The relationship between visual crowding and letter confusability: towards an understanding of dyslexia in posterior cortical atrophy.

Visual crowding is a form of masking in which target identification is hindered by excessive feature integration from other stimuli in the vicinity. It has previously been suggested that excessive visual crowding constitutes one specific form of early-visual-processing deficit, which may be observed in individuals with posterior cortical atrophy (PCA). This study investigated whether excessive visual crowding plays a significant role in the acquired dyslexia of two PCA patients, whose reading was characterized by visual paralexias. The patients were administered a series of letter, flanked letter, and word recognition tasks, and the effects of letter spacing and letter confusability upon response accuracy and latency were measured. In both patients, the results showed (a) evidence of excessive visual crowding, (b) a significant interaction between letter spacing and confusability on flanked letter identification tasks, and (c) effects of letter confusability affecting flanked but not unflanked letter identification. However, only mild improvements in reading accuracy were achieved in the experimental manipulations of interletter spacing within words because these manipulations had a dual effect: Increasing spacing improved individual letter identification but damaged whole-word form and/or parallel letter processing. We consider the implications of these results for the characterization of dyslexia in PCA, the design of reading rehabilitation strategies, and the relationship between visual crowding and letter confusability. In particular, we argue that the reading deficits observed in our patients cannot be accounted for solely in terms of a very low signal-to-noise ratio for letter identification, and that an additional crowding deficit is implicated in which excessive integration of fundamental letter features leads to the formation of incorrect letter percepts.