The study looked at how having a conversation with someone who is not present competes with those parts of the brain needed to perform visual tasks.

The report, which appears in the current issue of Experimental Psychology, was written by Amit Almor, an associate professor of psychology at the University of South Carolina.

For the study, volunteers were asked to take part in a series of visual tests on a computer while listening to information about, for example, setting up a fish tank or finding north by using the sun or stars. They were then asked questions about what they had heard.

The volunteers did much better on their visual tasks when they were just listening, as opposed to preparing to speak or speaking. When they were listening, if the demands on their brains became too much, they could just tune out what they were hearing.

The study found differences, although smaller ones, based on where the sound was coming from. When the audio came from the same direction that participants were facing to do their visual tasks, they did better on them.

It may be, the study said, that when people talk to someone who is not present, the visual-processing parts of their brain create a mental representation of where the other person might be. This suggests, Dr. Almor said, that using cellphones may be safer if the sound comes from the front.