Age-related changes in associative learning for olfactory and visual stimuli in rodents.

Memory for olfactory stimuli may be particularly affected by age-related brain changes in humans and may be an early indicator of cognitive impairment and Alzheimer's disease. Studies involving rats have offered insights into impaired cognition in aged animals, but few have examined odor memory. Therefore, it is unclear whether aged rats are a good model for possible age-related changes in odor memory in humans. Young (6-month-old) and old (24-month-old) rats were tested on associative learning tasks involving visual and olfactory stimuli. The first task examined age-related differences in discrimination and reversal learning for olfactory and visual stimuli; the second task utilized an associative contextual learning task involving olfactory and visual cues. Although old rats were able to perform the olfactory and visual discrimination tasks as well as young rats, old rats displayed significant age-related impairment on the reversal learning and contextual learning tasks. The results suggest that aging may have a similar deleterious effect on odor memory in rats and in humans. The findings may have important implications for the selection of memory paradigms for future research studies on aging. In addition, the use of an animal model to investigate the effects of aging on odor memory will allow researchers the ability to investigate how age-related neuroanatomical and neurochemical changes may result in impaired odor memory.

A comparison of discrimination and reversal learning for olfactory and visual stimuli in aged rats.

The present study investigated age-related differences in discrimination and reversal learning for olfactory and visual stimuli in 6-month and 24-month-old rats. Rats were trained to discriminate between two pseudo-randomly selected odors or objects. Once each animal reached a criterion on discrimination trials, the reward contingencies were reversed. Young and aged rats acquired the olfactory and visual discrimination tasks at similar rates. However, on reversal trials, aged rats required significantly more trials to reach the learning criterion on both the olfactory and visual reversal tasks than young rats. The deficit in reversal learning was comparable for odors and objects. Furthermore, the results showed that rats acquired the olfactory task more readily than the visual task. The present study represents the first examination of age-related differences in reversal learning using the same paradigm for odors and objects to facilitate cross-modal comparisons. The results may have important implications for the selection of memory paradigms for future research studies on aging.

Age-related changes in contextual associative learning.

The hippocampus plays a critical role in processing contextual information. Although age-related changes in the hippocampus are well documented in humans, nonhuman primates, and rodents, few studies have examined contextual learning deficits in old rats. The present study investigated age-related differences in contextual associative learning in young (6 mo) and old (24 mo) rats using olfactory stimuli. Stimuli consisted of common odors mixed in sand and placed in clear plastic cups. Testing was conducted in two boxes that represented two different contexts (Context 1 and Context 2). The contexts varied based on environmental features of the box such as color (black vs. white), visual cues on the walls of the box, and flooring texture. Each rat was simultaneously presented with two cups, one filled with Odor A and one filled with Odor B in each context. In Context 1, the rat received a food reward for digging in the cup containing Odor A, but did not receive a food reward for digging in the cup containing Odor B. In Context 2, the rat was rewarded for digging in the cup containing Odor B, but did receive a reward for digging in the cup containing Odor A. Therefore, the rat learned to associate Context 1 with Odor A and Context 2 with Odor B. The rat was tested for eight days using the same odor problem throughout all days of testing. The results showed no significant difference between young and old rats on the first two days of testing; however, young rats significantly outperformed old rats on Day 3. Young rats continued to maintain superior performance compared to old rats on Days 4-8. The results suggest that aging results in functional impairments in brain regions that support memory for associations between specific cues and their respective context.