Severity of spatial learning impairment in aging: Development of a learning index for performance in the Morris water maze.

Abstract

The Morris water maze task was originally designed to assess the rat's ability to learn to navigate to a specific location in a relatively large spatial environment. This article describes new measures that provide information about the spatial distribution of the rat's search during both training and probe trial performance. The basic new measure optimizes the use of computer tracking to identify the rat's position with respect to the target location. This proximity measure was found to be highly sensitive to age-related impairment in an assessment of young and aged male Long-Evans rats. Also described is the development of a learning index that provides a continuous, graded measure of the severity of age-related impairment in the task. An index of this type should be useful in correlational analyses with other neurobiological or behavioral measures for the study of individual differences in functional/biological decline in aging.A test of spatial learning introduced over a decade ago by Richard Morris (Morris, 1981;Morris, Garrud, Rawlins, & O'Keefe, 1982) has become widely used in neurobiological studies of limbic-cortical function and in the characterization of cognitive decline in aged rats. Originally devised as a test of "place" learning, the water maze task permits a number of variations that have made it useful for isolating cognitive deficits apart from nonspecific impairments in sensorimotor function. The purpose of the present article is to describe some new methods for behavioral analysis that were developed to supplement those measures commonly used for the water maze. As a framework for presenting the new methods, the following introduction provides a brief discussion of the traditional analysis of performance in this task. Measures Traditionally Used for Behavioral Analysis in the Water MazeThe Morris maze apparatus consists of a large, circular pool filled with water that has been made opaque through the addition of powdered milk or some other substance. In the typical "hidden-platform" version of the task, rats are trained to find a camouflaged escape platform Correspondence concerning this article should be addressed to Michela, Gallagher, Department of Psychology, CB# 3270, Davie Hall, Chapel Hill, North Carolina 27599-3270. HHS Public AccessAuthor manuscript Behav Neurosci. Author manuscript; available in PMC 2017 October 13. Author ManuscriptAuthor Manuscript Author ManuscriptAuthor Manuscript that is positioned just below the water surface. The location of this platform remains constant from trial to trial. Because there are no local cues that mark the position of the platform, the rat's ability to locate it efficiently depends on the rat's use of a configuration of extramaze cues surrounding the pool. Indeed, rats can learn to swim directly to the escape platform within relatively few training trials from any of a number of start locations at the perimeter of the pool. Learning is reflected in shorter latencies to escape and by decreases in the length of the path that the rat tra...

Spatial reference and working memory across the lifespan of male Fischer 344 rats

“…In agreement with previous studies, the cumulative search error measure appeared to be most sensitive for detecting age-related differences (particularly at middle-age). However, pathlength may be the optimal and most conservative choice for such analyses in aged F344 rats due to the lack of confounds with swim speed [19]. Barnes et al (1997) reported that, early in training, when performance on individual training trials was plotted on a frequency histogram, both young and aged rats exhibited a strong bimodality in performance such that on some trials rats of both ages had short swim paths whereas on other trials they had long swim paths.…”

“…To calculate cumulative search error, the rat's distance from the platform was sampled 10 times/s and these distances were averaged into 1 s bins. Cumulative search error is the sum of these 1 s bins minus the optimal path from the start location to the platform [19]. Additional measures of performance (e.g., latency, swim speed) also were recorded.…”

“…The Spatial Learning Index is calculated by weighting and summing mean search error from the interpolated probe trials to provide an overall measure of spatial learning ability for each rat. Weights for each probe trial were derived by dividing the mean search error in the young group on probe trial 1 by the mean search errors on probe trials 2-4 [19]. Using data from this F344 study population, the weights assigned to each probe trial were: probe trial 2: 1.25; probe trial 3: 1.60; probe trial 4: 1.70.…”

“…Indeed, both reference memory (memory for information that is held constant over time) and working memory (the ability to remember, usually for a relatively brief period of time, information that must be distinguished from previously learned similar information) are adversely affected during the aging process [17,[25][26][27]29,32,34,36]. Importantly, such impairments in medial temporal lobe functioning are not an inevitable consequence of the aging process [5,7,19,45]. At advanced chronological ages, a spectrum of mnemonic abilities exists such that some aged individuals perform on par with young adults whereas others experience deficits ranging from mild to profound.…”

“…Using a reference memory version of the Morris water maze, several naturalistic rat models of aging have been used to mimic the individual variability in mnemonic function associated with the medial temporal lobe system present in the human population [19,24,37]. Moreover, a variety of functional deficits in these structures and changes in neuronal encoding properties correlate with individual differences in spatial learning abilities [3,10,12,20,41].…”