Age differences in within-session habituation of exploratory behavior: effects of stimulus complexity.

The effects of age on the habituation of exploratory behavior of 8-month- and 28-month-old male C57BL/NNia mice were examined under three different stimulus complexity conditions. Increases in the degree of stimulus complexity resulted in an attenuation of between-session habituation and an initial disruption of within-session habituation by 8-month-old mice. Although increases in stimulus complexity also resulted in an increase in the overall level of exploration by aged mice, stimulus complexity was not found to have a systematic effect on between- or within-session habituation by aged mice. No between-session habituation was observed in aged mice under any of the stimulus complexity conditions. Further, aged mice exhibited significant within-session increases, rather than decreases, in exploration under each stimulus complexity condition. This disruption of within-session habituation in aged mice was found to persist over four daily test sessions. In view of the specific patterns of exploration by aged mice, the disruption of within-session habituation was attributed to age-related differences in reactivity to the arousal-inducing properties of novel stimuli.