Mediodorsal Thalamic Lesions Impair Long-Term Visual Associative Memory in Macaques

Six cynomolgus monkeys (Macaca fascicularis) were trained to associate visual stimuli with the delivery of various amounts of food reward. The animals had to choose correctly between pairs of stimuli drawn from a population of 16. Four of these stimuli were associated with 0 reward pellets, four with 1 pellet, four with 2 pellets and four with 4 pellets. Mediodorsal thalamic lesions including the medial part of the mediodorsal nucleus, similar to those which are frequently seen in Korsakoff amnesia, produced a severe impairment in this task. The impairment was seen both in memory for the quantity of reward, as expressed in choices between 1-pellet and 2-pellet stimuli or choices between 2-pellet and 4-pellet stimuli, and also in memory for the qualitative absence or presence of reward, as expressed in choices between 0-pellet and 1-pellet stimuli. The deficit in this task establishes that mediodorsal thalamic lesions in monkeys can impair long-term memory tasks, in addition to their known effects on several short-term memory tasks. The contrast between the present results and those of previous experiments on visual long-term memory in the monkey following mediodorsal thalamic lesions can be related to similar contrasts in studies of lesions in the amygdala, suggesting that the functions of these two structures are related.