Delayed matching by fornix-transected monkeys: The sample, the push and the bait

In the acquisition event of ordinary delayed matching to sample the monkey sees a sample, displaces it, and finds a food reward underneath; subsequently the retention test consists of a choice between that sample and a distractor, and the strength of the memory laid down by the acquisition event may be assessed by the correctness of choice at the retention test. The present experiments varied the acquisition events and examined the effect of those variations on normal and fornix-transected monkeys' memory. One variation was to proceed as normally but never to bait the sample at acquisition; this variation allowed assessment of the role of the food reward in ordinary matching. Another was to present the sample, baited and to be displaced, as normally, but to present also the distractor, baited but not requiring to be displaced, in a second acquisition event; this “push-match” variation allowed assessment of memory for the displacement. The main result from normal monkeys was that matching to unbaited samples was learned much faster than matching to baited samples. Following fornix transection, final performance levels in matching to baited and to unbaited samples were unimpaired but push-match showed a permanent deficit. These results support earlier indications that fornix-transected monkeys have normal sensory memory but are deficient in the memory of instrumental responses.