Physiological Studies of Attention in Autism: Implications for Autistic Cognition and Behaviour

As I mentioned, these visual stimuli are going by at eighteen frames a second. That means there are nine flashes a second on each side of the display. A stimulus flashed at this frequency generates brain electrical activity at the same rate - so we see an alternating voltage at the back of the head with a frequency of nine Hertz.

What we're interested in is the way in which the amplitude of this voltage changes during shifts of attention.

In order to get a better look at those changes, we separate electrical activity into a stimulus-driven component and a background component. Stimulus-driven EEG is a measure of the brain's response to the flashing stimulus, while background EEG represents unrelated processing. So if a person is paying a lot of attention to the task, we should see a lot of stimulus-driven activity and very little background.

For each of these components, we calculate amplitude as a function of time, and then subtract the amplitude during leftward shifts of attention from the amplitude during rightward shifts.

`Physiological Studies of Attention in Autism: Implications for Autistic Cognition and Behaviour', Matthew Belmonte, 26 January 2002