Hierarchical Coding of Letter Strings in the Ventral Stream: Dissecting the Inner Organization of the Visual Word-Form System

Abstract

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.

Breaking the symmetry: Mirror discrimination for single letters but not for pictures in the Visual Word Form Area

“…Together with a previous fMRI study from our group (Dehaene et al, 2010) the present work provides further evidence supporting the participation of this restricted region in mirror-orientation discrimination for both letters and words. This finding does not necessarily contradict the "local combination detector" (LCD) model of reading, which assumes a hierarchical coding of letters, bigrams, morphemes and short words within partially distinct sectors of ventral visual cortex , as supported by fMRI activation and priming experiments (Dehaene et al, 2004;Vinckier et al, 2007). First, when reanalyzing together the words and letters experiments, we did find stronger mirror-discrimination for words than for letters in a region just anterior to the VWFA, consistent with the proposed ventral gradient.…”

Neuroscience and education: prime time to build the bridge

“…This hypothesis was tested by measuring brain responses to visual strings that progressively disrupt the 'natural statistics' of the alphabet at different scales: JZWYZK (infrequent letters), QOADTM (frequent letters), QUMBST (frequent bigrams) and AVONIL (frequent quadrigrams). Results showed a gradient of selectivity spanning the left occipitotemporal cortex, with increasing selectivity for higher level stimuli toward the anterior fusiform region 70 .…”

The left occipitotemporal system in reading: Disruption of focal fMRI connectivity to left inferior frontal and inferior parietal language areas in children with dyslexia

“…More recently, a posterior to anterior VWF-System gradient of increasing print specificity was found in adults and adolescents (Brem et al, 2006;Vinckier et al, 2007) as well as in children (Brem et al, 2009;Van der Mark et al, 2009). …”

“…Recently, the VWFA was shown to be part of a larger Visual Word Form (VWF) system that plays a vital role in processing orthographic representations of visual letter-strings (Brem et al, 2006;Mechelli et al, 2005;Van der Mark et al, 2009;Vinckier et al, 2007).…”