Abstract
In both humans and monkeys not all individuals show the same rate of age-related cognitive decline. One important factor to influence the rate of decline is extended exposure to elevated levels of glucocorticoids, which play a central role in the response to stress. Furthermore, studies with humans have suggested that
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the social environment influences such exposure to glucocorticoids. However, the complex social structure of human society complicates the investigation of the effects of the life time social environment on cognitive decline. The stable social structure of many monkey species does allow this investigation. The colony of long-tailed macaques (Macaca fascicularis) of the Utrecht University is especially suited for this. The social dominance relations of the females are stable and have been known for decades. Moreover, a low dominance status has been related to exposure to elevated levels of glucocorticoids. We aimed to confirm previously found results supporting the hypothesis that a history of low dominance is related to an acceleration of age-related cognitive decline.
Fourteen female monkeys of various age and dominance classes were tested in a four-choice discrimination-reversal test. Subjects were temporarily isolated from their group based on voluntary co-operation, and tested with a computer operated touch screen.
The results do not support this hypothesis. A number of theoretical and methodological issues are discussed in relation to this result. Further analyses revealed that high dominance status was related to relatively low levels of attention, which in turn were related to poor cognitive performance. This finding confounded the possibility to find the hypothesised effect. In an additional study the social groups were exposed to a loud noise to assess individual differences in the temperamental aspect of anxiety, which were then related to differences in behaviour and performance in the previous cognitive test. More anxious subjects paid less attention to the task and had a poor performance. However, this effect was found only for the experimentally naïve subjects.
In the second part of this thesis the effects of stimulus parameters on cognitive performance are investigated. We hypothesised that biologically relevant 2-dimensional stimuli are more easily discriminated than pictures that the animals has no prior associations with. Also, bright and contrasting colours may improve performance compared to dull colours. In a relatively difficult cognitive task bright colours and high levels of contrast enhanced performance, while no such effect was found in a similar but less difficult task. Biological relevance of stimuli did not influence performance.
This thesis underlines the possibilities and difficulties that testing individual cognitive performance of socially housed monkeys offer. Social dominance relations and individual characteristics strongly influence cognitive performance and complicate the study of other differences in cognitive abilities. Moreover, the parameters of the stimuli are important to consider in the design of a study as these may profoundly affect performance in more difficult tasks.
It is recommended that in future studies a situation should be created in which the test is an integrated part of the animals' daily foraging behaviour, allowing larger sample sizes and longer study periods.
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