Their research found that study participants who needed to conduct two visual tasks tended to do worse than study participants who needed to fulfill a visual task and an audio task.

In the study, led by Zheng Wen, PhD, from the University of Ohio, 32 college students were asked to determine whether two grids on the screen were a match. After completing the task without multitasking, they were asked to complete the same task while they communicated with "Jennifer," a "student" who needed help getting to an interview within six minutes. Half of the participants communicated with Jennifer through voice chat (Google Talk, with a microphone and headset); the other half communicated with her through IM (Google Chat).

In general, multitasking hurt thr ability to conduct two different tasks. But the level of decreased ability varied widely by task.

The study used eye-tracking technology to monitor subjects' gaze. Researchers found that people who had two visual tasks spent less time on any individual task, and that their eyes moved a lot more, in comparison with people with a visual and an audio task.

Multitasking of any kind hurt performance but the participants who communicated with Jennifer via audio had a 30 percent drop in ability. Participants who talked with Jennifer through Google Chat had a 50 percent drop. In addition, participants who communicated via audio completed more steps in their matching task.

Perhaps more troublingly, participants who conducted two visual tasks felt that they had done better than the participants who had conducted a visual and an audio task.

Wang said in a statement that the difference in confidence came from participants' ability to respond to Jennifer when they wanted, as opposed to having audio cues to which they needed to pay attention. She also thought that people felt more efficient when having multiple visual cues.

Researchers believe that their study has various implications – most widely with regards to driving. They think that drivers should focus on audio cues rather than visual ones from the GPS. They also said that drivers who text tend to feel more in control of the vehicle than drivers who are talking on the phone, even though the opposite is true.

Most of the United States has laws in place with certain bans on cell phone use while driving. In total, 10 states have laws against handheld cell phone use; 32 states and the District of Columbia ban cell phone use by new drivers; and 43 states ban texting while driving for all drivers or for novice drivers.

The results of the study were published in Computers in Human Behavior.