Double dissociation of social and environmental stimulation on spatial learning and reversal learning in rats.

Environmental enrichment induces structural and biochemical changes in the brains of mammals that correlate with improved learning and memory. Research in rats suggests that social compared to inanimate stimulation might affect behavior differently, by acting upon dissociable neural substrates. Here we tested this hypothesis at the behavioral level by examining whether social and inanimate stimulation affect spatial memory formation and non-spatial discrimination reversal learning selectively. Spatial memory formation is known to depend on hippocampal-neocortical pathways, whereas reversal learning depends primarily on prefrontal cortico-striatal pathways. Male Lister hooded rats were housed singly or in groups of three in either small barren or large enriched cages, from weaning onwards. After 10 weeks of differential housing, spatial learning and memory were examined in the Morris water maze, followed by a series of tactile and odour discriminations, including discrimination reversal, in a two-choice discrimination task. Regardless of inanimate stimulation, social deprivation affected neither the acquisition of simple or complex discriminations, nor spatial memory formation, but was associated with impaired reversal learning in the two-choice discrimination task. By contrast, inanimate deprivation, regardless of social stimulation, affected neither acquisition nor reversal of two-choice discriminations, but selectively delayed the acquisition of spatial memory in the Morris water maze. This is the first demonstration of a double dissociation of early social and inanimate stimulation on two distinct behavioural functions that are mediated by dissociable underlying neural pathways. These findings strengthen the view that social and inanimate stimulation act, at least in part, upon dissociable neural substrates. Copyright 2003 Elsevier B.V.