From Scientific American:
Since the mid-1990s scientists have floated the idea that representations of numeric quantities, whether expressed as digits or as written words, are codified by the parietal cortex, a higher-processing region in the brain located just above the forehead. The notion is supported by calculation deficits observed in patients with damage to that brain region and in low birth weight children who exhibit reduced gray matter in adulthood.
Still, it is unknown whether the parietal cortex—specifically its left hemisphere—processes numeric value independent of notation. New research indicates that the brain region may prefer symbolic notation to other numeric representations—a finding that could open the door to helping kids plagued by dyscalculia, a learning disability characterized by severely impaired mathematical ability.
More here.