Learning performance varies with brain weight in heterogeneous mouse lines.

Three lines of unselected heterogeneous stocks of mice were tested for learning and activity in active avoidance acquisition and extinction, water-maze discrimination learning and reversal learning, operant discrimination, and passive avoidance acquisition tasks. Ambulation in the open field was also measured. Small to moderate correlations (absolute values of .17 to .42) between brain weight and learning measures were obtained for all tasks except passive avoidance. A moderate correlation between brain weight and activity was found only in the open field (r = .39). Partialing out differences in operant level and body weight and learning performance. When ambulation in the open field was partialed out, however, all correlations between brain weight and learning performance decreased. Previous research has suggested a positive relation between brain weight and learning scores across mammalian orders and species. The results reported here extend this relation to within-species variation in brain size. The results also emphasize the limitations of estimating genetic associations between brain and behavior from comparisons between small numbers of inbred strains or selected lines.