Spared short-term memory in monkeys following medial temporal lobe lesions is not yet established: a reply to Alvarez-Royo, Zola-Morgan and Squire

Abstract

It is important to know whether or not short-term memory (STM) is preserved in monkeys, as sometimes claimed, following lesions to medial temporal lobe that disrupt longer term memory. As examined herein, the magnitude of the longer term deficit in the delayed matching-to-sample task is well correlated with slower learning at short delays. This learning deficiency with short delays can be severe, e.g., failure to reach criterion despite ten times the number of trials required by control animals, yet the same animals can perform some visual discriminations normally. Such slow learning may thus be most parsimoniously attributed to a STM deficit. Studies are also reviewed which compare delayed (non)matching-to-sample performance in lesioned monkeys at short and long delays. For those groups that received equal training at all delays, the short-term deficit is as large as the longer term deficit. For those groups trained only at the short delay the short-term deficit is small. Caveats for future studies are discussed.