Monkeys with selective damage to the hippocampus are often unimpaired in matching‐to‐sample tests but are reportedly impaired in visual paired comparison. While both tests assess recognition of previously seen images, delayed matching‐to‐sample may engage active memory maintenance whereas visual paired comparison may not. Passive memory tests that are not rewarded with food and that do not require extensive training may provide more sensitive measures of hippocampal function. To test this hypothesis, we assessed memory in monkeys with hippocampal damage and matched controls by providing them the opportunity to repeatedly view small sets of videos. Monkeys pressed a button to play each video. The same 10 videos were used for six consecutive days, after which 10 new videos were introduced in each of seven cycles of testing. Our measure of memory was the extent to which monkeys habituated with repeated presentations, watching fewer videos per session over time. Monkeys with hippocampal lesions habituated more slowly than did control monkeys, indicating poorer memory for previous viewings. Both groups dishabituated each time new videos were introduced. These results, like those from preferential viewing, suggest that the hippocampus may be especially important for memory of incidentally encoded events.
Evidence that the hippocampus is critical for spatial memory in nonnavigational tests is mixed. A recent study reported that temporary hippocampal inactivation impaired spatial memory in the nonnavigational Hamilton Search Task in monkeys. However, several studies have documented no impairment on other nonnavigational spatial memory tests following permanent hippocampal lesions. It was hypothesized that transient, but not permanent, hippocampal disruption produces deficits because monkeys undergoing transient inactivation continue to try to use a hippocampal‐dependent strategy, whereas monkeys with permanent lesions use a nonhippocampal‐dependent strategy. We evaluated this hypothesis by testing five rhesus monkeys with hippocampal lesions and five controls on a computerized analogue of the Hamilton Search Task. On each trial, monkeys saw an array of squares on a touchscreen, each of which “hid” one reward. Retrieving a reward depleted that location and monkeys continued selecting squares until they found all rewards. The optimal strategy is to remember chosen locations and choose each square once. Unlike the inactivation study, monkeys with hippocampal damage were as accurate as controls regardless of retention interval. Critically, we found no evidence that the groups used different strategies, as measured by learning rates, spatial search biases, perseverative win‐stay errors, or inter‐choice distance. This discrepancy between the effect of inactivations and lesions may result from off‐target effects of inactivations or as‐yet‐unidentified differences between the physical and computerized tasks. Combined with previous evidence that hippocampal damage impairs navigational memory in monkeys, this evidence constrains the role of the hippocampus in spatial memory as being critical for navigational tests that likely involve allocentric spatial memory but not nonnavigational tests that likely involve egocentric spatial memory.
more » « less- Award ID(s):
- 1632477
- NSF-PAR ID:
- 10074203
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Hippocampus
- Volume:
- 29
- Issue:
- 2
- ISSN:
- 1050-9631
- Page Range / eLocation ID:
- p. 93-101
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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