DRIVERS who listen to cellphone conversations are distracted even if they don’t talk, exhibiting similar behavior as people who have been drinking, authors of a new study said.

Conducted at the Center for Cognitive Brain Imaging at Carnegie Mellon University in Pittsburgh, the study used brain imaging to document that when driving and listening, activity in the parietal lobe, the portion of the brain associated with driving, was reduced by 37 percent.

Using a driving simulator, 29 volunteers steered a car along a virtual winding road at a fixed, challenging speed while inside an MRI brain scanner. Participants were tested while undisturbed or while deciding whether a sentence they heard was true or false. When listening was involved, more lane errors were committed, such as hitting simulated guardrails and weaving out of lanes, similar to behavior that occurs while under the influence of alcohol.

The findings were reported in February in the journal Brain Research.

Marcel Just, a cognitive neuroscientist and the center’s director, said the process of listening to spoken language requires more mental effort than is apparent.

“The brain comprehends language without much strategic control,” he said. “It goes into the brain and gets worked on; you can’t stop it once it gets in.”

Additionally, there is a false sense that a driver is not doing anything, but the combined demands can be misleading.

“The human brain isn’t really geared to process many streams of information at a time,” Dr. Just said. And listening to a cellphone may be more distracting than chatting with a passenger, who would most likely sense challenging driving situations and stop talking. Because of this, he said, making cellphones hands-free or voice-activated is not sufficient in eliminating distractions.