Brain's visual area may help scientists understand how behaviour is organized
If the brain is thought of as an army, the new signals may give scientists a unique opportunity to trace how messages from the high command reach all the way down to individual soldiers in a particular platoon and affect their activities.
That's because the brain region in question, called V1, has already been the focus of detailed studies at the level of individual brain cell interactions and how they encode and analyze data from the eyes.
"To really understand how a control signal works, you first have to know how the mechanism being controlled works, and we already have a fairly detailed feel for that in V1," says Anthony I. Jack, Ph.D., a postdoctoral fellow and lead author of a study that appeared last month in the journal Neuron. "This provides us with a potential way of understanding a major puzzle: on a minute scale, how do control signals change how neurons process incoming information?"
Much of the human brain's power derives from its ability to take one stimulus and process it in different ways to meet a variety of needs. Different parts of the brain have specialized abilities that can contribute in various ways to completion of different tasks. They just need to be told when to shift from one task to the next.
Scientists have long recognized V1 as the place where visual data first enters the cortex, the area responsible for many of the higher functions of human thought, analysis and decision-making. Aspects of the visual signal analyzed by V1 include the orientation of edges and lines.
"Edges form the boundaries of visual objects," Jack explains. "By encoding this information, the neurons in V1 provide the brain with the information that it uses to visually distinguish one object from another."
Jack points out that the new results apparently show V1 responding to another, more abstract type of boundary: the divisions between mental tasks.
In their analysis of a listening task, Jack and colleagues found a puzzling pair of strong signals coming into V1. Subjects working on the task had to listen to a sequence of tones, determine whether the sequence was ascending or descending in frequency, and then, after an eight-second delay, indicate their choice. This produced two large spikes in activity in V1.
The finding led to three years of follow-up studies, most devoted to showing that the signals coming into V1 were not a product of other, lower-level cognitive processes.
"We went on a long whodunit where we ruled out all the usual suspects," Jack explains. "We haven't yet proven who did it or why, but we have some ideas."
The new signals seem to be timed to a subject's need to do something: listen for the tones or press the button to indicate a response.
"This link between the signals and the need to do something suggests to us that the signals may be preparing the area for directions on its role in a cognitive task, sort of a 'standby for further instructions' message," Jack says.
As an alternative theory, Jack and his colleagues have also speculated that the V1 signals may help the brain prime itself for learning, possibly aiding efforts to learn to process visual information in new ways.
They are currently conducting follow-up studies to determine which, if either, of the hypotheses, can be validated through additional evidence.
"What is exciting about this finding is the potential it presents for learning more about how control signals work at the neuronal level," says Jack. "At present, most ideas about how control signals work are based on theoretical models that seem plausible but have little detailed experimental support. That is problematic because nature often surprises us. It certainly did in this case."
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