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No one is quite sure what to make of these findings. There had already been hints that spawning of brain cells, a process called neurogenesis, occurs in animals with more primitive nervous systems. For years, Fernando Nottebohm of Rockefeller University has been showing that canaries create a new batch of neurons every time they learn a song, then slough them off when it's time to change tunes.
But it was widely assumed that in mammals and especially primates (including the subset Homo sapiens), this wholesale manufacture of new brain parts had long ago been phased out by evolution. With a greater need to store memories for the long haul, these creatures would need to ensure that the engrams weren't disrupted by interloping new cells.
Not everyone found this argument convincing. (Surely birds had important things to remember too.) When neurogenesis was found to occur in people, the rationalizations began to take the tone of special pleading: there was no evidence that the new brain cells had anything to do with memory or that they did anything at all.
That may yet turn out to be the case with the neurons found by the Princeton lab. The mechanism Gould and her colleagues uncovered in macaque monkeys could be nothing more than a useless evolutionary leftover, a kind of neurological appendix. But if, as many suspect, the new neurons turn out to be actively involved with inscribing memories, the old paradigm is in for at least a minor tune-up--and maybe a complete overhaul.
It is telling that the spawning ground for the neurons is the hippocampus, which is indisputably crucial to memory. Patients with hippocampal injuries lose their ability to acquire new facts, though they can still recall impressions laid down in the years before the damage occurred. Maybe, Gould speculates, the newly generated hippocampal neurons are especially agile in forming connections with one another. As in the canaries, the new cells would readily join hands to encode a new memory. Then, when they were no longer needed, they would be flushed from the system, and the engram would be transferred elsewhere for safekeeping.
That explanation fits pretty well with the old theories. More puzzling, though, is another of the study's findings: the steady migration of new neurons from the hippocampus to the cerebral cortex. Could these neurons be somehow involved in ferrying information into permanent storage--storing short-term memories for the long term?
Perhaps, Gould and her colleagues ventured in a recent paper, this purported transport mechanism provides a means of time-stamping memories, helping us keep track of when we learned what. Older memories would be somehow associated with older neurons. No one is even guessing how this might work. But if memories are indeed flowing through the brain in rivulets of new neurons, then all the old ideas will have to be reconsidered.
The brain is so complex and neuroscientific experiments are so difficult to interpret that this whole picture could change in a year. Whatever happens with neurogenesis, the fundamental notion that engrams are made by stringing together neurons--whether new ones or old ones or a combination of the two--is likely to survive in some form.