Dyscalculia is a severe and persistent disability in learning arithmetic that is often highly selective, in that it can affect children with normal intellectual ability.
Karin Landerl and her team at the University of Graz, Austria, investigated the development of numerical processing in elementary school children with dyscalculia and a control group with good arithmetic skills. Children from grades 2 through 4 were asked to repeatedly perform simple computer tasks, for example selecting the larger number or set of dots, counting dots, or placing numbers on a number line.
While the efficiency of numerical processing generally improved over time, children with dyscalculia showed marked and persistent deficits which were specific to the domain of number. Dyscalculic children also had particular difficulty to "subitize", that is, to enumerate between 1 and 3 dots as quickly as possible.
Landerl and colleagues conclude a biologically driven deficit causes children with dyscalculia to have an imprecise internal representation of numbers, which explains these children's difficulty with the number line task.
Development of numerical processing in children with typical and dyscalculic arithmetic skills - a longitudinal study, Karin Landerl, Front. Psychol. 4:459. doi: 10.3389/fpsyg.2013.00459