To find out what brain areas are involved in intentional forgetting, they asked 40 participants to learn word pairs; consisting of reminder-memory words and reminder-substitute words. For example, the reminder ‘Beach’ was paired with the memory word ‘Africa’ and the substitute word ‘Snorkel’. Participants practiced direct suppression (they avoided thinking about the memory word) and thought substitution (they thought of the substitute word). During a functional magnetic resonance imaging (fMRI) scan, participants were shown reminder words and asked to recall or suppress the memory word. 50% were instructed to use direct suppression and 50% thought substitution, before recalling both the memory and substitute words.
What did they find?
The final sample consisted of 36 participants, having excluded four for excessive movement or falling asleep during scanning (who thought it possible?!)
To confirm participants had followed their instructions for forgetting, Anderson and Benoit compared participant reports and their recall of substitute words (figure 2b/c). As expected, those using thought substitution recalled significantly more substitute words than those using suppression; after all, they’d been thinking about them! Statistical analysis showed that in comparison to baseline forgetting, people remembered significantly less memory words following both thought substitution and direct suppression (2d/e).
fMRI results showed that:
Direct Suppression led to increased activation in the right dorsolateral prefrontal cortex and decreased activation in the right hippocampus (figure 3a/b). A dynamic model supported their theory that the dorsolateral prefrontal cortex actually inhibits hippocampal activity. With the hippocampus previously identified as important in conscious recall (Eichenbaum et al, 2007), it’s hardly surprising that reduced activity in this area would result in lower recall.
Thought Substitution led to increased activation in the left caudal and mid ventrolateral prefrontal cortex (3c/d). People who had more difficulty remembering the substitute word showed greater activation in these areas; two memories were competing for awareness. People with less activation remembered more easily; they’d already forgotten the competing word, so it didn’t interfere with recall.
Their conclusion then, seems relatively straightforward: both methods are equally effective but the brain processing in each differs. Direct suppression inhibits memory retrieval whereas thought substitution selects a different memory!
Could it really be that easy to forget? A few things to consider about the study…
On a general note (no fMRI study is complete without it!) we have to be wary when interpreting fMRI data as there are differing views on its meaning; active processing or active inhibition?
– Is it new? Yes. The study builds upon previous work (Anderson et al, 2004) to show that the dorsolateral prefrontal cortex has an inhibitory effect on the hippocampus during suppression. It also looks at brain activity in a second method of forgetting: substitution.
– Is it useful? Maybe. It gives us an idea of how the brain works. Unfortunately, it doesn’t tell us the methods most frequently used to forget unwanted memories, or if suppression and substitution are the usual methods. My concern: the lack of control group as a measure of ‘normalcy’. Benoit and Anderson used only two conditions and in both they instructed people to forget in specific ways. Thus, providing no evidence that either of these methods are the ones used in everyday life (when we aren’t being instructed) or that they are more effective.
I’m sure everyone reading this has experienced their own unwanted thoughts or memories. So, how do you attempt to forget them? Using suppression, substitution, or a different method entirely? If so, you’re not the only one: Anderson and Levy (2008) found participants reported various strategies, the most common being mental imagery (albeit only 23% of the time, figure 4).
– Is it (ecologically) valid? Debatable. Is forgetting unwanted emotional memories and trauma really comparable to forgetting random words? It’s useful to know you can probably use these methods to forget an annoying earworm, but can the same be said for a traumatic experience? One might imagine it’s a little more difficult:
The results can’t account for a known rebound effect – the white bear phenomenon. Many studies (showing arguably greater ecological validity) demonstrate this, including: Shipherd and Beck (1999), who found participants suffering post-traumatic stress disorder (PTSD) experienced a decrease, followed by an increase in unwanted thoughts when using direct suppression and a lower level of perceived control over thoughts. Participants without PTSD didn’t experience this rebound; this isn’t suggesting that people without PTSD found the experience less traumatic, rather, people suffering PTSD are less likely to benefit from direct suppression.
More recently, however, some research has shown support for Anderson’s method. Depue (2006) found similar results using different stimuli, such as aversive images. This suggests that Anderson’s method is not only generalisable to the forgetting of random words but also to more emotive stimuli.
So… maybe there’s hope for all of us that when things start to go horribly wrong, we won’t have to get our memories ‘erased’ like Jim Carrey and Kate Winslet…
Anderson, M.C., Ochsner, K.N., Kuhl, B., Cooper, J., Robertson, E., Gabrieli, S.W., Gabrieli, J.D.E. (2004). Neural Systems Underlying the Suppression of Unwanted Memories. Science, 303(5655), 232-235. doi: 10.1126/science.1089504
Benoit, R. G., & Anderson, M.C. (2012). Opposing Mechanisms Support the Voluntary Forgetting of Unwanted Memories. Neuron, 76(2), 450-460. doi: 10.1016/j.neuron.2012.07.025
Depue, B. E., Banich, M. T., & Curran, T. (2006). Suppression of Emotional and Nonemotional Content in Memory: Effects of Repetition on Cognitive Control. Psychological Science, 17(5), 441-447. doi: 10.1111/j.1467-9280.2006.01725.x
Eichenbaum, H., Yonelinas, A. P., & Ranganath, C. (2007). The medial temporal lobe and recognition memory. Annual Review of Neuroscience (Vol. 30, pp. 123-152). Palo Alto: Annual Reviews. doi: 10.1146/annurev.neuro.30.051606.094328
Levy, B. J., & Anderson, M. C. (2008). Individual differences in the suppression of unwanted memories: The executive deficit hypothesis. Acta Psychologica, 127(3), 623-635. doi: 10.1016/j.actpsy.2007.12.004
Shipherd, J. C., & Beck, J. G. (1999). The effects of suppressing trauma-related thoughts on women with rape-related post traumatic stress disorder. Behaviour Research and Therapy, 37(2), 99-112. doi: 10.1016/S0005-7967(98)00136-3