =.25) versus divided attention word lists (M = 4.46, SE =.30). There was no interaction between categorization and attention (p =.23). Importantly, neither a main effect of list type (p =.25) nor an interaction between list type and attention (p =.49) was found. There was no effect of emotion. A 2x5 ANOVA (full versus divided attention, all the word lists) was then run to see how this changed, if at all, with the inclusion of the taboo word lists. Notice this analysis ignores categorization (since, as mentioned before, categorization did not apply to the taboo word lists). This analysis yielded, in line with the previous one, a main effect of attention [F(1, 18) = 17.88, p =.001] on the number of target words recalled. There was also a main effect of list type [F(4, 15) = 21.85, p <.001]. Taboo words were recalled the best (M = 6.71, SE =.20), then categorized 12

13 neutral words (M = 5.32, SE =.29), then categorized negative words (M = 5.26, SE =.30), then uncategorized negative words (M = 4.92, SE =.27), and finally uncategorized neutral words (M = 4.47, SE =.39). Unlike the former analysis, an interaction between list type and attention was found [F(1, 18) = 4.59, p =.036]. As can be seen in Figure 1, the interaction occurred because taboo word recall did not decline from full to divided attention but all other list types did. Paired samples t-tests comparing recall in the full and divided attention conditions for each list type found no significant difference for taboo word lists (one-tailed, p =.17), but a significant difference for uncategorized neutral (one-tailed, p =.001), categorized neutral (onetailed, p =.038), uncategorized negative (one-tailed, p =.023) word lists, and a marginally significant difference for categorized negative word lists (one-tailed, p =.07) Taboo Neutral Categorized Negative Categorized Neutral Uncategorized Negative Uncategorized 3 Full Attention Divided Attention Figure 1. Target words recalled No statistically significant difference was found between performances on the tone task across all list types [F(3.41, 61.33) =.49, p =.71]. The mean of percent correct responses was best on the categorized negative word lists (M =92.98, SE = 7.49), then on the uncategorized 13

14 negative word lists (M =92.55, SE = 7.80), then on the categorized neutral word lists (M =92.54, SE = 7.81), then on the uncategorized neutral word lists (M =91.23, SE = 7.59), and finally on the taboo word lists (M =90.98, SE = 7.49). Discussion Consistent with the findings of Kensinger and Corkin (2004), participants consistently recalled the taboo word lists better than the other word lists in both the full attention and divided attention conditions. Recall that in the 2x5 ANOVA, which included taboo words, a main effect of list type was found, and this could only be attributed the taboo words being better recalled, because in the 2x2x2 ANOVA run beforehand, taboo words were excluded and no effect of list type was found. Also, an interaction between list type and attention was found in the 2x5 ANOVA, but no interactions between categorization and attention and list type and attention were found in the 2x2x2 ANOVA. Once again, only the taboo words could explain the interaction in the 2x5 ANOVA, as no interactions were found in the 2x2x2 ANOVA (when taboo words were excluded). All of this supports the hypothesis that memory enhancement from arousal, being dependent on the amygdala-hippocampal network, is automatic and independent of available attentional resources at encoding. Note that because the delay between encoding and recall was so small (practically nonexistent), this effect could only be the product an encoding, as opposed to a post-encoding consolidation, process. Unlike Kensinger and Corkin (2004), however, there was no improved memory effect due to valence. That said, there was an improved memory effect due to categorization, in line with Talmi and Moscovitch (2004). It is likely that because Kensinger and Corkin (2004) failed to control for semantic relatedness in the word lists they used, the improved memory effect due to 14

15 valence they found was, in actuality, a product of categorization. In other words, their negative, low arousal words were more semantically related than their neutral words, and this is what caused the negative words to be better remembered. This prompts the question of whether all supposed memory enhancements from valence are really just effects of categorization. The results of this experiment support this hypothesis, which is consistent with the neural substrates uncovered by Kensinger and Corkin (2004). Memory performance for both neutral and negative, low arousal words was correlated with activation of the PFC-hippocampal network, suggesting that the underlying mechanism for encoding all low arousal stimuli is the same. Note that the improved memory effect from categorization found in this study was only present in the full attention condition. It was not present in the divided attention condition, in line with the hypothesis that memory enhancement of this sort, being dependent on the PFChippocampal network, relies on conscious focus, and thus is dependent on available attention resources at encoding. Importantly, the improved memory effect found for high arousal stimuli cannot be attributed to a decreased performance on the secondary task. The claim that participants only fared better on recall for taboo words in the divided attention condition because they neglected the secondary task is a non-starter, because no statistically significant difference was found between performances for the tone task across all word lists. To take stock, results confirmed the hypothesis that memory enhancement from emotional arousal is automatic, independent of attentional resources available at encoding, and the result of (at least in some cases) an encoding process. It also supported the hypothesis that the supposed memory enhancement from valence is really one of categorization, and this effect, unlike the emotional arousal effect, is dependent on attentional resources available at encoding. 15

16 An important area for future research would be to implement the same sort of experiment with older adults. One would expect, given that the PFC atrophies more than the amygdala with age, that the difference between memory for arousing material versus nonarousing material to be that much greater. In other words, older adults would experience a greater benefit for memory of emotionally arousing stimuli compared to low arousal stimuli than would younger adults. Another avenue for future research would be to monitor RTs on responses to the secondary task. The current experiment was only concerned with correct responses on the secondary task, not how fast these responses were issued, so it is a standing question whether encoding of arousing stimuli interferes with RTs on the secondary task. Binding theory, depending on how it is construed, might predict this (especially if it is proposed as a general attentional resource model). Taking a more fine-grained look at responses to the secondary task would shed light on this issue. References Anderson, A. K., and Phelps, E. A. (2001). Is the human amygdala critical for the subjective experience of emotion? Evidence of intact dispositional affect in patients with amygdala lesions. Journal of Cognitive Neuroscience, 14, Cahill, L., Babinsky, R., Markowitsch, H. J., and McGaugh, J. L. (1995). The amygdala and emotional memory. Science, 377, Christianson, S. A. (1992). The Handbook of Emotion and Memory: Research and Theory. Hillsdale, NJ: Lawrence Erlbaum Associates. Critchley, H. D., Elliott, R., Mathias, C. J., and Dolan, R. J. (2000). Neural activity relating to generation and representation of galvanic skin conductance responses: A functional magnetic resonance imagining study. Journal of Neuroscience, 20, Dolcos, F., LaBar, K. S., and Cabeza, R. (2004). Interaction between the amygdala and the medial temporal lobe memory system predicts better memory for emotional events. Neuron, 41,

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