Pure Alexia: A Nonspatial Visual Disorder Affecting Letter Activation

Several different interpretations have been offered to explain the mechanism giving rise to the linear relationship between word length and reading time shown by patients with pure alexia or letter-by-letter reading. One interpretation attributes this word length effect to a spatial impairment in which there is a left-right gradient of processing efficiency. This fundamental resource limitation requires that the patient focus on each letter in turn to increase its signal-to-noise ratio and discriminability, especially for letters towards the end of the string. An alternative view attributes the word length effect to a letter activation deficit that disrupts the rapid and efficient processing of single letters. In this paper, we examine these two hypotheses in relation to DS, a letter-by-letter reader. DS is able to distribute her attention to multiple locations in parallel and her performance is unaffected by the absolute or relative spatial location of the letters in a string. She is, however, impaired at reporting the identity of a letter independent of its spatial location and requires an abnormally long time to process each letter. Furthermore, investigations of DS's reading, using Howard's (1991) analyses of reaction time distributions, suggest that she processes each letter in a sequential order. Based on the results of these studies, we propose that prototypic pure alexia is a nonspatial visual disorder that affects the activation of individual letters