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Abstract:
We have investigated the cognitive capacity of visual working memory in two macaque monkeys performing a visual matching to sample task at four different load conditions. Our objectives were, first, to determine whether monkeys can generalize without prior training from a simple to a more demanding task and, second, through testing cognitive limits, uncover possible differences in cognitive strategies.
Both monkeys were trained to master (performance >80% correct responses) a short term memory task at load 1 (L1) which required the monkey to indicate by differential button presses whether test stimuli matched or did not match sample stimuli. We then tested their ability to resolve the task at higher load conditions (L2-L4) which were subsequently introduced during a single session. Our results confirm that both monkeys were able to generalize from the simple L1 to higher load conditions. Even in the cognitively more demanding non-match trials, in which the monkey needs to check the internal representation of samples exhaustively during test stimulus processing, both monkeys performed significantly different from chance level from the first session on. We also found inter-individual differences between monkeys: monkey J, who was trained for several years more expressed balanced performance in match and non-match trials, even under high load conditions, but needed more time to solve the task (mean RT = 747 ms). Monkey L, who was trained for only two years, displayed significantly faster reaction times (mean RT=594 ms). While performance was good in match trials (L3 76.51, L4 75.22%), he reached cognitive limits in the non match condition already at L3 (57.0%, L4 51.18%).
These results suggest that macaque monkeys are able to generalize impromptu from a simple to a more demanding visual working memory task. In testing cognitive limits we observed that the monkeys may have acquired different strategies during extended training periods, which are reflected in different reaction times and performance levels and which influence the way in which individuals can cope with sudden behavioral challenges. We conclude that untrained task variations are a promising alternative for investigating non-human primate cognition which allow for the study of neuronal mechanisms of cognitive strategies in primates.
