Redeeming Freud: Memory Suppression

Freud suggested that humans can repress unwanted or traumatic memories, and many still think of this idea as simply an unproven Freudian hypothesis. However, the fact is that we can intentionally forget stimuli, as seen in directed forgetting and think/no-think paradigms. In these studies, subjects learn several paired associations (between two words, or two pictures), and are then repeatedly presented with one of each pair of stimuli and asked to either remember or forget it's associate. Memory for the items is then tested through explicit (free-recall, cued-recall, or recognition) or implicit measures (wordfragment completion, reptition priming), and the difference between recall accuracy of to-be-remembered items and to-be-forgotten items is assumed to reflect the effects of an intentional "forgetting" process.

There are several different theories of how directed forgetting actually works, all of which have implications for the functions of working memory and executive control. One unlikely hypothesis is that subjects are able to somehow inhibit the stimuli in working memory, without conscious awareness. A second more likely hypothesis is that subjects are able to think of and focus on something else which then displaces the to-be-forgotten item from working memory ("diversionary thought" and "associative interference" hypotheses). A third hypothesis is that subjects are actually able to shut down areas outside of working memory that would otherwise process the items.

In deciding between these hypotheses, it's important to take a closer look at the data. Some have reported that even novel cues still do not elicit recall of the to-be-forgotten items, suggesting that the directed forgetting is not simply due to "erasing" the association between items. A network of brain areas has been shown through fMRI to be more active during suppression than recall (e.g., dorso- and ventrolateral prefrontal cortex, anterior cingulate cortex) suggesting that this is an active suppression process. However, data from hippocampus show a mixed response, in that items that are later remembered show more hippcampal activation than items that were simply forgotten, whereas items that were actively suppressed showed the highest activation of all.

Research is just beginning to shed light on the functional contributions of each of these brain areas to the task of directed forgetting, and any conclusion about their true function is of course premature. Nonetheless, it appears that there are active suppression processes which involve a network of brain regions coordinated by DLPFC and ACC, and which may show an advantage for suppressing emotional as opposed to neutral stimuli.

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