It appears our brains have a separate area used for counting, a finding that may help explain why some people struggle with arithmetic.

Researchers in Britain and the United States say an area of the brain thought be involved in processing numbers actually has two separate functions: one for counting "how many" and the other for knowing "how much."

People with dyscalculia often have trouble with mental math, such as making change or calculating sales tax, and abstract concepts such as keeping track of time. However, their language skills may be normal or above average.

People with dyscalculia may have trouble mentally calculating how much change they should receive.

Prof. Brian Butterworth of University College London's Institute of Cognitive Neuroscience and his colleagues focused their experiments on brain activity in the intraparietal sulcus, an area of the brain thought to be involved in processing number information.

Using a functional magnetic resonance scanner (fMRI), the researchers found subjects used one area when counting and another when estimating amounts.

The study looks at the two ways people count, Butterworth said.

Using the first method, you could count how many men versus women are in a room by counting them as they enter.

In the second, you could try to assess the difference once everyone is in the room.

Instead of men and women, subjects saw blue and green squares on a screen, either in a sequence or at the same time.

Both activated the same region of the brain, the researchers report in this week's online issue of the Proceedings of the National Academy of Sciences.

However, when the colours were merged and appeared as a continuously changing square of a cloudy coloured rectangle, a different part of the brain lit up because the subjects could no longer count objects.

"By comparing these two types of stimulus, we identified the brain activity specific to estimating numbers of things," Butterworth said in a release.

"We think this is a brain network that underlies arithmetic and may be abnormal in dyscalculics."

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It's hoped the finding could help in diagnosing dyscalculia, he said, just as similar brain research has helped scientists better understand dyslexia.