CAMBRIDGE, Mass. Researchers at the Massachusetts Institute of Technology have confirmed that the more a driver is distracted, regardless of the distraction, the less frequently he or she scans the road for hazards.

The MIT researchers say that if a drivers mind is deeply focused on any topic, including trouble at home or even a cellphone conversation, the lack of attention to the road can result in a "narrowing of vision" (less visual scanning), accompanied by a slower response to incidents.

The MIT team studied 108 volunteers in three age ranges -- 20-29, 40-49 and 60-69 -- driving a midsize sport utility vehicle on Route 93 north of Boston, while they were given low-, medium- and high-demand cognitive tasks.

In the easy task, drivers were given a series of single-digit numbers, 0-9, and asked to repeat aloud each as it was given. In the medium-demand task, participants were asked to repeat the number given a digit earlier in the sequence; for the high-demand task, the number given two digits earlier.

"Even the easy task reduces the amount you're scanning," said Bryan Reimer, a research scientist at MITs AgeLab. "You're scanning the road a little less.

"At the medium- and high-demand level, you're scanning even less. It's called a loss of situational awareness. At the low level, you're less aware. At medium and high levels, you're even less aware," said Reimer.

Interestingly, the MIT researchers found no age difference in scanning patterns.

Reimer says distracted drivers, who are focused on the road ahead but not on the surrounding environment, react slower to a vehicle braking in front of them.

The MIT research into levels of distraction is unusual.

"In the past, the emphasis (of distracted driving research) was on whether youre distracted or not distracted," said Reimer. "This is too simple of a categorization. There are levels of cognitive demand, and those levels are statistically distinguishable.

"The level of thought going on has a relationship to how much a driver is aware of the driving environment," he said.

The MIT research, published in Human Factors: The Journal of the Human Factors and Ergonomics Society, comes as automakers and transportation officials are considering whether smartphone and GPS-device makers should be included in federal efforts to combat driver distraction.