Abstract
The spatial water maze is routinely used to investigate hippocampal-dependent spatial memory and the biological mechanisms that underlie variability in cognitive decline during aging. The utility of the task for repeated testing in order to examine the trajectory of cognitive decline and to prescreen animals prior to therapeutic interventions maybe limited due to carryover effects of repeated training. The current study examines the role of carryover effects, as well as the reliability of individual differences, in determining age-related impairment on episodic and reference memory versions of the water maze task. Results indicate that impaired acquisition of episodic spatial information emerges in middle-age and the propensity for impairment increases with advancing age. While learning was variable across animals, acquisition deficits for episodic information were reliable across training sessions in middle-age and aged rats. A significant impairment in the 24~h retention of episodic spatial information was observed in aged animals. When animals were trained to the same location (i.e., reference memory), an impairment was limited to the rate of acquisition in aged animals. However, with continued training, all aged animals were able to acquire a reference memory and no age differences were observed in the 24~h retention of a spatial reference memory. Together, the results point to a progressive impairment in episodic spatial memory with advancing age and suggest that tests of episodic spatial memory are reliable and more sensitive than reference memory for detecting cognitive decline.
Highlights
Episodic memory for information that encompasses a specific temporal and spatial context is vulnerable to agerelated decline in humans (Sharps and Gollin, 1987; Zelinski and Light, 1988; Cherry and Park, 1993; Uttl and Graf, 1993; Spencer and Raz, 1995; Newman and Kaszniak, 2000; de Jager et al, 2002; Iachini et al, 2005)
The utility of the task depends on a PROCEDURAL MEMORY IS INTACT DURING AGING All groups, regardless of age, exhibit better performance for the second training session 10 days following the first training session indicating memory for certain aspects of the task
These carryover effects likely include memory for the procedural aspects of the task, memory for the spatial cues that remain constant across sessions, and the use of a spatial mapping strategy
Summary
Episodic memory for information that encompasses a specific temporal and spatial context is vulnerable to agerelated decline in humans (Sharps and Gollin, 1987; Zelinski and Light, 1988; Cherry and Park, 1993; Uttl and Graf, 1993; Spencer and Raz, 1995; Newman and Kaszniak, 2000; de Jager et al, 2002; Iachini et al, 2005). Episodic spatial memory deficits emerge in middle-age (Jucker et al, 1988; Ando and Ohashi, 1991; Markowska and Savonenko, 2002; Bizon et al, 2009; Kumar and Foster, 2013) With advancing age, this initial deficit may progress toward a more serious cognitive deficit that mimics a hippocampal lesion, including an inability to learn about spatial relationships that remain constant across days of training (i.e., spatial reference memory deficits). Reference memory is trial-independent, and depends on learning the spatial aspects that remain constant This includes the location of the escape platform, which remains in the same spatial location across training sessions
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