Environmental enrichment enhances spatial cognition in rats by reducing thigmotaxis (wall hugging) during testing

Rats, Rattus norvegicus, housed with 'environmental enrichment' do better in tests of spatial cognition than rats housed in barren cages. The leading hypothesis is that exposure to `social and inanimate complexity' leads to better cognitive-processing abilities, which directly enhances performance in a spatial task. However, enrichment is associated with reduced stress responses and anxiety in novel or acutely stressful situations (cognitive tasks are typically both). Therefore, a plausible alternative hypothesis is that experience of enrichment indirectly enhances performance by reducing a rat's anxiety levels during cognitive testing. We found that, irrespective of sex, enriched rats outperformed barren-housed rats in the Morris water maze. However, after accounting for the effects of thigmotaxis (a behavioural anxiety measure during testing), there was no significant difference in performance between enriched and barren-housed rats. Enriched rats were simply less thigmotactic and this indirectly improved their performance. This was true for both males and females. We conclude that enrichment reduces anxiety outside the home cage, in a cognitive test situation, and, subsequently, the cognitive benefits of enrichment occur because enriched animals are less anxious during cognitive testing. (C) 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.