Researchers say they have found a connection between mobile phone use and increased brain activity.
American scientists found that 50 minutes of mobile phone use was associated with increased "brain glucose metabolism" - a marker of brain activity - in the region closest to the phone antenna.
However the health implications of the finding are unknown, they say in the Journal of the American Medical Association.
The article points out that there have been concerns regarding potential harmful effects of exposure to radiofrequency-modulated electromagnetic fields (RF-EMFs) of mobile phones.
But studies of the association between mobile phone use and prevalence of brain tumours have been inconsistent and remain unresolved, they say.
Nora D Volkow, of the National Institutes of Health in Bethesda, Maryland, and colleagues conducted a study involving 47 people.
Mobile phones were placed on the left and right ears and brain imaging was performed to measure brain glucose metabolism twice, once with the right phone activated - but with the sound muted - for 50 minutes and once with both phones deactivated.
The researchers found that whole-brain metabolism did not differ between the on and off conditions - but there were significant "regional" effects.
Metabolism in the brain region closest to the antenna (orbitofrontal cortex and temporal pole) was significantly higher (approximately seven per cent) for 'mobile phone on' than 'mobile phone off' conditions.
“Results of this study provide evidence that acute cell (mobile) phone exposure affects brain metabolic activity,” the researchers wrote.
"However, these results provide no information as to their relevance regarding potential carcinogenic effects (or lack of such effects) from chronic cell phone use.
"Further studies are needed to assess if these effects could have potential long-term harmful consequences," they said.
Professor Patrick Haggard, of the Institute of Cognitive Neuroscience at University College London, said the “study reports a seven per cent increase in brain metabolism close to the antenna of a commercial mobile phone during transmission, compared to when the phone is off”.
"It is not the first study to report such effects, but it is considerably larger than any of the previous studies, including many more participants.
"This is a very interesting result, since it suggests a possible direct effect of mobile phone signals on brain function. However, the result should be treated with some caution.
"First, independent replication of results in a different laboratory is generally considered very important in this area of research.
"Second, it would be useful to know whether participants could tell if the phone was on or off. Simply knowing that the phone is on could influence brain activity, so the results can only definitively be attributed to the mobile phone signals if this can be ruled out.
"Third, as the authors point out, the implications for health remain unclear. Much larger fluctuations in brain metabolic rate occur naturally, for example during thinking.
"However, if further studies confirm that mobile phone signals do have direct effects on brain metabolism, then it will be important to investigate whether such effects have implications for health," Haggard said.
Professor Malcolm Sperrin, director of medical physics and clinical engineering at Royal Berkshire Hospital, said the “paper is particularly interesting in that it reports an increase in brain metabolism as a result of the use of mobile phones”.
"The causality is important since a direct link is reported as opposed to coincidental events which may have an independent origin.
"The authors and institution where the research was carried out are all of the highest calibre and the paper is written in an authoritative manner.
"The fact that there is an increase in metabolism as a result of phone use is not claimed to be clinically significant; more work is required to establish any possible link between RF energy deposition in the brain and a consequential health risk.
"It is reasonable to assume that the small increase in metabolism results from a deposition of energy, which may result in turn from local temperature changes or perhaps magnetic or electrical stimulation that does not involve heat at all.
"Furthermore, it is known that sensory input (e.g. hearing) does elevate brain metabolism although the asymmetric nature of the finding does suggest that this is not the case here.
"It is important to fully appreciate that no health risk is identified in this paper," Sperrin said.