RELIABILITY, DISTRIBUTION, AND VALIDITY OF AGE-RELATED COGNITIVE DEFICITS IN THE MORRIS WATER MAZE

In the present study, F-344 rats throughout 1.5 to 26 months of age we re tested in the reference memory version, a moving-platform repeated acquisition version, and in a cued platform version of the Morris wate r maze. The results suggest that: (1) performance in the water maze de clines continuously, beginning at the earliest age, and very closely f its a linear function; (2) there are robust, reliable differences betw een individuals in terms of their performance in the Morris water maze , but chronological age accounts for only a fraction of the variance b etween individuals; (3) there is no evidence of a bimodal distribution among aged rats-there is no distinct subgroup of individuals that per forms so poorly that they are qualitatively different from the majorit y of the population, and distinctions between 'impaired'' and ''unimpa ired'' subjects must be based on arbitrary criteria that may not be co nsistent from one study to the next; (4) age-related deficits in the M orris water maze may not be restricted to learning and memory, but may also include deficits in attention, the ability to process spatial in formation, and/or the ability to develop efficient spatial search stra tegies; and (5) swim distance is the most appropriate measure of cogni tive function in the Morris water maze, but the relationship between t his measure and other measures of noncognitive function make it clear that swim distance may not be a pure measure of cognitive function. Al though the Morris water maze remains a valuable preclinical test with better validity and specificity than many other behavioral tests, meas ures of performance in the Morris water maze should not be considered synonymous with cognitive function. (C) 1997 Academic Press.