Like living and forgetting
And if I pick you up
I'll be sure to let you down
-Living and Forgetting, Glasstown (mp3)
Forgetting Emotional Information Is Hard
Our memory for emotional events is generally better than our memory for neutral events. This is a key issue in developing treatments for post-traumatic stress disorder. How do we rid ourselves of unpleasant memories? In structured laboratory environments, the best way to forget is intentional inhibition during the encoding phase, when exposed to the material for the first time. In other words, engage in a deliberate strategy to forget while the event is actually occurring, as shown in a recent study by Nowicka and colleagues. This process is effortful, and it engages a larger proportion of the brain when the material is emotionally laden (i.e., negative pictures from the International Affective Picture System, or IAPS), relative to when it is neutral (Nowicka et al., 2010).
In the study phase, intention to forget and successful forgetting of emotionally negative images were associated with widespread activations extending from the anterior to posterior regions mainly in the right hemisphere, whereas in the case of neutral images, they were associated with just one cluster of activation in the right lingual gyrus [occipital cortex]. Therefore, forgetting of emotional information seems to be a demanding process that strongly activates a distributed neural network in the right hemisphere. In the test phase, in turn, successfully forgotten images—either neutral or emotionally negative—were associated with virtually no activation... These results suggest that intentional inhibition during encoding may be an efficient strategy to cope with emotionally negative memories.However, "directed forgetting" is usually not a practical strategy when real life events are unfolding. Whether it can effectively occur at all during horrible tragedies is highly controversial (e.g., Terr vs. Loftus). The phenomenon is more often studied when applied to the retrieval of traumatic or unwanted memories (Anderson & Levy, 2009; Geraerts & McNally, 2008; Levy & Anderson, 2008), not during the encoding phase.
How to Forget
Obviously, it’s unethical to expose people to traumatic events for experimental purposes. Instead, the present study used an item-method directed forgetting task in the lab and measured brain activity with fMRI (Nowicka et al., 2010). Twenty-three participants1 viewed a set of images from the IAPS that were either negative or neutral in content. During the initial encoding phase, participants were instructed to either REMEMBER or FORGET each picture by means of a cue that was presented after the item appeared on the screen. Then in the memory test phase, these previously presented pictures were intermixed with new ones, and the subjects were told to indicate whether they recognized them or not, regardless of the previous task cue.
Trials were sorted according to task instruction (Remember or Forget) and memory outcome (Remembered or Forgotten). Behavioral data showed that the directed forgetting manipulation was successful. Participants remembered fewer pictures in the To-Be-Forgotten (TBF) condition than in the To-Be-Remembered (TBR) condition. The valence manipulation appeared to be successful as well: recognition was better for emotional pictures, especially in the TBF condition. However, the rate of "false alarms" (incorrect responses to new items) was higher for emotional pictures as well (see figure below). This suggests a bit of a response bias: participants were more likely to say "yes I saw it before" for emotional images than for neutral.
Figure 1B (modified from Nowicka et al., 2010). Percentage of correctly recognized TBR and TBF images (TBR_R and TBF_R, respectively) and percentage of false alarms for the group of 16 subjects included in the fMRI analyses. Bars represent SD; E, emotionally negative images; N, neutral images.
When corrected for false alarm rate, it appears that recognition accuracy was actually lower for the To-Be-Forgotten emotional pictures, meaning they were easier to forget [unless I'm missing something here]. Hmm.
On to the fMRI data. The major analysis was done in relation to the FORGET vs. REMEMBER cue. Was there differential activity when trying to forget an emotional picture compared to a neutral picture? Figure 2A shows the answer: yes, there was greater activity in the bilateral occipital cortex and elsewhere in the right hemisphere for emotional pictures, with only a small occipital focus of activation for the neutral ones.
Figure 2 (modified from Nowicka et al., 2010). The study phase. (A) Effect of memory instruction: intention to forget contrasted with intention to remember (F instruction > R instruction for all trials). Significant group activations are superimposed on a normalized single subject's T1 image.
This indeed suggests that the intention to forget an emotional image (such as a car crash or mutilated body) is more effortful for the brain than trying to forget a neutral landscape scene. During the memory test, however, it didn't matter if you forgot the picture on purpose or by accident -- the neural response to forgotten items was identical to the response produced by entirely new images. Nary a trace [at least as a change in BOLD signal]. Have other investigators found this as well? What does it all mean?
In conclusion, the findings of this item-method directed forgetting fMRI study reveal that forgetting of emotional information is supported by a widely distributed neural network, indicating more effort than forgetting of neutral information. These differences were observed in the study phase but not the test phase, which suggests that the directed forgetting effect is mainly based on inhibition at the encoding level rather than at retrieval (but see: Ullsperger et al. 2000; Nowicka et al., 2009). More generally, our results suggest that flexible control of memory may be effective even in case of unpleasant memories, but still it requires more effort than in case of neutral ones.
--Review in The Journal of the Royal Anthropological Institute
1 Seven were excluded for insufficient task performance.
Anderson MC, Levy BJ. (2009). Suppressing unwanted memories. Curr Dir Psychol Sci. 18:184-194.
Geraerts E, McNally RJ. (2008). Forgetting unwanted memories: directed forgetting and thought suppression methods. Acta Psychol (Amst). 127:614-22.
Joslyn S, Carlin L, Loftus EF. (1997). Remembering and forgetting childhood sexual abuse. Memory 5:703-24.
Levy BJ, Anderson MC. (2008). Individual differences in the suppression of unwanted memories: the executive deficit hypothesis. Acta Psychol (Amst). 127:623-35.
Nowicka, A., Marchewka, A., Jednorog, K., Tacikowski, P., & Brechmann, A. (2010). Forgetting of Emotional Information Is Hard: An fMRI Study of Directed Forgetting Cerebral Cortex DOI: 10.1093/cercor/bhq117
Terr LC (1991). Childhood traumas: An outline and overview. Am J Psychiatry 148:10–20.
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