Impaired verbal associative learning after resection of left perirhinal cortex.

Some patients considered for left temporal lobectomy for epilepsy present with normal verbal learning and no MRI evidence of hippocampal pathology. In order to preserve learning function, the surgical approach in these cases often aims at sparing the hippocampus. Parahippocampal structures, including the left perirhinal region, however, also appear to contribute to some forms of verbal learning. We studied aspects of verbal learning in four patients with left temporal lobe resections that preserved the hippocampus, but which included perirhinal/entorhinal cortices in two cases. Pre- and postoperative T1-weighted MRI scans were spatially normalized and residual mesial temporal structures identified. The two patients whose resection included perirhinal and entorhinal cortices exhibited a marked decrement in the ability to acquire arbitrarily related word pairs that persisted at 12-month follow-up. Word list learning showed an early postoperative impairment, but recovered to normal levels within 12 months. In two patients, resection encompassed anterolateral and inferior temporal neocortex but spared the perirhinal and entorhinal cortices, amygdala and hippocampus. No postoperative change in verbal learning was evident. We concluded that hippocampal-sparing left temporal lobe resections result in task-specific verbal learning deficits when perirhinal/entorhinal tissue is included in the resection.