Enrichment-dependent differences in novelty exploration in rats can be explained by habituation

In rats, exploratory activity and emotional reactivity towards novel stimul i reflect independent biological functions that are modulated differently b y rearing experiences. Environmental enrichment is known to improve perform ance in exploratory tasks, while having inconsistent effects on emotionalit y. This study examined the effect of environmental enrichment on the behavi our of rats in two exploratory tasks. Male rats were reared under one of fo ur conditions, differing in social and non-social complexity. At 9 weeks of age, exploration of a novel open field, and exploration of novel objects i n the same open field following 24 h habituation, was assessed. Differences in social and non-social complexity of the rearing environment had inconsi stent effects on exploration in the novel open field. In contrast, when rat s were faced with novel objects in an otherwise familiar environment, explo ration habituated faster with increasing stimulus complexity of the non-soc ial environment. The social environment had no effect on this latter test. These findings indicate that environmental enrichment affects exploratory a ctivity primarily through its effect on habituation to novelty. This effect depends on relative stimulus complexity of the rearing environment, but is independent of social factors. The present results further suggest that av ersive tasks can obscure the expression of enrichment-dependent differences in habituation to novelty. (C) 2001 Elsevier Science B.V. All rights reser ved.