Intracranial dissection of word reading mechanisms

Abstract

Reading a word gives access, within a fraction of a second, to a rich variety of information stored in memory, including meaning, grammatical features, and pronunciation. In parallel to memory retrieval, we can convert any string of letters into a sequence of speech sounds, provided that the string conforms sufficiently to the regularities of orthography (1). A prerequisite to the operation of both those reading routes is that letters and their order be rapidly and accurately identified by the visual system. This critical step is achieved in the ventral occipitotemporal (VOT) cortex, a broad region devoted to visual object recognition. The orthographic encoding process is strongly lateralized to the left hemisphere, a bias correlated to and probably resulting from the usual left lateralization of language areas (2). The causal role of the left VOT cortex in reading was revealed by 19th century neuropsychology through the study of patients with a reading impairment following left VOT lesions (3). Among other tags, this deficit has been termed pure alexia, a label that emphasizes that even in severe cases preventing the identification of single letters, all other facets of visual perception may be spared, including the recognition of faces, objects, and places. One century later, pioneering functional imaging studies elucidated this finding by showing that the VOT cortex area harbors patches of cortex selectively activated by different categories of items, including a word-selective region. Thus, Nobre et al. (4), using intracranial electroencephalography (iEEG) in epileptic patients implanted for diagnostic purposes, observed evoked potentials in the posterior fusiform region about 200 ms after stimulation by strings of letters, but not by pictures of faces, butterflies, or cars. In PNAS, Hirshorn et al. (5) report novel findings on the left VOT cortex contribution to reading, combining, in a kind of methodological tour de force, … [↵][1]1Email: laurent.cohen{at}aphp.fr. [1]: #xref-corresp-1-1