HIPPOCAMPAL SCLEROSIS AND VERBAL ENCODING ABILITY FOLLOWING ANTERIOR TEMPORAL LOBECTOMY

Previous research has shown that the degree of verbal memory decline f ollowing left anterior temporal lobectomy (ATL) is inversely related t o the extent of neuronal dropout in resected left hippocampus. The goa l of this investigation was to clarify further the nature of the free recall impairment and to determine the relative contribution of verbal retrieval and encoding processes. Seventy-six patients who underwent left (n = 46) or right (n = 30) ATL were classified according to the p resence or absence of hippocampal sclerosis and pre- to postoperative changes in free recall, cued recall and recognition memory for verbal material were examined. Surgically induced free recall impairments wer e selectively associated with resection of nonsclerotic left hippocamp us and represented a 29-35% decline in verbal learning ability. These free recall deficits were due to postoperative impairment in verbal en coding efficiency, not retrieval difficulties. Assessment of false pos itive recognition errors indicated that resection of nonsclerotic left hippocampus selectively impaired the ability to encode stimulus uniqu eness within correct semantic fields. The clinical and theoretical sig nificance of these results are discussed. Copyright (C) 1996 Elsevier Science Ltd.