Driving a vehicle requires you to monitor the situation around you, keeping an eye out for any potential changes. You need to check mirrors, blind spots, your speedometer, and the road ahead, adjusting your speed and steering as necessary.

With such a reliance on vision, it can be extremely hazardous if a driver allows his or her vision to narrow to a smaller field. You might not even perceive an object if you don't expect it. Several studies have warned that this effect is prevalent among drivers who talk on both cell phones and hands-free devices while driving.

Siri Carpenter, writing for the American Psychological Association, says this phenomenon is known as inattention blindness. People will fail to notice something if they are focusing on another cognitive task. In one study, participants at a computer who were focused on measuring which arm of a cross was longer often failed to notice when other objects appeared on the screen.

Daniel Simons, writing for Smithsonian Magazine, says inattention blindness was also apparent in a more amusing study he helped to conduct. Participants were asked to study a video and determine how many times a basketball was passed among players who were wearing white shirts. Most people were so intent on this task that half of them failed to notice when a woman in a gorilla suit walked through the game, even after she looked right at the camera and thumped her chest.

The National Safety Council says the risk of inattention blindness is apparent when drivers are using cell phones or otherwise distracted. A driver might look at something but not mentally perceive that it is there.

According to University College London, the brain's visual cortex stops responding to unattended information when the brain is processing a high information task. The brain's capacity becomes dedicated to one task and ceases responding to information that is considered irrelevant.

Such a response means multitasking puts more of a strain on your brain and that one function you are trying to perform is likely to suffer. A study by Western Washington University notes that some driving tasks, such as maintaining your lane, are relatively automatic. Other responsibilities, particularly attention to unexpected hazards, are compromised by inattention blindness.

The study says drivers who use a cell phone while driving are more likely to experience this condition. The study found that these drivers are less likely to respond to external stimuli and more likely to disrupt their automatic driving functions.

The NSC says inattention blindness also contributes to a narrowed field of vision. A study by Transport Canada's Ergonomics Division found that the narrowed field of vision made drivers less likely to respond to unexpected situations and monitor their surroundings for potential hazards. Reduced awareness of the road situation, in turn, will make a driver less able to brake or steer to avoid a hazard.

Though most states allow the use of hands-free devices while driving, including Connecticut, the NSC says these devices can also lead to inattention blindness. The Transport Canada study found that drivers using a hands-free device reduced or stopped their monitoring of mirrors and instruments, were more likely to miss visual cues such as traffic signals and exit signs, and less likely to use turn signals at intersections.

A University of Utah study was also critical of the allowance of hands-free devices while driving. Researchers concluded that inattention blindness problems "are attributed in large part to the distracting effects of the phone conversations themselves, effects that appear to be due to the direction of attention away from the  external environment and towards an internal, cognitive context associated with the phone conversation."

The NSC recommends that drivers be aware of the potential risks of inattention blindness while on the road and not assume that their awareness of the situation around them will remain acute during a phone conversation.