Confusing confusability: on the problems of using psychophysical measures of letter confusability in neuropsychological research.

The effect of letter confusability on reading has received increasing attention over the last decade. Confusability scores for individual letters, derived from older psychophysical studies, have been used to calculate summed confusability scores for whole words, and effects of this variable on normal and alexic reading have been reported. On this basis, letter confusability is now increasingly controlled for in stimulus selection. In this commentary, we try to clarify what letter confusability scores represent and discuss several problems with the way this variable has been treated in neuropsychological research. We conclude that it is premature to control for this variable when selecting stimuli in studies of reading and alexia. Although letter confusability may play a role in (impaired) reading, it remains to be determined how this measure should be calculated, and what effect it may have on word and letter identification.