Scientists say they have isolated the area of the brain linked to the maths learning disability dyscalculia.
Dyscalculia can cause children to struggle with simple sums
A research paper published in the Proceedings of the National Academy of Sciences in the US shows a separate part of the brain is used for counting.
The researchers, in California and London, say the area that processes numbers has two functions - counting "how many" and knowing "how much".
Prof Brian Butterworth of UCL said this could be key to diagnosing dyscalculia.
People with dyscalculia may have better than average language skills and be good at sciences, the creative arts and even some aspects of mathematics.
But they tend to have difficulty visualising number sequences and the passage of time. They can find handling money problematic.
The new research used scans of brain activity in the intraparietal sulcus - the area known to be involved in processing number information.
The first was when subjects were counting, the second when they were assessing quantities.
Professor Butterworth said: "There are two ways of counting things. Imagine assessing how many men versus women are in a room by counting them at the door as they enter the room, let's say three women and four men.
"Then try assessing the difference by looking at the room when everyone is present.
"Both methods of assessing the number of people should produce the same result."
In the experiment, blue and green squares were used either in a sequence or at the same time.
Both activated the same brain region.
They then merged them either as a continuously changing square or a cloudy coloured rectangle.
A different part of the brain was being used - instead of counting it was trying to assess how much of each colour was present.
"By comparing these two types of stimulus, we identified the brain activity specific to estimating numbers of things," Prof Butterworth said.
"We think this is a brain network that underlies arithmetic and may be abnormal in dyscalculics."
Discrete and analogue quantity processing in the parietal lobe: A functional MRI study by Fulvia Castelli at the California Institute of Technology and Daniel E Glaser and Brian Butterworth at UCL.