Subjects actively tried not to think about something that they had learned while being while being reminded of it. This suppression was effective at degrading memory. fMRI scanns showed that active suppression “was associated with increased dorsolateral prefrontal activation, reduced hippocampal activation, and impaired retention of those memories. Both prefrontal cortical and right hippocampal activations predicted the magnitude of forgetting.”
“Subjects learned word pairs …[and then, on each trial,]… were presented with one member of a pair and asked either to think about the [other member of the pair] or to prevent [that] word from entering consciousness at all (Suppression condition) for the entire four seconds that the stimulus was presented… subjects were tested to determine whether attempts to stop retrieval during Suppression trials had impaired memory for the response, when given the originally trained cue …. Suppression did impair memory.”
Anderson, Michael C., Ochsner, Kevin N., Kuhl, Brice, Cooper, Jeffrey, Robertson, Elaine, Gabrieli, Susan W., Glover, Gary H., Gabrieli, John D. E.
Neural Systems Underlying the Suppression of Unwanted Memories.
Science 2004 303: 232-235