Successfully overcoming the inhibitory impact of the "forget" instruction: a voxel-based morphometric study of directed forgetting.

In directed-forgetting studies, test items are followed by an instruction either to "remember" or to "forget" (F). Many to-be-forgotten (TBF) stimuli are not retrieved at the recognition phase. However, some subjects are able to correctly recollect a high number of TBF stimuli. We examined whether this ability is reflected in the structure of brain regions involved in memory and the control of retrieval processes. In subjects with high recognition rates for TBF stimuli, voxel-based morphometry revealed increased gray matter (GM) volume in the left ventrolateral prefrontal cortex (VLPFC) and the right hippocampus (H). GM volume in these regions correlated positively with the TBF recognition rate. No significant differences were detected in subjects who forgot many TBF stimuli. Our findings indicate that the right H and left VLPFC are of particular relevance in releasing TBF items from inhibition caused by the F instruction.