According to a study, brain activity is increased significantly after using a handset for 50 minutes, in the area closest to the antenna.
The results come after a controversial decade-long project suggested that heavy mobile use could increase the risk of brain cancer by up to 40 per cent.
However the new investigation did not look at the potential carcinogenic effects of the electromagnetic radiation emitted by handsets, and absorbed by the brain when held next to the ear.
Professor Patrick Haggard, Institute of Cognitive Neuroscience at University College London, said: "This is a very interesting result, since it suggests a possible direct effect of mobile phone signals on brain function."
He said there were several limitations to the study, but added: "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."
In the study, Dr Nora Volkow of the US government's National Institutes of Health conducted an experiment on 47 people to establish if mobile phone exposure affected regional activity in the brain.
They placed mobile phones on both ears of the participants and measured brain activity once when the right-hand mobile was on for 50 minutes with the sound muted, and then with both phones off.
Scans were studied to see what effect they had on glucose metabolism - the process whereby the brain processes sugar for energy.
They found that metabolism across the whole brain did not alter in the two tests, but metabolism in the brain region closest to the antenna - the orbitofrontal cortex and temporal pole - was 7 per cent higher when the mobile was switched on.
Dr Volkow said: "Results of this study provide evidence that acute cell phone exposure affects brain metabolic activity.
"However, these results provide no information as to their relevance regarding potential carcinogenic effects - or lack of such effects - from chronic cell phone use."
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