Monkey See, Monkey Don't Remember

Neurogenesis declines in the aging primate brain

Marmoset trying to remember if this bug was tasty or not.

Image: Gerald Durell
We’ve all heard that you can’t teach an old marmoset new tricks, but researchers now understand why in a study that hopes to narrow in on the cause of neurodegenerative illness. Writing in the early edition of Proceedings of the National Academy of Sciences, Benedetta Leuner and colleagues at Princeton determined that neurogenesis, or the growth of new neurons, diminishes once these South American monkeys reach sexual maturity. This decline in neurogenesis is particularly noticeable in the hippocampus, a region of the brain central to learning and memory.

This research has been demonstrated previously in rats (I participated in some of this work as an undergraduate student) but there has always been a question as to whether or not the more complex primate brains undergo the same phenomenon:

As the authors reported:

No previous studies have investigated whether primates exhibit a similar decline in hippocampal neurogenesis with aging. . . These data demonstrate that a substantial decrease in neurogenesis occurs before the onset of old age in the adult marmoset brain, suggesting the possibility that similar alterations occur in the human brain.

This is why languages are so difficult to learn as an adult while children seem to absorb them readily. Without the growth of new neural connections in the hippocampus the accumulation of information slows dramatically.


Imo, the Japanese macaque who invented potato washing,
a trait that was easily picked up by the young but not older individuals.

Image: Franz de Waal
These results have also been shown in the adoption of new cultural traditions in apes and monkeys. In the famous “potato washing” findings in Japanese macaques, it was younger females who were quickest to learn the new technique while the older males sat around on the periphery wondering what was wrong with the kids these days.

However, the authors point out the situation isn't a hopeless descent from vigorous mental youth to gum-smacking confusion in old age (although Noam Chomsky should be evidence enough of that). As author Elizabeth Gould explains to Science Daily:

"This news isn't entirely negative, though it seems to be at first glance," Gould said. "The silver lining here is that neurogenesis continues long past puberty and does not stop entirely, even in older primates. What's more, it can be stimulated with experience."

So keep keep struggling through Dostoevsky and Foucault. The mental effort will pay off in the long run.

Reference:

Benedetta Leuner, Yevgenia Kozorovitskiy, Charles G. Gross and Elizabeth Gould (2007). Diminished adult neurogenesis in the marmoset brain precedes old age. Proceedings of the National Academy of Sciences, early online edition Oct. 15.