Amnesia after transection of the fornix in monkeys: long-term memory impaired, short-term memory intact.

Monkeys in whom the fornix had been transected and controls were trained on two versions of delayed non-matching to sample (DNMS). On DNMS-LSS the stimulus pair for each trial was selected from a large set of junk objects so that the animals saw particular stimuli infrequently. On DNMS-RS the same two stimuli were presented repeatedly. Memory for the sample was assessed with retention intervals of 10, 70 and 130 sec spent either in the dark or with levels of illumination designed to promote interference effects. On DNMS-LSS the lesioned animals were impaired, being more susceptible than controls to increases in the retention interval. However, neither group was particularly sensitive to interference. In contrast, on DNMS-RS the lesioned monkeys were unimpaired, both groups showing a marked decrement in performance with increasing retention interval and a high sensitivity to the effects of interference. These data suggest that monkeys possess a short-term memory which is intact after transection of the fornix and a long-term memory which is not.