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"What" and "Where" in Word Reading: Ventral Coding of Written Words Revealed by Parietal Atrophy

¹ Hôpital de la Salpêtrière, France, ² INSERM, U562, CEA/DSV, France, ³ Université Paris VI, Paris, France

Reprint requests should be sent to Prof. Laurent Cohen, Service de Neurologie 1, Hôpital de la Salpêtrière, 47/83 Bd de l'Hôpital, 75651 Paris CEDEX 13, France, or via e-mail: laurent.cohen{at}psl.aphp.fr.

The visual system of literate adults develops a remarkable perceptual expertise for printed words. To delineate the aspects of this competence intrinsic to the occipitotemporal "what" pathway, we studied a patient with bilateral lesions of the occipitoparietal "where" pathway. Depending on critical geometric features of the display (rotation angle, letter spacing, mirror reversal, etc.), she switched from a good performance, when her intact ventral pathway was sufficient to encode words, to severely impaired reading, when her parietal lesions prevented the use of alternative reading strategies as a result of spatial and attentional impairments. In particular, reading was disrupted (a) by rotating word by more than 50°, providing an approximation of the invariance range for words encoding in the ventral pathway; (b) by separating letters with double spaces, revealing the limits of letter grouping into perceptual wholes; (c) by mirror-reversing words, showing that words escape the default mirror-invariant representation of visual objects in the ventral pathway. Moreover, because of her parietal lesions, she was unable to discriminate mirror images of common objects, although she was excellent with reversible pseudowords, confirming that the breaking of mirror symmetry was intrinsic to the occipitotemporal cortex. Thus, charting the display conditions associated with preserved or impaired performance allowed us to infer properties of word coding in the normal ventral pathway and to delineate the roles of the parietal lobes in single-word recognition.