Using a hands-free device to have a conversation while driving is generally considered to be less distracting than talking on a handheld cell phone. However, a new study says hands-free devices can still distract a driver by affecting brain activity and creating "visual tunneling."

Psychologists at the University of Sussex in England recently published their findings in the journal "Transportation Research." They found that drivers whose conversations affected visual imagination focused on a smaller area of the road and were less likely to detect road hazards, even when they were looking directly at them.

"A popular misconception is that using a mobile phone while driving is safe as long as the driver uses a hands-free phone," said Graham Home, senior lecturer in psychology at the University of Sussex. "Our research shows this is not the case. Hands-free can be equally distracting because conversations cause the driver to visually imagine what they're talking about. This visual imagery competes for processing resources with what the driver sees in front of them on the road."

In one experiment, researchers asked participants to react to hazards displayed on a video screen. They distracted some participants by having them listen to sentences and say whether they were true or false. Half of the distracted participants were asked to determine the veracity of a sentence that used visual imagery. For example, one question asked participants whether a five pound note was the same size as a ten pound note.

All distracted participants were slower to respond to hazards displayed on the screen, detected fewer hazards, and made more errors where they looked at a hazard but did not perceive it. These errors were more serious for participants who were distracted by statements that used visual imagery.

"Our findings have implications for real-life mobile phone conversations," said Hole. "The person at the other end of the phone might ask, 'Where did you leave the blue file?", causing the driver to mentally search a remembered room. The driver may also simply imagine the facial expression of the person they're talking to."

The study suggests that phone conversations may require the use of more of a brain's visual processing ability than previously thought. When a conversation creates a visual image, it can compete with the brain's processing capacity and cause them to miss hazards on the road.

The National Safety Council, which supports research and education initiatives to reduce the number of preventable deaths in the United States, says the brain can quickly alternate between different tasks but cannot adequately focus on two things at the same time. The organization says brain activity in the area that processes moving images falls by about one-third during a phone conversation.

A second experiment by University of Sussex researchers distracted some participants by giving them verbal instructions to move around an imaginary grid. Compared to undistracted participants, these people were more likely to miss hazards in their peripheral vision due to "visual tunneling." The National Safety Council says a driver's field of vision narrows when talking on the phone and can shrink by up to 50 percent.

"Conversations are more visual than we might expect, leading drivers to ignore parts of the outside world in favor of their inner 'visual world' – with concerning implications for road safety," said Hole.

Distractions that can affect a driver's focus are classified as cognitive, manual, or visual. Cognitive distractions involve taking your attention away from the task of driving. Manual distraction occurs when you take your hands off the wheel, while visual distraction occurs when you turn your eyes away from the road.

Hands-free devices allow a driver to make a call while keeping their hands on the wheel. These options include the speakerphone setting on a cell phone, an earpiece, or an in-vehicle system. In Connecticut, driving while using a handheld cell phone is prohibited for all drivers; using a hands-free device while driving is allowed for drivers ages 18 and older.

A conversation with a passenger can also be distracting, but not as much as a phone conversation. A passenger is more cognizant of the road conditions and tend to limit the conversation when drivers arise. They also offer another set of eyes and can alert the driver to hazards.