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May 25, 1998

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Hopkins Scientists Clock The Speed Of Comprehension

Capitalizing on an opportunity presented by a patient scheduled for tests using electrodes surgically placed on his brain, Johns Hopkins scientists have clocked the speed of thought, measuring the time the patient took to understand what everyday objects are in pictures.

The Hopkins team, led by neurologist John Hart, M.D., discovered that it took their volunteer patient about 250 to 300 milliseconds, or a fourth of a second, to begin to understand a pictured object, and another 250 to 450 milliseconds to fully comprehend what the object was. He appeared to complete the process more quickly when the object shown was familiar to him, Hart notes.

"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," Hart says.

Understanding the individual steps that cause this accumulation, and how they relate to each other functionally and temporally, could help scientists better understand comprehension and word loss from disorders like stroke or Alzheimer's disease, he notes.

In an article describing the unusual experiment in the May 25 Proceedings of the National Academy of the Sciences, Hart and Hopkins specialists in epilepsy, biomedical engineering, neurosurgery, neuropsychology and cognitive science said knowing the time sequence and speed of language processing and other "cognitive operations" is "critical for building theories of higher mental activity."

"This information has been difficult to acquire," Hart notes, "even with different combinations of behavioral tests, electrical recordings and imaging studies such as PET scans."

Their task was somewhat simplified by an adult epilepsy patient preparing for surgery to control intractable seizures experienced since age 8. The patient was scheduled to have a grid of 174 electrodes temporarily implanted on the surface of his brain to pinpoint the source of his seizures for removal in surgery.

The patient, a 22-year-old, agreed to participate in a series of language experiments while the electrodes were still implanted.

Scientists asked him to name and categorize a variety of pictures and words. During some of these tests, scientists used the grid to monitor brain activity; in others, they applied a mild electrical current through two electrodes.

"The current temporarily and safely depolarizes the nerve cells in the vicinity of the electrodes," says Hart. "We've shown previously that this will temporarily disable the area of the brain where it's applied, but the effect doesn't spread to other nearby areas and is safely reversible."

That makes the current a good test of whether a brain area is critical for a task, Hart notes.

"We were able to show that an area known as the left occipitotemporal gyrus, located on the base of the brain, is linked in our patient to many of the word recognition activities we tested," says Hart. "Applying current here as we presented the patient with a new test could cause problems in understanding what objects are; misinterpretations of verbs, colors, or shapes; and word-finding difficulties in spontaneous speech."

Hart says the finding suggests that there may a stage in language processing where different kinds of language activities, all performed in various parts of the brain, come to the same brain area to complete a common step.

To time brain activity, researchers applied the current at various times while the subject was recognizing a picture, noting when the current disabled his ability to name a picture and when it didn't, a technique Hart calls "timeslicing."

"Applying the current anytime before 400 milliseconds after the picture was shown disrupted his ability to name it," says Hart. "This disruption started to fade away for familiar objects at 450 milliseconds, and for less familiar objects at 750 milliseconds."

Other authors on the study were Nathan Crone, Jeffrey Sieracki, Diana Miglioretti, Charles Hall, David Sherman and Barry Gordon.


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