Pure Alexia as a Visual Impairment: A Reconsideration

Abstract

We contrast two types of explanation for the syndrome of pure alexia: the visual impairment hypothesis, according to which the reading difficulty is just the most obvious manifestation of an impairment in visual perception; and the word form hypothesis, according to which the reading difficulty results from damage to reading-specific mechanisms. We argue that the word form hypothesis cannot account for certain findings in the literature on pure alexia, and that the visual impairment hypothesis is consistent with the available data. In addition, we present two new studies in support of the visual impairment hypothesis. In the first study, a pure alexic patient was found to be impaired on tests that assess the specific type of visual impairment hypothesised to underlie pure alexia. He was not impaired on the less specific tests that had been used previously to argue that perception is intact in pure alexia. In the second study, the additive factors method was used to identify the locus of impairment during reading in the same pure alexic patient. An interaction was found between the visual quality of the words and the length of the words. This implies that the abnormal word length effect in pure alexia (i.e. the letter-by-letter reading strategy) originates in the same processing stage as the visual quality effect, that is, in visual perception. Finally, we attempt to generalise about which kinds of processes make use of dedicated brain hardware and which do not, and suggest that the visual impairment hypothesis of pure alexia is more consistent with this generalisation than is the word form hypothesis.