28 May 1998

Clocking The Speed Of Thought

Around 300 milliseconds. That's how long it took a volunteer to begin to understand a pictured object. Add to that another 250 to 450 milliseconds to fully comprehend what it was. Total speed of thought: between 550 and 750 milliseconds.

Such are the results coming out of work conducted by John Hopkins scientists seeking to measure rates of comprehension. "This information has been difficult to acquire," says neurologist and team leader John Hart, "even with different combinations of behavioural tests, electrical recordings and imaging studies such as PET scans."

Yet by taking advantage of a unique opportunity afforded by a patient scheduled for tests using electrodes surgically placed on his brain, the researchers have moved one step closer to "building theories of higher mental activity." Until now, the speed of cognitive operations (including language processing) has been the missing ingredient.

Reporting their findings in the May 25 Proceedings of the National Academy of the Sciences, the researchers describe how the patient was asked to name and categorize a variety of pictures and words. By way of a grid of 174 electrodes, his brain activity was then monitored. The speed of comprehension was far quicker for objects that were already familiar to him.

"The data, obtained within a single stage at a single site in the brain, are further evidence that information accumulates gradually in the brain, rather than in a strictly all-or-none fashion," says Hart.

He adds that understanding this process of accumulation could help scientists understand comprehension and word loss from disorders such as stroke or Alzheimer's disease.