Understanding medial temporal activation in memory tasks: evidence from fMRI of encoding and recognition in a case of transient global amnesia.

We used fMRI to examine the activation patterns of patient AE during encoding and recognition of visual scenes during an episode of transient global amnesia (TGA) and 3 months later. Controls (n = 5) showed bilateral (R > L) activation in parahippocampal and fusiform gyri during encoding and right-sided activation in the same regions associated with recognition of previously viewed scenes. AE showed a similar pattern at follow-up. During acute TGA, when performance was profoundly impaired, AE showed no medial temporal activation associated with encoding of new scenes or recognition of old scenes. In both contrasts, the percent signal change in relevant medial temporal regions was more than three standard deviations below the control sample mean. She did, however, show striking bilateral hippocampal activation for recognition attempts (old + new scenes > baseline) even though retrieval was unsuccessful (55% recognition accuracy). This finding was unique to AE on this occasion. This is the first study to document normalization of both encoding and recognition activation patterns in TGA. Furthermore, the strong hippocampal activation during unsuccessful retrieval highlights important issues in interpreting memory-related activations, particularly in dysfunctional systems.