rCBF to the hippocampal complex covaries with superior semantic memory retrieval.

The study of medial-temporal amnesics has established that much of the brain's store of general knowledge can be retrieved without the hippocampus. Several brain imaging studies, however, have reported that the hippocampus is active during the performance of semantic memory tasks. Although this activity may reflect memory encoding, it is also possible that it is due to the process of retrieving the semantic representations of the stimuli. Here we show in two H(2)(15)O positron emission tomography studies that cerebral blood flow (CBF) to the hippocampus covaries with superior performance during the retrieval of general knowledge. Left hippocampal CBF was associated with faster word meaning retrieval and right hippocampal CBF with better picture naming. A follow-up experiment failed to show that faster responding in the word meaning task was associated with better episodic memory for the stimuli and so weakens the proposal that the hippocampal activity was strictly related to memory encoding. In both experiments CBF to the lateral temporal cortex covaried with hippocampal CBF in a way that suggests that the hippocampus provides complementary support to activation within the association cortex.