Hierarchical coding of letter strings in the ventral stream: dissecting the inner organization of the visual word-form system.

Visual word recognition has been proposed to rely on a hierarchy of increasingly complex neuronal detectors, from individual letters to bigrams and morphemes. We used fMRI to test whether such a hierarchy is present in the left occipitotemporal cortex, at the site of the visual word-form area, and with an anterior-to-posterior progression. We exposed adult readers to (1) false-font strings; (2) strings of infrequent letters; (3) strings of frequent letters but rare bigrams; (4) strings with frequent bigrams but rare quadrigrams; (5) strings with frequent quadrigrams; (6) real words. A gradient of selectivity was observed through the entire span of the occipitotemporal cortex, with activation becoming more selective for higher-level stimuli toward the anterior fusiform region. A similar gradient was also seen in left inferior frontoinsular cortex. Those gradients were asymmetrical in favor of the left hemisphere. We conclude that the left occipitotemporal visual word-form area, far from being a homogeneous structure, presents a high degree of functional and spatial hierarchical organization which must result from a tuning process during reading acquisition.