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.

The role of the CA3 hippocampal subregion in spatial memory: a process oriented behavioral assessment.

Computational models, behavioral data, and electrophysiological data suggest that the CA3 subregion of the hippocampus may support multiple mnemonic processes critical to the formation and subsequent retrieval of spatial memories. Multiple researchers have proposed that the CA3 subregion contains an autoassociative network in which synaptic connections between CA3 neurons that represent different components of a memory are strengthened via recurrent collateral connections. As a result, it has been suggested that the CA3 autoassociative network may support multiple processes including the formation of spatial arbitrary associations, temporary maintenance of spatial working memory, and spatial pattern completion. In addition, the CA3 subregion has been suggested to be involved in spatial pattern separation. The separation of patterns is hypothesized to be accomplished based on the low probability that any two CA3 neurons will receive mossy-fiber input synapses from a similar subset of dentate gyrus cells. The separation of patterns also may be enhanced by competitive inhibition within CA3 and dentate gyrus. This review will focus on the mnemonic processes supported by CA3 neurons and how these processes may facilitate the encoding and retrieval of spatial information. Although there is growing evidence indicating that the hippocampus plays a role in the processing of nonspatial information as well, the scope of the present review will focus on the role of the CA3 subregion in spatial memory.

Differential effects of normal aging on memory for odor-place and object-place associations.

Odor-place and object-place associative memory were compared in healthy older (over the age of 65) and young (18 to 25 years of age) adults. Twelve spatial locations were defined on a tabletop board. Either six odors or six objects were presented one at a time and each was paired with a location on the board. The participant then was presented with each stimulus individually and asked to place it in its paired location. Older adults showed impaired odor-place associative memory but unimpaired object-place memory compared to young adults. Item recognition memory for the individual stimuli or locations used on the associative memory task was similar in both groups. The results suggest that odor-place associative memory is particularly affected by age-related brain changes.

Effect of encoding condition on source memory for odors in healthy young and older adults.

BACKGROUND: Source memory has been shown to be more affected by aging than item memory. Aging also has been shown to result in impairments in odor memory. OBJECTIVE: The aim of the study is to examine the effects of explicit encoding instructions on source memory for olfactory stimuli in healthy older and young adults. METHODS: Source and item memory for odors were assessed in two conditions. In the uninformed condition, young (18-30) and older adults (65+) were presented with 16 odors by two sources (male and female) without instruction at encoding and no warning of a subsequent memory task. In the informed condition, young and older adults were instructed to encode the stimuli and their respective sources. To assess item memory, the participant was presented with an odor from the task and a new odor and was asked to indicate which odor had been presented previously. On source memory trials, the participant was presented with an odor from the task and was asked to indicate whether the male or female presented the odor. RESULTS: A 2 x 2 x 2 analysis of variance revealed that older adults were significantly impaired relative to young adults on the source memory trials in both the uninformed and informed conditions, F(1, 52) = 18.15, p < 0.001. However, older adults matched the performance of young adults on item memory trials, regardless of encoding condition. CONCLUSIONS: Even with conscious effort to encode the sources associated with the odors, older adults show significant source memory impairments. The mnemonic processes used to integrate contextual source information with item memory during encoding may fail to initiate due to the amount of effort required to encode the olfactory stimulus. The difficulty of encoding and subsequently retrieving the source may be increased due to the difficulty of encoding the odors. The results demonstrate the robust effects of aging on source memory for odors.

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.