Differential involvement of the hippocampus and temporal lobe cortices in rapid and slow learning of new semantic information.

The present study examined the rapid and slow acquisition of new semantic information by two patients with differing brain pathology. A partial double dissociation was found between the patterns of new learning shown by these two patients. Rapid acquisition was impaired in a patient (YR) who had relatively selective hippocampal damage, but it was unimpaired in another patient (JL) who, according to structural MRI, had an intact hippocampus but damage to anterolateral temporal cortex accompanied by epileptic seizures. Slow acquisition was impaired in both patients, but was impaired to a much greater extent in JL. The dissociation suggests that the mechanisms underlying rapid and slow acquisition of new semantic information are at least partially separable. The findings indicate that rapid acquisition of semantic, as well as episodic information, is critically dependent on the hippocampus. However, they suggest that hippocampal processing is less important for the gradual acquisition of semantic information through repeated exposure, although it is probably necessary for normal levels of such learning to be achieved.