http://blog.sciam.com/index.php?title=where_new_neurons_go_to_work_1&more=1&c=1&tb=1&pb=1

The above link goes to a short series of posts on one of Scientific American’s seminar blogs. Contributors review and discuss an article by Kee et al. that appeared in Nature Neuroscience on Feb. 4, 2007. The name of the article is “Prefrontal incorporation of adult-generated granule cells into spatial memory networks in the dentate gyrus.”

Doug Fields, a principal investigator at the NIH, explains that new neurons are born in particular parts of the brain throughout our lives, but it’s still unclear how these new neurons are incorporated into established neural networks. In fact, many scientists believe that the new neurons are functionless or, if anything, are used by the brain only as simple substitutes for worn-out or damaged neurons.

The Key et al. (2007) article that’s reviewed in the blog suggests that newly generated neurons are integrated into existing neural networks in the hippocampus, and in fact play a distinct functional role in encoding new memories. The Key et al. study was carried out on mice that were trained to master the Morris water maze. In the Morris water maze, a mouse swims around to find an underwater platform that it can stand on and catch a breather. After doing the water maze a few times, a normal mouse will remember where the platform is and will swim right to it. If the mouse really has learned something new, certain memory-related genes in hippocampal neurons will be activated so that the neurons will build proteins that reinforce relevant synapses (edges). To identify newly generated neurons, Key et al. injected the mouse with a genetic marker that would light up in all the new neurons.

Key et al. found that after four to six weeks, memory-related genes were indeed activated in the newly generated neurons. In fact, there were more new neurons than old ones active in the new memory formation. While existing neurons were already established in neural networks and circuits, the new neurons were apparently more flexible in encoding new memories and forming new edges.

In lecture we learned about network exchange theory, and how adding new nodes to an economic-exchange network will affect the network’s set of balanced outcomes. Analogously, adding new nodes to the hippocampal neural network changes the network dynamics and functionality. Just as with characterizing the evolution of economic networks, understanding the ways in which new nodes change neural network dynamics is a hot research topic.