Experimental Evidence for Memory Suppression

If the Larry Summers deba(cle)te has taught me anything, it's that where issues of science are concerned, very few opinions are worth anything. Too often have I seen appeals to the (often baseless) opinions of "authority figures" (e.g., Stephen Pinker) treated as evidence for one position or another. But this is not how scientific debates are supposed to work. We're supposed to reference facts, not names, reputations, or degrees. This is particularly true for issues with serious practical implications. In these cases, suddenly everyone is an expert, though in most cases the opinions of these "experts" are based on selective readings of the literature, especially when the "experts" aren't actually experts in the area on which they are voicing their opinions (as is the case for Pinker and sex differences in mathematical ability).

All of this aplies to me as much as anyone. While I do study memory (long-term, schematic memory), I am not an expert on repressed, recovered, false, traumatic, or emotional memories, and therefore my opinions on these topics should not be substituted for a careful review of the available scientific research. My responsibility, as a scientist who is making public statements about these topics, is to try to present the facts. While in my previous two posts I tried to be fair to both sides, even discussing at length (by blog standards) two of the most often cited defenses of the pro-repression position, I have not presented all of the evidence. I feel justified in leaving out some research (e.g., that of Bessel van der Kolk) because it is either inferior or irrelevant. In other cases, however, my ommissions may not have been justified. Because I've recently received a lot of visits (and as a result, a couple emails) from Google searches on recovered memories, I feel like it's incumbent upon me to present the issue in a more balanced fashion, sticking to the facts, in the interested of fairness and reasoned scientific debate. With that in mind, I'm going to describe one line of laboratory research that has produced findings that may be taken as evidence for memory mechanisms that could lead to repression.

Before I get to that, though, I want to briefly note that I am not a dispassionate observer when it comes to recovered memory phenomena. One of the charges leveled against cognitive psychologists (including me, in an email), who have generally taken anti-recovered memory positions, is that they are drawing conclusions without any real contact with the real-life effects of repressed and recovered memories. Clinicians, on the other hands, see their effects on a daily basis, and are therefore less inclined to deny their existence or dismiss recovered memories as false or implanted. However, I have seen their effects up close and personal. My son's mother, with whom I lived for several years, is an abuse survivor who, while she has always had memories of intense physical and psychological abuse, only "recovered" memories of sexual abuse, which she believes took place between the ages of 3 and 6, in her early 20s. The same is also true of my closest friend of many years, who has always had memories of parental abuse, but "recovered" memories of sexual abuse at the hands of a non-family member in her late 20s¹. The effects of these "recoveries" on both of these people has been staggering, and for the people close to them, like me, heartbreaking. Their lives have been forever altered. Personal experiences like these are the impetus for my own attempt to learn and understand the research on repressed memories.

OK, so on to the science. Michael Anderson and his colleagues² have developed experimental paradigms that they feel provide tests for the existence of memory suppression mechanisms. In one paradigm, which they call the "retrieval practice paradigm," (Anderson et al., 1994; Anderson & Spellman, 1995) participants are given listes of pairs of words (e.g., "fruits-banana"), the first of which names a category, and the second of which names a member of the category. After learning the word pairs, they complete a stem-completion task for half of the category member words from half of the categories. This involves being presented with the category word (e.g., "fruit") and the beginning of one of the category members (e.g., "ba_____"). The task is to complete the stem with one of the words from the word pairs they had previously learned. They do this three times for each of the words in the stem-completion task, in order to practice retrieving these words from memory. Finally, they are given a recall test for all of the original word pairs, in which they were given the category word and asked to recall the words that had been paired with it. In this paradigm, there are three types of category member words: words from categories used in the stem-completion task that were practiced, words from those same categories that weren't practiced in the stem-completion task, and words from categories that weren't used in the stem-completion task. They found that recall was best for practiced words and worst for unpracticed words from categories used in the stem-completion task, with words from categories not used in the stem-completion task in between the two. They argue that this demonstrates the existence of a mechanism for inhibiting the recall of related items.

There is an alternative explanation for this finding, namely that with practice, the stem-completion category members are strengthened to such an extent that they completely overshadow the memory traces for the non-practiced items from the same category, making it very difficult to recall the latter. To rule this explanation out, they ran a modified version of the same experimental paradigm in which some of the category member words were actually members of two separate studied categories (e.g., participants studied the categories "red" and "food," and the word "tomato," which is a member of both). For these words, one of the categories was used in the stem-completion task, while the other wasn't. If memory inhibition is the cause of the difficulty in recalling non-practiced words from stem-completion categories, then words like tomato should be inhibited regardless of the cue (practiced category or unpracticed category) used during the retrieval task. And that is what they found: practiced words were again recalled the most, with unpracticed words from two categories were recalled the least regardless of whether the cue was the practiced or unpracticed category.

Interestingly, the effects demonstrated in these two experiments (called retrieval-induced forgetting) have been replicated using "mock crime scenes"³. In these experiments, participants view images of a crime scene, and are subsequently interviewed about some of the details of that scene. Aftewards, their recall for information about the crime scene that was not discussed in the interview is inhibited, and thus recalled at a lower rate.

While these show that there may be inhibition processes recruited during memory retrieval, they do not show that unwanted memories can be actively inhibited. To show this, Anderson and Green (2001) used the following paradigm. Participants first learned fourty word pairs, this time containing unrelated words (e.g., "ordeal-roach"). Immediately after the completion of this task, they were given a list of words to recall (recall pairs), and a list of words not to think about when they received the associated cue (suppression pairs). They were then given the cue words (the first word from each pair), and either recalled the associated word, or avoided thinking about it, depending on whether it was from a recall or suppression pair. Finally, they were given another recall task, and told to ignore the previous instructions to not think about some of the words when they received the cue. In other words, in the final recall task, they were to recall all of the words when given their associated cues. Recall for the suppression pairs was significantly poorer than recall for the words from the recall pairs. Furthermore, the amount of recall for a word from a suppression pair depended on how many times they had suppressed thoughts of a word when receiving its associated cue, with more suppression leading to poorer recall. As with the retrieval practice paradigm, they ran several variants of this experiment designed to rule out alternative explanations.

The results of these (and other related) experiments appear to indicate that people can actively inhibit unwanted memories by suppressing thoughts of those memories when the relevant retrieval cues are present. Levy and Anderson (2002) speculate that the inhibitory mechanisms involved in these tasks may involve the anterior cinculate cortex and the dorsolateral prefrontal cortex or left inferior prefrontal cortex (depending on the type of information being inhibited). The anterior cinculate cortex, they claim, likely sends a signal to the prefrontal cortex that executive control is needed, and the actual inhibition takes place in the prefrontal cortex.

Before the repression advocates start jumping up and down, they should be aware of several limitations to these studies. First of all, the Anderson and Greene results have proved difficult to replicate in other labs⁴. Since it is the Anderson and Greene experiments that provide the most direct evidence for the suppression of unwanted memories, failure to replicate them is problematic. In addition, none of the Anderson studies use emotional memories, or memory under conditions of high arousal. Since research has shown that it is much more difficult to suppress emotional memories, this makes the generalizability of these studies to traumatic memories difficult. It may turn out that the suppression of unwanted traumatic memories is much more difficult, a finding that would be consistent with previous research. A third problem is that in all of the Anderson experiments, and in the Anderson and Greene experiments in particular, recall for the inhibited or suppressed memories is still quite high (never dropping below 70%). In fact, when the task is changed from a cued recall task to a stem completion task (a more implicit measure of memory), the difference between suppressed and non-suppressed words becomes quite small, with suppressed words being recalled about 80% of the time. If the type of suppression Anderson argues his experiments demonstrate can account for the long-term and complete repression of traumatic memories, research will have to show how these memories can be completely forgotten. This hints at another problem with the study: they all take place over the course of about an hour. We cannot know from these studies how long-term suppression of unwanted memories will effect them. Will it reduce recall even more, perhaps to the point where people are compeltely unable to recall the unwanted memories? Or is there a limit to the effects of suppression? In order to apply the findings to traumatic memories, we would need to answer these questions. Finally, there is no indication of how suppressed memories might be recovered. As of yet, there is absolutely no empirical evidence for anything resembling a memory-recovery mechanism for suppressed memories.

In conclusion, then, while the research of Anderson and his colleagues may provide the first evidence of suppression mechanisms, which Anderson believes may be related to Freud's concept of repression, it is yet to be seen whether their data is relevant to the recovered memory debate. I do think it's the best evidence repression advocates have to offer, because it is the only real evidence from carefully controlled experiments, but the fact that this is all they really have is just one more indication that the repression hypothesis has no direct empirical support.







¹ If you're still wondering how I could be a skeptic after having observed the effects of recovered memories on two people close to me, then let me add this. In one case, implantation is very likely. Both highly suggestive therapy and the revelation by a sibling that she had been sexually abused "triggered" the recovered memories. In the other case, there is strong evidence that the "recovery" is actually a case of forgetting that she remembered. Other friends who knew here before me report that even before the age at which she "recovered" the memories of abuse, she discussed the abuse with them, indicating that the "recovery" may have been of memories that had never actually been forgotten.
² Anderson,M.C., et al. (1994). Remembering can cause forgetting: retrieval dynamics in long-term memory. Journal of Experimental Psychology: Learning, Memory, and Cognition, 20, 1063-1087. Anderson, M.C., & Spellman, B.A. (1995). On the status of inhibitory mechanims in cognition: memory retrieval as a model case. Psychological Review, 102, 68-100. Anderson, M.C. & Neely, J.H. (1996). Interference and inhibition in memory retrieval. in E.I. bjork & R.A. Bjork (eds.) Memory. Handbook of Perception and Cognition (2nd edition), pp. 237-313, Academic Press. Anderson, M.C., et al. (2000a). Similarity and inhibition in long-term memory: evidence for a two-factor model. Journal of Experimental Psychology: Learning, Memory, and Cognition, 1141-1159. Anderson, M.C., et al. (2000b). Retrieval-induced forgetting: evidence for a recall-specific mechanism. Psychological Bulletin and Review, 3, 380-384. Anderson, M.C., & Greene, C. (2001). Suppressing unwanted memories by executive control. Nature, 410, 131-134. For a short review, see Levy, B.J. & Anderson, M.C. (2002). Inhibitory processes and the control of memory retrieval. Trends in Cognitive Sciences, 6(7), 299-305. All subsequent references will be to these papers, by authors and year.
³ Macleod, M. (2002). Retrieval-induced forgetting in eyewitness memory forgetting as a consequence of remembering. Applied Cognitive Psychology, 16, 135-149.
⁴ I know of no published descriptions of replication attempts, but I do know of at least two labs that have attempted to replicate the Anderson and Greene suppression findings, and failed to do so.