Are cellphones safe? That question has gotten a lot of attention, but so far there has been no convincing evidence that those ubiquitous devices cause health problems. A new federal study may stir things up further, but the bottom line again is that the study raises more questions than it answers.

Nora Volkow of the National Institutes of Health and colleagues conducted positron emission tomography, or PET, scans on the brains of 47 subjects throughout 2009 as the subjects held phones up to their left or right ears for 50 minutes at a time, sometimes on but muted and other times off.

The researchers found that the activity of the entire brain did not differ between when the phone was on or off. But brain activity in the brain region closest to the antenna, known as the orbitofrontal cortex and temporal pole, was significantly higher - about 7 percent more active - when the phone was on than when it was off, the researchers found.

"The increases were significantly correlated with the estimated electromagnetic field amplitudes, both for absolute metabolism and normalized metabolism," the authors wrote. "These results provide evidence that the human brain is sensitive to the effects of . . . acute cellphone exposures."

They add, however, that "these results provide no information as to their relevance regarding potential carcinogenic effects (or lack of such effects) from chronic cellphone use. Further studies are needed to assess if these effects could have potential long-term harmful consequences."

The study is published in the latest issue of the Journal of the American Medical Association. In an accompanying editorial, Henry Lai of the University of Washington and Lennart Hardell of University Hospital in Orebro, Sweden, said the meaning of the findings remains far from clear but warrants "further investigation."

"An important question is whether glucose metabolism in the brain would be chronically increased from regular use of a wireless phone with higher radiofrequency energy than those used in the current study. Potential acute and chronic health effects need to be clarified. Much has to be done to further investigate and understand these effects," they wrote.

The findings may indicate that other changes in brain function occur from exposure to radiofrequency emissions, they said.

"If so, this might have effects on other organs, leading to unwanted physiological responses. Further studies on biomarkers of functional brain changes from exposure to radiofrequency radiation are definitely warranted," the editorial said.