Role of the hippocampus in memory

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Drawing of the neural circuitry of the rodent hippocampus. S. Ramón y Cajal, 1911.

Psychologists and neuroscientists dispute the precise role of the hippocampus, but generally agree that it has an essential role in the formation of new memories about personally experienced events (episodic or autobiographical memory). Some researchers prefer to consider the hippocampus as part of a larger medial temporal lobe memory system responsible for general declarative memory (memories which can be explicitly verbalized—these would include, for example, memory for facts in addition to episodic memory).

Some evidence exists that, although these forms of memory often last a lifetime, the hippocampus ceases to play a crucial role in the retention of the memory after a period of consolidation. Damage to the hippocampus usually results in profound difficulties in forming new memories (anterograde amnesia), and normally also affects access to memories prior to the damage (retrograde amnesia). Although the retrograde effect normally extends some years prior to the brain damage, in some cases older memories remain - this sparing of older memories leads to the idea that consolidation over time involves the transfer of memories out of the hippocampus to other parts of the brain. However, experimentation has difficulties in testing the sparing of older memories, and in some cases of retrograde amnesia the sparing apparently affects memories formed decades before the damage to the hippocampus occurred, so its role in maintaining these older memories remains controversial.

Damage to the hippocampus does not affect some aspects of memory such as the ability to learn new skills (playing a musical instrument, for example), suggesting that such abilities depend on a different type of memory (procedural memory) and different brain regions. And there is evidence (e.g. O'Kane et al 2004) that patient HM (who had his medial temporal lobes removed bilaterally as a treatment for epilepsy) can form new semantic memories.

Role in spatial memory and navigation

Some evidence implicates the hippocampus in storing and processing spatial information. Studies in rats have shown that neurons in the hippocampus have spatial firing fields. These cells are called place cells. Some cells fire when the animal finds itself in a particular location, regardless of direction of travel, while most are at least partially sensitive to head direction and direction of travel. In rats some cells, termed splitter cells, may alter their firing depending on the animal's recent past (retrospective) or expected future (prospective). Different cells fire at different locations, so that by looking at the firing of the cells alone, it becomes possible to tell where the animal is. Place cells have now been seen in humans involved in finding their way around in a virtual reality town. The findings resulted from research with individuals who had electrodes implanted in their brains as a diagnostic part of surgical treatment for serious epilepsy.

The discovery of place cells led to the idea that the hippocampus might act as a cognitive map — a neural representation of the layout of the environment. Recent evidence has cast doubt on this perspective, indicating that the hippocampus might be crucial for more fundamental processes within navigation^{[How to reference and link to summary or text]}. Regardless, studies with animals have shown that an intact hippocampus is required for simple spatial memory tasks (for instance finding the way back to a hidden goal).

Without a fully functional hippocampus humans may not successfully remember the places they have been to and how to get where they are going. Researchers believe that the hippocampus plays a particularly important role in finding shortcuts and new routes between familiar places. Some people exhibit more skill at this sort of navigation than do others, and brain imaging shows that these individuals have more active hippocampi when navigating.

London's taxi drivers must learn a large number of places — and know the most direct routes between them (they have to pass a strict test, The Knowledge, before being licensed to drive the famous black cabs). A study at University College London showed that part of the hippocampus is larger in taxi drivers than in the general public, and that more experienced drivers have bigger hippocampi. It may be that having a bigger hippocampus helps you to become a cab driver. It also seems that finding shortcuts for a living may make your hippocampus grow.

A study on rats at Indiana University suggested that the sexual dimorphism in the hippocampus morphology is tied to a sexual dimorphism in repeated maze performance. Males seem to be better at contexualizing their whereabouts because they have more hippocampus to work with.

Key texts

Papers

• Alkire, M. T., Haier, R. J., Fallon, J. H., and Cahill, L. (1998). Hippocampal, but not amygdala activity at encoding correlates with long-term, free recall of nonemotional material , Proceedings, National Academy of Sciences, USA 95, 14506-10.
• Barbizet, J. (1963). Defect of memorizing of hippocampal-mammillary origin: a review , Journal of Neurology and Neurosurgery Psychology 26 ,127-3S.
• Broadbent N.J., Squire L.R. and Clark R.E. (2004). Spatial memory, recognition memory, and the hippocampus. Proceedings of the National Academy of Sciences of the United States of America. 101 (40): 14515-20. DOI:10.1073/pnas.0406344101.
• Da Cunha, C., Wolfman, C., Huang, C., Walz, R., Koya, R., Bianchin, M., Medina, J. H. and Izquierdo, I. (1991). Effect of posttraining injections of flumazenil into the amygdala, hippocampus and septum on retention of habituation and of inhibitory avoidance in rats , Brazilian Journal of Medical and Biological Research 24 301-6.
• Fortin, N. J., Agster, K. L. and Eichenbaum, H. B. (2002). Critical role of the hippocampus in memory for sequences of events , Nature Neuroscience 5, 458-62.
• Gould E, Beylin A, Tanapat P, Reeves A, Shors TJ (1999) Learning enhances adult neurogenesis in the hippocampal formation. Nature Neurosci 2: 260-265.
• Izquierdo, I., Quillfeldt, J. A., Zanatta, M. S., Quevedo, J, Schaeffer, E., Schmitz, P. K. and Medina J. H. (1997). Sequential role of hippocampus and amygdala, entorhinal cortex and parietal cortex in formation and retrieval of memory for inhibitory avoidance in rats , European Journal of Neuroscience 9, 786-93
• McClelland, J. L., McNaughton, B. L., and O Reilly, R. C. (1995). Why there are complementary learning systems in the hippocampus and neocortex: insights from the successes and failures of connectionist models of learning and memory , Psychological Review 102, 419-57.
• Maguire, E.A., et al (2000). Navigation-related structural change in the hippocampi of taxi drivers. Proceedings of the National Academy of Sciences of the United States of America. 97 (8): 4398-4403. DOI:10.1073/pnas.070039597.
• Maguire E. A., Frackowiak, R. S. J. and Frith, C. D. (1997). Recalling routes around London: activation of the right hippocampus in taxi drivers , Journal of Neuroscience 17, 7103-10.
• Maguire, E. A., Gadian, D. G., Johnsrude, I. S., Good, C. D., Ashburner, J., Frackowiak, R. S. J., and Frith, C. D. (2000). Navigation related structural change in the hippocampi of taxi drivers , Proceedings, National Academy of Sciences, USA 97 4398-403
• Mizuno K. and Giese K.P. (2005). Hippocampus-dependent memory formation: do memory type-specific mechanisms exist?. Journal of Pharmacological Sciences. 98 (3): 191-7. DOI:10.1254/jphs.CRJ05005X.
• Packard, M. G. and McGaugh, J. L. (1996). Inactivation of hippocampus or caudate nucleus with lidocaine differentially affects expression of place and response learning , Neurobiology of Learning and Memory 65, 65-72.
• Packard, M. G, Cahill, L. and McGaugh, J. L. (1994). Amygdala modulation of hippocampal-dependent and caudate nucleus-dependent memory processes , Proceedings, National Academy of Sciences, USA 91, 8477-81
• Packard, M. G., and Teather, L. (1998). Amygdala modulation of multiple memory systems: Hippocampus and caudate-putamen , Neurobiology of Learning and Memory 69, 163-203.
• Packard, M. G. (1999). Glutamate infused post-training into the hippocampus or caudate-putamen differentially strengthens place and response

learning , Proceedings, National Academy of Sciences, USA 96, 12881-6.

• Teng, E. and Squire, L. R., Memory for places learned long ago is intact after hippocampal damage , Nature 400 (r9991, 675-7.
• Teyler, T. J. and DiScenna, P. (1986). The hippocampal memory indexing theory , Behavioral Neuroscience 100 147-54;

Additional material

Papers

• Scoville, W. B. and Milner, B.,(1957). Loss of recent memory after bilateral hippocampal lesions , Journal of Neurological Neurosurgery and Psychiatry 20 11-21.
• Bunsey, S., and Eichenbaum, H. (1960). Conservation of hippocampal memory function in rats and humans , Nature 379, 255-7

External links

• • Brain Systems Underlying Declarative and Procedural Memories in "Neuroscience" second edition by Dale Purves, et al (2001) Published by Sinauer Associates, Inc. ISBN 0-87893-742-0

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