Turning Up the Volume
The ebb and flow of neuronal activity and sensitivity in different areas of the brain is called neural dynamics. The basic idea is simple: different parts of the brain specialize in processing different types of information. Depending on what you’re concentrating on at a given moment, some neurons are very sensitive and others are merely on call.
“You might imagine there are conditions under which you want [certain nerves] to be very sensitive–you’re just trying to detect a dot of light or something barely touching your finger,” says Moore. In those situations, regions of the brain responsible for vision or touch become more sensitive. In other situations, heightened sensitivity is a disadvantage. “You’re at a cocktail party and there’s lots of noise all around. The last thing you want to do is amplify all that noise,” says Moore. “What you really want to do is focus just on one person’s voice.” But when a friend across the room starts calling out your name, you need to regroup and amplify the noise again.
“Neural dynamics are what allow you to be inventive on a moment-to-moment timescale, to see information and act on it,” explains Moore. You’d never be able to stay asleep if you were as attuned to noises as you are when you ride a bike or wait for an important phone call. But it’s crucial to be able to shift back into a waking state in the morning–or when the fire alarm goes off in the middle of the night. These shifts are accomplished as neurons in different areas of the brain become more or less sensitive.
Moore studies the neural dynamics of sensory perception, and one of his favored subjects is the ebb and flow of electrical activity among a group of 60,000 neurons in the human cortex. This group processes touch sensations from the tip of the left middle finger. Moore put subjects inside a brain scanner called an MEG, which records electrical activity, and tapped that finger with a mechanical actuator. Subjects were then asked whether they felt anything.
Strangely, sometimes you will be conscious of having been tapped on the finger, and sometimes you won’t–even though the force and timing of the tap are identical. Moore has shown that the difference depends on the electrical activity of the 60,000 fingertip neurons a fraction of a second before the tap occurs. Certain patterns predict sensitivity to the tap, while others are associated with failure to notice it.
What is the source of these differences in sensitivity? That isn’t completely understood, but they’re known not to be random. Moore believes that an increase or decrease in blood flow to the dedicated fingertip neurons might function as a sensitivity dimmer switch that determines whether you feel a tap. That situation is a very simple one–a model system that’s easy to study in the lab–but similar dynamics govern all our thinking and perception.