Like I told you in this post here, our brain begins to shrink in our thirties and continues to do so progressively faster throughout our lifespan. One brain-structure that seems to be especially sensitive to the impact of aging is the hippocampus.
The hippocampus is a part of the cerebral cortex, belongs to the limbic system and is underneath the cortical surface in the medial temporal lobe on each side of the brain. Research has shown that hippocampal volume decreases only slightly before the age of 50, but the largest decline occurs after the age of 60. One study showed the hippocampal decline to be 6% in each decade between the age of 50 and 80.
Functions of the hippocampus
Henry Molaison is probably the most intensively studied medical case in history (known as patient HM in the medical literature). HM’s hippocampus was surgically removed in each cerebral hemisphere in an attempt to relieve him from epileptic seizures. He was 27 years old at the time. From then on until his death at the age of 82, HM was unable to form any new conscious memories but kept his earlier memories. HM was unable to learn new words, new facts or new faces and every experience he had was forgotten as soon as he turned away from it. Sadly, for him time had stopped. HM’s case and other patients who have sustained damage to their hippocampi show clearly that hippocampus plays an important role in the formation of new autobiographical memories, but does not play a role in keeping or retrieving older memories.
Hippocampus has also been implicated in spatial navigation. Lots of studies have shown that neurons in the rat hippocampi show activity related to the rat’s location within its environment. O’Keefe and Nadel (1978) theorized that the hippocampus might act as a cognitive map, some kind of neural representation of the layout of the environment. A famous study on taxi drivers in London showed that a part of the hippocampus was larger in taxi drivers than in the general public, and that more experienced drivers had bigger hippocampi. The more time spent as a taxi driver, the bigger was the volume of the right hippocampus.
Education and hippocampal size
As I mentioned earlier, several studies have shown an age-related decline in the size of the hippocampus. A brand-new research indicates that this trend is not the same across all levels of socioeconomic status. In a sample of 275 individuals, aged 17 to 87, MRIs showed that years of education moderated the effect of age on hippocampal size. Among people 35 years and older, those with less education tended to have a marked age-related decrease in hippocampal size. The age-related reductions were less pronounced among more highly educated individuals.
This result is in accordance with the fact that the first pathological signs of Alzheimer’s disease in the brain are typically found in or near the hippocampus and the first behavioral signs of the disease are accordingly, memory problems and disorientation. Some studies have shown that Alzheimer’s disease is less prevalent among those with higher educational levels. Possibly the greater hippocampal size of those with higher education can present itself as some kind of cognitive reserve, i.e. the clinical symptoms of the disease (memory problems) may not manifest themselves until higher levels of pathology in the brain is reached.
The effect of stress on hippocampal size: Stress and hippocampal size
The effect of excercise on brain volume decline: Cardiovascular exercise is probably the simplest way to protect your brain against the influence of aging
Here are four videos from youTube that show very clearly the devastating effects of a damaged hippocampus. Clive Wearing thinks he is seeing his wife for the first time in years, every time he sees her:
Here is a link to Deborah Wearing’s book about her husband:
Raz, N., Gunning-Dixon, F., Head, D., Rodrigue, K. M., Williamson, A. og Acker, J. D. (2004). Aging, sexual dimorphism, and hemispheric asymmetry of the cerebral cortex: Replicability of regional differences in volume. Neurobiology of Aging, 25 (3), 377-396.
O’Keefe, J; Nadel L (1978). The Hippocampus as a Cognitive Map. Oxford University Press.
Maguire, E.A., Gadian, D.G., Johnsrude, I.S., Good, C.D., Ashburner, J., Frackowiak, R.S., Frith, C.D. (2000). Navigation-related structural change in the hippocampi of taxi drivers. Proceedings of the National Academy of Sciences 97 (8): 4398–403.
Noble, K. G., Grieve, S. M., Korgaonkar, M. S., Engelhardt, L. E., Griffith, E. Y., Williams, L. M. o.fl. (2011). Hippocampal volume varies with educational attainment across the life-span. Frontiers in Human Neuroscience, 6.