H.M., word knowledge, and aging: support for a new theory of long-term retrograde amnesia.

This study develops a new theory of long-term retrograde amnesia that encompasses episodic and semantic memory, including word knowledge. Under the theory, retrograde amnesia in both normal individuals and hippocampal amnesics reflects transmission deficits caused by aging, nonrecent use of connections, and infrequent use of connections over the life span. However, transmission deficits cause severe and irreversible retrograde amnesia only in amnesics who (unlike normal persons) cannot readily form new connections to replace nonfunctioning ones. The results of this study are consistent with this theory: For low-frequency but not high-frequency words, a famous "hippocampal amnesic" (H.M.) at age 71 performed worse than memory-normal control participants in a lexical decision experiment and a meaning-definition task (e.g., What does squander mean?). Also as predicted, H.M.'s lexical decision performance declined dramatically between ages 57 and 71 for low-frequency words, but was age-invariant for high-frequency words.