Bottleneck in the Brain

The chronically uncoordinated have long endured taunts about their allegedinability to walk and chew gum simultaneously. But put the jokes on hold: Psychologists say that doing two things at the same time can be much harder than we think.

That's true even when the tasks are simple, says University of California at San Diego psychologist Harold Pashler, Ph,D. A three year-old might be able to perform either task alone. But because of a bottleneck in our brain, even a Nobel Prize winner can't do them simultaneously.

The major hurdle, apparently, lies in the brain's decision-making and memory-retrieval systems. When simultaneous tasks both require us to make a choice or summon a memory, a mental traffic jam occurs. The second decision must await completion of the first (although we can act on the first decision while making the second).

Pasher's work with split-brain patients, described in Neuroreport (Vol. 5, No. 17), suggests the bottleneck occurs in the brain's more primitive regions. In these patients the corpus callosum, a bundle of fibers that relays information between the two cerebral hemispheres, is severed. Yet when the patients attempted simultaneous tasks tailored to involve different hemispheres, interference still occurred.

Even when we seem to be performing two tasks at once, often we are really switching rapidly between them. That's why we can talk while driving. "If you need to take half a second to decide where to steer, your short-term memory holds on to your conversation," says Pashler. Your brain switches gears like an English roadster, shifting back to the discussion so smoothly that you don't notice the interruption. If, however, you need to slam on the brakes to make an unexpected turn, conversation also comes to a screeching halt while you decide what to do next.