More than 100 years ago, Sigmund Freud, the father of psychoanalysis, proposed a mechanism called repression, whereby desires and impulses are actively pushed into the unconscious mind. For Freud, repression was a defence mechanism – the repressed memories are often traumatic in nature, but, although hidden, they continue to exert an effect on behaviour.
Many of Freud’s theories have long since been discredited, but they remain influential to this day. Repression in particular has proven to be extremely controversial. It has attracted much attention in the context of the many legal cases involving allegations of childhood sexual abuse, especially in light of the knowledge that false memories can be created with ease.
Nevertheless, we now know that unwanted memories can indeed be actively forgotten, and some of the brain mechanisms underlying voluntary memory suppression have been identified. Now a group of German researchers report that anticipating the suppression of a memory is associated with specific patterns of electrical activity in the brain, and that these patterns can be used to accurately predict forgetting.
There are a number of reasons why one might want to prevent memories from entering consciousness. Most obviously, memories of certain events can be extremely emotional, and their recall can therefore contribute to post-traumatic stress disorder or other similar conditions. Less obviously, irrelevant information is suppressed when one is performing a cognitive task, so that it does not interfere the information that is needed for that task.
In 2004, a group of researchers led by Michael Anderson, who was then at the University of Oregon, investigated the neural basis of voluntary memory suppression using functional magnetic resonance imaging (fMRI). Participants were first presented with, and asked to learn, pairs of words. They were then placed inside the brain scanner, and asked to perform a “think/no’think” task; in each trial, they were shown one member of one of the word pairs, and asked to either recall and think about the word associated with it, or to actively prevent the associated word from entering their conscious awareness.
This study showed that suppressing a memory was associated with increased activity in a diffuse network of brain regions in the frontal cortex, including the dorsolateral and ventrolateral prefrontal cortex (VLPFC and DLPFC, respectively), anterior cingulate cortex and pre-supplementary motor cortex. By contrast, activity in the hippocampus, a region of the medial temporal lobe known to be crucial for memory formation, was found to be reduced reduced. This all suggests that active suppression can override or interfere with memory recall – engagement of the network in the frontal lobes inhibits hippocampal activity, so that the memory cannot be retrieved and is instead forgotten.
The new study, carried out in Karl Heinz-Bäuml‘s laboratory in the Department of Experimental Psychology at Regensburg University, builds on these initial findings. In a modification of the same paradigm, 24 participants were first presented with 27 face-word pairs on a screen, and then shown some of the faces one by one, and asked to say the words associated with each. In this test phase, the participants remembered the face-word pairs well, scoring an average of almost 81% on the recall test.
The participants were shown some of the same face-word pairs in the think/no-think phase of the experiment. In these trials, the participants first fixated on a small black cross which appeared on the screen just before each face-word pair, and changed colour. A change from black to green indicated to the participants that they should try to remember the word associated with the face in the stimulus they were about to see (the “think” condition), whereas a change to red indicated that they should actively avoid thinking about it (the “no-think” condition).
In these trials, the coloured cross therefore enabled the participants to prepare themselves to either actively remember or suppress the word associated with the face in each of the trials. Afterwards, another cued recall test was performed and, as expected, recall was often ound to be significantly reduced in the no-think condition, confirming that instructing the participants to avoid thinking about a face-word pair effectively promoted forgetting. This suppression-induced forgetting did not occur in all trials, and was observed in some participants more than others.
As the participants performed these tasks, the researchers used electroencephalography (EEG) to monitor the electrical activity of their brains. In this way, they identified two event-related potentials (ERPs), or electrophysiological signatures, which were associated with the no-think conditions. One of these occurred around 300 milliseconds after presentation of the “no-think” cue (the red cross), but before the appearance of the memory cue itself, from the electrodes overlying right frontal and left parietal lobes. The other was recorded from the same electrodes, at the time when the memory cue itself was presented.
The researchers found that these the presence of these ERPs was an accurate predictor of whether or not the participants would subsequently recall the words associated with the faces in each trial. The earlier ERP was consistently followed by the later one, and both effects were specific for the “no-think” condition. This study therefore shows for the first time that there are neural mechanisms which anticipate voluntary memory suppression. The source of the ERPs could not be determined with EEG; however, fMRI could be used in the future to investigate the precise location of the activity observed here.
This study also confirms some of the findings of a number of earlier studies. First, it confirms that repeated, active suppression of specific encoded information can lead to subsequent forgetting of that information. Here, as in the earlier studies, suppression-induced forgetting only occurred after a large number of trials, indicating that the brain mechanisms involved are activated with repeated suppression. The new study also showed that the second ERP was found to be slightly stronger than the first; this suggests that the “no-think” cue induces anticipatory mechanisms which later increase in strength when the information to be suppressed is presented.
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Hanslmayr, S. et al (2009). Anticipatory signatures of voluntary memory suppression. J. Neurosci. 29: 2742-2747. [PDF]
Anderson, et al (2004). Neural systems underlying the suppression of unwanted memories. Science 303: 232-235. [PDF]