Good division, but bad addition, subtraction and multiplication. A “leftmost-first” bug?

The present investigation reports the case of patient AS, ambidextrous, who showed a selective problem with arithmetical procedures in addition, subtraction and multiplication, contrasting with complete sparing of division. AS displayed a specific and never described “bug” error, involving the selection of digits that have to be added, subtracted or multiplied. This bug consisted in inverting the order of the numbers that have to be selected to correctly solve the operation. In particular, AS selected the numbers beginning from the leftmost position. This bug spared division, since it is the only operation that requires starting from the leftmost digit(s). The present case would suggest that some aspects of arithmetical procedures are operation independent. Moreover, an account of the nature of the syntactic rule getting lost in AS’s performance has been proposed.