Finally, they adjusted the amount of light downward until they found the lowest number of brain cells needed to evoke a measurable response in the mice. That number turned out to be less than 50 -- much fewer than the wide-flung networks of cellular activation neuroscientists had previously assumed would be necessary, Svoboda said.
The mouse brain's ability to tap into a mere 50 cells is even more remarkable when you consider that the activity of this cluster of cells takes place amid a background roar of other neurological "noise" from millions of cells, he said.
"At the same time, the functional brain area just chatters along and produces perhaps a hundred thousand spontaneous action potentials [electrical signals]," he noted. "So, the brain can actually distinguish the tiny, tiny number of action potentials from that huge background."
According to Svoboda, the experiment strongly supports a theory of brain function called "sparse coding," in which "neurons that listen to the neurons that we have activated have to be able to pull out very sparse subsets of activity."
In another study, Svoboda and co-researcher Christopher Harvey, also of the HHMI and Cold Spring Harbor Laboratory, focused on the synapse -- the microscopic gap separating individual neurons. Messages are passed neuron-to-neuron across the synapse by a complex mechanism of electrochemical signaling.
"Scientists had shown that synapses behave rather independently," Svoboda said, so that long-term electrical activation ("potentiation") of one synapse didn't directly affect a neighboring synapse. Long-term potentiation is, in essence, the key cellular step in how the brain lays down memory.
However, computer models had suggested that activation at one synapse might more subt
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