The electromagnetic radiation emitted by a cellular phone's antenna appears to activate nearby regions of the brain to unusually high levels, according to a study published Tuesday in the Journal of the American Medical Assn. that is likely to spark new concerns about the health effects of wireless devices. The preliminary study, led by a respected neuroscientist at the National Institutes of Health, raises many more questions than it answers. But by providing solid evidence that cellphone use has measurable effects on brain activity, it suggests that the nation's passionate attachment to its 300 million cellphones may be altering the way we think and behave in subtle ways.