Visual complexity in letter-by-letter reading: "pure" alexia is not pure.

Standard accounts of pure alexia have favoured the view that this acquired disorder of reading arises from damage to a left posterior occipital cortex mechanism dedicated to the processing of alphanumeric symbols. We challenge these accounts in two experiments and demonstrate that patients with this reading deficit are also impaired at object identification. In the first experiment, we show that a single subject, EL, who shows all the hallmark features of pure alexia, is impaired at picture identification across a large set of stimuli. As the visual complexity of pictures increases, so EL's reaction time to identify the stimuli increases disproportionately relative to the control subjects. In the second experiment, we confirm these findings with a larger group of five pure alexic patients using a selected subset of high- and low-visual complexity pictures. These findings suggest that the deficit giving rise to pure alexia is not restricted to orthographic symbols per se but, rather, is a consequence of damage to a more general-purpose visual processing mechanism.