One study correlated MTL theta power during free recall of items encoded in a virtual environment with the virtual spatial distance between the encoding locations of successively recalled items [14]. They observed increased theta power for recalls that were followed by recall of an item encoded in spatial proximity. Assuming that ‘spatial clustering’ during recall indexes successful spatial context retrieval, this analysis specifically implicated theta in retrieving item-location associations. When applying the same rationale to the relation of theta power and temporal distances during encoding (i.e. indexing memory for temporal context), two studies found no effects of temporal clustering on theta power during encoding [47] or retrieval [14]. More recent results, however, suggest that hippocampal theta power does increase during temporally clustered recalls, but that this effect can only be observed on high-performance trials [54]. In free recall, temporally clustered recalls can reflect contextually mediated recall or other non-contextual processes, such as rehearsal of the first few or the last few items in a study list. Therefore, excluding lists that are likely to reflect significant rehearsal strategies may be crucial
