ABSTRACT
The evidence for selective impairment of arithmetical calculation reported in the neurological literature is at best qualitative and often merely anecdotal. Arithmetical calculation is a complex accomplishment and is likely to comprise many subcomponents. The neurological case descriptions of disorders of numeracy provide some pointers to the variety of functional systems subserving arithmetical calculation. E. Guttman described a patient who knew the multiplication tables and could carry out simple arithmetical calculations, yet had an unexpectedly grave difficulty with number estimation and number knowledge. There was a very clear dissociation in Dr. G. Hitch’s numerical abilities test which specifically compares number operations with number computations. A proportion of a fraction is removed, or rather the proportion is fractioned and the fraction is then divided and a decision is made as to which of the fractions is to be kept and which rejected.
