| LOSS OF RECENT MEMORY AFTER BILATERAL HIPPOCAMPAL LESIONS | |
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| H. M.'s medial temporal lobe lesion: findings from magnetic resonance imaging | |
| Medial Temporal Lobe Activity Associated with Active Maintenance of Novel Information |
1. Bilateral hippocampal removal leads to profound anterograde amnesia
• Patients who had extensive bilateral medial temporal lobe resections (especially H.M.) exhibited a severe and persistent inability to form new memories, despite intact general intelligence and personality.
• This condition is referred to as anterograde amnesia, the inability to form new long-term declarative memories.
2. Hippocampus and hippocampal gyrus are critical for recent memory
• Memory loss occurred only when the resection included the hippocampus and hippocampal gyrus bilaterally.
• Patients whose resections spared these structures (e.g., limited to uncus and amygdala) retained normal memory.
3. Severity of memory loss correlated with lesion extent
• The more posterior the hippocampal resection, the more severe the memory impairment.
• H.M. and two other patients who had ~8 cm resections showed the most drastic memory deficits.
4. Early memories and IQ remained intact
• Despite severe anterograde amnesia, patients retained early-life memories, language, reasoning, and motor skills.
• For example, H.M. had a Wechsler IQ of 112 post-operatively, even as his memory quotient was only 67.
5. This study established the medial temporal lobe’s role in declarative memory