Anatomy And Physiology

The Role of Hippocampal Size in Spatial Memory



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Studies in birds, rats, mice, voles and humans all show a connection between the size of the hippocampus and spatial memory such that a larger hippocampus facilitates a higher spatial memory capacity. Comparisons among species of birds that hide/store their food for later use, those whose food is harder to find requiring foresight in where to look for it, and those who do not store and whose food is readily available, have consistently confirmed the difference.

Similar results have been seen in mammals such as kangaroo mice where the species that caches its food in scattered locations by necessity has a better-developed special memory than the species that stores all its food in the burrow and this difference is reflected in the larger hippocampus of the former. Specially bred messenger pigeons with a particular talent for quickly getting to the designated location and back home not surprisingly also show larger hippocampi than other pigeon breeds.

There are interesting special learning and memory differences between sexes observed in rats, mice, and humans, such that males tend to show a higher level of related ability accompanied by appropriately differing hippocampal size. And while some have suggested that the difference is biologically assigned to the sexes, many studies have shown that the difference seems behaviorally mediated such that the sex whose social/behavioral role requires more spatial memory performs better on tests measuring it and also has a larger hippocampus.

Between two species of American voles the one showing a marked special memory and hippocampal difference between males and females is the one where the males’ polygynous nature requires them to keep track of movement patterns of as many females as they can. The vole species where males and females are monogamous show no such differences, while among brown-headed cowbirds the female shows a superior special memory and hippocampal size as it’s her job to monitor nests and movements of other birds in order to plant one of her own eggs to be fed and raised by the unsuspecting pair.

The behavioral connection is further supported by studies with crows whose large hippocampus is only achieved through spatial learning and memory behaviors such that birds that were deprived of the usual food storage and retrieval behavior and instead fed by feeders in the lab setting had smaller hippocampi then controls allowed their usual feeding methods. Studies with London taxi drivers whose on-the-job daily demands on spatial learning and memory are well above average all show significantly enlarged posterior hippocampi while the anterior part was interestingly significantly smaller compared to the control subjects.

It was further concluded that the differences seen were not inborn but rather the direct result of the driver’s career choice as the changes in the right (but not left) hippocampus became more pronounced the longer the taxi drivers had been on the job. Cases such as that of the famous patient H.M. who has lost his ability to make new long term memories after having his entire hippocampus surgically removed further testify of this brain region’s crucial involvement in forming long term declarative memories (but not their storage), and this certainly includes spatial learning.

Source:

Breedlove, S.M., Rosenzweig, M.R., & Watson, N.V. (2007). Biological Psychology: An Introduction to Behavioral, Cognitive, and Clinical Neuroscience. Fifth Edition.  Sunderland, MA: Sinauer Associates, Inc.

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