Rate of Forgetting in H.M: 6-Month Recognition

The picture-recognition performance of H.M. and six control subjects was evaluated 6 months after initial learning, using materials from an earlier study in which H.M. received additional study time in order to equate his yes-no and delayed-match-to-sample (DMS) performance 10 min after learning to that of control subjects. In the study detailed here, 6-month recognition performance was assessed with no intervening exposure to the target items. H.M. performed at chance levels when tested using the standard yes-no recognition procedure. When the yes-no procedure was modified so that distractor stimuli required positive responses, H.M.'s performance was comparable with that of control subjects. In addition, H.M.'s DMS and delayed-nonmatch-to-sample (DNMS) performance were comparable with that of control subjects 6 months after learning. Conclusions regarding H.M.'s 6-month recognition performance are thus dependent on the procedures used to assess memory.