There have been worries about cell phones causing brain cancer. And certainly everyone worries about driving behind the guy who's holding the steering wheel with his knees while tapping in a message on a wireless e-mail device.

But now hear this: Mobile phone use may cause a slowing of brain activity.

Before anyone panics, the suggestion that frequent mobile phone use makes us behave a little unbalanced is, so far, based on a study of 300 people conducted by researchers in Australia, England and the Netherlands.

The study, published in the International Journal of Neuroscience this month, looked at the group of 300 people over 2.4 years, but researchers plan to expand the study over a longer period and with data involving 17,000 people.

According to the study, frequent mobile phone users demonstrated slowed brain function, but with the caveat that the slowed brain effects are still considered within normal brain functioning. A longer study with a larger sample group would consider whether the slowed brain activity should be considered an adverse health effect, according to a statement from Brainclinics Diagnostics in Nijmegen, the Netherlands, one of the groups involved in the study.

The noted slowed brain function could not be explained by differences in personality, according to researchers. "In Alzheimer's dementia you also find a severely slowing of brain activity," said Martijn Arns, the main investigator for Brainclinics Diagnostics, in a statement. "However, the slowing found in this study, with mobile phone users, can still be considered within 'normal' limits." Still, Arns predicted that a longer-term study would show more severe effects.

Of the 300 people in the study, only 100 were frequent mobile phone users, while 100 were non-mobile phones users and the third group of 100 were an intermediate user group. Differences in brain activity, as measured with quantitative electroencephalographic (EEG) studies, and neuropsychological functions such as attention, memory, executive function and personality, were assessed. Among the results, frequent users scored higher on ratings as extraverts and were found to be less open-minded.

The study also found that frequent users also showed improved focused attention, which was explained by a learning effect due to making more phone calls in busy places where users had to focus better on a phone call while filtering out background noise and other distractions.

Despite this improved focus and the findings about personality, the frequent users showed more instances of slowed activity as measured by delta and theta EEG power, as well as a slowdown in a measurement called alpha peak frequency.

The researchers cited several other studies going back to 1998 on the short-term effects of mobile phone use, some of which showed that frequent users improved their scores on cognitive tests. Those positive outcomes were linked to small increases in brain temperature, which led to faster metabolic activity and thus faster reaction times. However, the researchers in the current study said the previous studies are inconclusive.

In the recent study, Brainclinics was joined by researchers at Radbound University in Nijmegen, the Institute of Psychiatry in London and The Brain Resource in Sydney.