Applying a technique perfected by Hopkins neuroscientist Vernon Mountcastle, researchers used seven electrodes to simultaneously monitor individual brain cell activity in the monkeys as they worked. They originally analyzed the data they gathered for changes in the firing rate of brain cells as the animals switched attention between tasks.
When Niebur arrived at Hopkins a few years ago, researchers started talking about taking another look at the data.
Niebur and other theoretical neuroscientists were speculating that the brain might encode information both in the firing of individual brain cells and in the timing of those firings.
"It's been shown in animals that the firing rate of neurons can go up by a factor of 2 or 3 when they start to pay attention to a stimulus," Niebur says. "But it seems to run the risk of confusing signals if you try to code for two different things -- the stimulus itself and the degree that one should pay attention to it -- with one type of signal, the rate at which neurons are firing."
Niebur says the two different signals have to be connected. What your senses perceive will influence how much attention you pay to them, but, he said, "it seems like a good idea if you can have two different but related signals that you can use to represent these two things." An increase in the number of nerve cells firing in unison could represent just such an independent, but related, second signal.
Hsiao and Johnson had data from three earlier experiments appropriate for testing the theory. Steinmetz, now a post-doctoral scholar at Caltech, combined currently available computer power with a cutting-edge statistical technique to determine if nerve cells were firing synchronously and if the strength of that synchrony changed when the monkeys needed to pay attention.