Memory and the Medial Temporal Lobe
The next article in our series is a report published in 1957 by Scoville and Milner1 describing the effects of bilateral medial temporal lobe resection on memory function. One of the subjects in the report, H. M., is considered the index case of amnesia resulting from temporal lobe injury.2
H. M. suffered with frequent generalized seizures that were not adequately controlled by anticonvulsant therapy. The surgery was a radical experiment that was felt to be justified by the patient's incapacitation and lack of response to available treatment. H. M.'s bilateral medial temporal lobe resection included the hippocampal formation and adjacent structures including most of the amygdaloid complex and entorhinal cortex.3
The effect of the surgery on his memory was dramatic and immediately evident. Formal neuropsychological testing demonstrated dense amnesia for all events following the surgery and intact memory for events that occurred prior to 19 months preceding surgery.
Scoville and Milner concluded that the hippocampus was crucial for the formation of memory to recent events, and they had the prudence to consider that adjacent structures such as the amygdala might also be involved. Amnesia from ischemic injury limited only to the hippocampus has also been found to impair acquisition of new memory, but not to the severe degree experienced by H. M.2
1 Scoville WB, Milner B: Loss of recent memory after bilateral hippocampal lesions. J Neurol Neurosurg Psychiatry 1957; 20:11–21Crossref, Medline, Google Scholar
2 Squire LR, Zola-Morgan S: The medial temporal lobe memory system. Science 1991; 253(5026):1380–1386Google Scholar
3 Corkin S, Amaral DG, González RG, et al: H. M.'s medial temporal lobe lesion: findings from magnetic resonance imaging. J Neurosci 1997; 17:3964–3979Google Scholar