The audience for Sunday morning’s keynote lecture at CNS 2013 got to play the part of monkeys during a talk by William Newsome of Stanford University, though our task was a bit easier than what his test monkeys usually experience.
Normally, in Newsome’s experiment, monkeys have 750 milliseconds to determine either whether a flashing field of dots is moving left or right, or whether green or red is the predominant dot color. To receive a gulp of juice as a reward, they must indicate the correct answer by fixing their eyes on a single dot on the right or left side of the screen for the directional test, or on the green or red dot for the color test. The audience at Newsome’s lecture had 2 seconds to watch the dot field, and of course, we could respond verbally with our choice.
By examining the brains of these monkeys while these tests are under way and modeling what’s occurring in both individual and whole populations of neurons, Newsome’s team has shed light on how the brain separates relevant from irrelevant information to make context-dependent decisions.
In the monkey test case, both color and movement information make their way down the network of neurons, until the monkey has to decide if it’s being asked for an answer about color or direction. As individual neurons are stimulated, each one hangs onto all the information that the monkey is gathering. It’s not until much later in the process, when the signals of many neurons are integrated into a network on their way to the frontal eye fields (FEF) portion of the prefrontal cortex, that “gating” occurs – when the brain starts to select what information is relevant and what is not based on the decision it has to make. In the motion test, which direction the dots move is the relevant information, but whether the dots are green or red is irrelevant.
Because all of this information comes down the pipeline at once, it’s very difficult to home in on the exact point when gating occurs. To address this issue, Newsome collaborated with post-doctoral student David Sussillo to build a computer model that closely mimics the response of the monkeys’ brains to the dot tests. By plotting the data on 3-D axes, they have begun to tease apart where gating occurs and which mechanisms play a role.
Newsome says he was startled to learn from the model this gating mechanism actually selects for relevant information and that gating doesn’t occur as previously thought, where a mechanism suppresses irrelevant information, allowing just the relevant details to proceed. “This is a totally different view of how selection and gating can happen in the prefrontal cortex. It’s not one the field [of cognitive neuroscience] has entertained before,” Newsome said during his talk.
Acknowledging that more work lies ahead, he added, “The question is now, ‘Is this the way the brain is really working?’ because we don’t know.” As Sussillo and Newsome have scaled up their tests, the model’s outputs have remained robust, suggesting that this selection mechanism has much broader implications in the prefrontal cortex. Newsome believes it will play an important role in this line of research for years to come.
For more on Newsome’s research, read a recent Q&A with the scientist.