Water-maze probe trial, spatial retention score for target zones 1-3 comprising 5, 10 and 10% of the pool surface versus remaining 75% of surface (ANOVA: zone F3,78=36.04 p<0.0001, genotype x zone F3,78=7.807 p=0.0001). Spatial selectivity declined with age in controls and was prematurely reduced in mutants at 9- and 14-months. 