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.
Vinckier
Fabien
F
INSERM, U562 Orsay, France.
Dehaene
Stanislas
S
Jobert
Antoinette
A
Dubus
Jean Philippe
JP
Sigman
Mariano
M
Cohen
Laurent
L
D000328:Adult; D002540:Cerebral Cortex / Q000502:physiology*; D005260:Female; D005625:Frontal Lobe / Q000502:physiology; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D008297:Male; D009415:Nerve Net / Q000502:physiology; D009778:Occipital Lobe / Q000502:physiology; D010775:Photic Stimulation; D011932:Reading; D021641:Recognition, Psychology / Q000502:physiology*; D013702:Temporal Lobe / Q000502:physiology