The concerns about the health impacts of cellphone use are likely to resurface today with the publication of a study in JAMA, the Journal of the American Medical Association. The study doesn't uncover any health risks associated with cellphone use, but it does indicate that holding a phone to one's head for an extended call seems to be enough to boost the brain's metabolic rate. The boost, however, is smaller than that seen in the visual system when it's processing images.

The new report comes against a backdrop of persistent fears about possible health risks associated with heavy cellphone use. Different studies that have tested for associations between heavy usage and cancer have generally produced negative results, although there are some notable exceptions, and cell phones haven't been around long enough for there to be good measures of risk associated with multi-decade use. Even if the epidemiological data has remained slightly ambiguous, however, the biology hasn't: there's no known mechanism that could lead from low-energy, long-wavelength radiation to cancer.

That doesn't mean that cell phones do nothing; radiation in these frequencies is likely to heat nearby tissues. A few other biological effects have been proposed but, so far at least, they haven't been seen consistently. (The authors cite some of these other findings, assigning them a bit too much credibility, in my judgement).

The new study attempts to show there's more going on than simply heating by tying cell phone use to changes in the brain's metabolism. Even here, however, past experiments have caused a bit of confusion. Studies based on tracking blood flow or metabolic activity in the brain have found just about any result imaginable, from increases near the site of the cell phone to decreases there and increases elsewhere in the brain. It's difficult to make any sense of them at all.

To get around some of these problems, the authors simply recruited nearly 50 participants; previous studies they cite typically involved 20 people or less. For measuring metabolic activity, they went with the PET scan. PET involves giving the subject a form of the sugar glucose, in which one of the atoms is radioactive. Its decay emits a small bit of antimatter—a positron—that in turn emits a gamma ray when it collides with a regular electron. By figuring out where the gamma rays are coming from, we can tell where the glucose is going, and thus which cells are most active. PET provides a picture of metabolic activity that covers a longer time period than the transient responses seen with blood flow imaging.

(Appreciate, for the moment, the idea of looking into potential health implications of weak cellular radiation by triggering matter-antimatter annihilations inside the brain.)

You should also appreciate the experimental procedure. To blind the participants, the authors strapped two cell phones on their heads, one to each ear (the cellphone used in this work is a standard Samsung CDMA flip phone). Both were kept muted, and only one was activated by a call—the side that was activated was flipped in two different recording sessions. The calls started 20 minutes before a dose of radioactive glucose, and kept going for a half an hour afterwards to provide a long-term picture of metabolic activity. The data from one of the subjects ended up not being used because the cell company dropped the call.

After recording the results, the authors looked for differences across the experimental and control tests. To simplify the calculations, however, they narrowed their focus to regions that are likely to be exposed to substantial amounts of radiation from the cell phone's antenna—only areas projected to receive 50 percent or more of the maximum power were compared. This might explain why the results were a bit less confused than earlier work, but it could also mean that areas of the brain that could produce confusing results were ignored.

In any case, on a whole-brain level, there wasn't any significant difference between having an active cell phone strapped to your head and having an unused one. But, when individual areas are compared, some differences do leap out: the signal increased in the areas that received the strongest cellular signal. Since the antenna was in the base of the handset, this meant that the bottom-front of the brain lit up.

What does this mean? It's not entirely clear. Typically, these signals increase in response to enhanced activity in the neurons there, and the authors propose that the same is true here, meaning that localized exposure to a cell phone causes the neurons there to fire more often. But that has not been demonstrated by this work, and there's still no obvious mechanism by which it would occur.

Does it represent a health risk? The authors have no idea, and say as much. However, they also note that the increase in activity seen here is actually less dramatic than that seen when the brain goes to work on a visual task. And there has been no indication that excessive mental activity causes health problems; in fact, the exact opposite appears to be the case.

JAMA, 2011. DOI: 10.1001/jama.2011.186 (About DOIs).