Study show we can control our forgetfulness

07 Giugno 2011

Forgetting can be highly functional when unwanted or irrelevant memory representations interfere with cognitive or motivational goals. Current theory assumes that forgetting can be brought about by the intentional or unintentional inhibition of interfering memory representations.

In four studies, the present thesis investigated this claim by employing behavioral testing, and neuroimaging methods based on the electroencephalogram (EEG), event-related potentials (ERPs) and EEG oscillations. Studies I-III employed adaptations of the think/no-think paradigm. Participants first engage in paired associate learning. Then, during the think/no-think phase, one member of the paired items (cues) is repeatedly presented with the instruction to retrieve (think condition) or suppress (no-think condition) the associated target item.

Study I showed that intentional suppression can reduce subsequent recognition memory performance for no-think items. This finding suggests that intentional suppression leads to a reduction of item memory strength by inhibition. Correct recognition of no-think items was accompanied by late positive ERPs, implying more effortful post-retrieval monitoring processes.

Study II investigated ERPs during the think/no-think phase. In no-think trials, we observed a reduction of parietal positive-going ERPs between 500-600 ms, indicating that intentional suppression hampered the conscious recollection of unwanted memories. In addition, intentional suppression correlated with a pronounced negative ERP amplitude peak between 350-450 ms and a positive ERP slow wave at frontal electrode sites from 700-1000 ms. The negative peak correlated with a N2 ERP component observed in a stop-signal-task in the same participants, suggesting that memory suppression and motor stopping recruit similar inhibitory control networks.

Study III explored the influence of memory strength, task-switching, suppression strategy, and compliance on the ERP correlates of memory suppression. A decrease of item memory strength over the time course of the think/no-think phase reflected in more negative going ERPs between 225-450 ms after cue onset. Besides mirroring the findings of Study I, the results also implied that negative going ERP components, assumed to reflect inhibitory control processes, may be superimposed on such neural correlates of strength reduction. The frontal positive slow-wave as observed in Study II was found to be enhanced after a switch from think to no-think trials, suggesting that the think/no-think manipulation also entails task switching processes reflecting in neural markers of executive control. Finally, a direct suppression strategy and higher compliance correlated with more negative going ERPs throughout the epoch. Memory inhibition can aid the very act of remembering. Trying to retrieve a particular experience or fact often leads to the activation of other currently irrelevant memories. In order to remember the desired information, inhibitory control mechanisms are thought to be recruited in order to dampen the strength of interfering memories.

Study IV was designed to track inhibition to the brain networks housing the traces of interfering memories. Participants learned abstract shapes that were paired with two different colors, one appearing in the left and the other in the right visual half-field. Subsequently, participants practiced retrieval of one color, selecting it over the other, competing color. In a final recall test, memory for competing colors was impaired. Between 90-430 ms, we observed an increase of oscillatory power in the alpha/beta band of the EEG at posterior electrode sites over the brain hemisphere housing the sensory representation of the competitor.

In the light of strong evidence connecting EEG alpha/beta oscillation to inhibition, we regard this finding as direct neural evidence for the inhibition of interfering memories during selective retrieval. Taken together, the results give strong evidence for the claim that undesired memories can be inhibited. Memory inhibition appears to act similar to inhibition in selective attention and motor control. The present thesis mainly aimed at investigating the underlying mechanisms in controlled laboratory settings and has implications for future studies in the field. The findings also contribute to understanding the symptoms and psychological processes in populations that display memory distortions and biases, such as the elderly, and patients suffering from post-traumatic stress disorder (PTSD) and depression.

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