Let's face it: reading acquisition, face and word processing.

Abstract

A TIGHT LINK BETWEEN READING ACQUISITION AND CHANGES IN FACE PROCESSING The invention of writing is one of the most important cultural changes of mankind. Notably, because reading was invented only 5000 years ago, there was not sufficient time to evolve a brain system devoted to visual word recognition. Nevertheless, learning to read leads to the development of a strong response to written materials in the left fusiform gyrus, in the “visual word form area” (VWFA, e.g., Dehaene, 2009). Consequently, reading must rely on pre-existing neural systems for vision and language, which may be partially “recycled” for the specific problems posed by reading (Dehaene, 2005; Dehaene and Cohen, 2007). This consistent localization is related to prior properties of the corresponding tissue, which make it particularly suitable to the specific problems posed by the invariant visual recognition of written words (Dehaene, 2009): bias for foveal stimuli (Hasson et al., 2002), posterior-toanterior increase in perceptual invariance (Grill-Spector et al., 1998; Lerner et al., 2001), and more direct projection fibers to language areas (Cohen et al., 2000; Epelbaum et al., 2008). The neuronal recycling model predicts that, as cortical territories dedicated to evolutionarily older functions are invaded by novel cultural objects, their prior organization should slightly shift away from the original function (though the original function is never entirely erased). As a result, reading acquisition should displace whichever evolutionary older function is implemented in the site of the VWFA. In a recent fMRI study (Dehaene et al., 2010) comparing illiterate to literate adults, we showed that learning to read competes with the cortical representation of other visual objects, especially faces. With increasing literacy, cortical responses to faces decrease slightly in the left fusiform region, while increasing strongly in the right fusiform area (FFA). Thus, right-hemispheric lateralization for faces is increased in literates compared to illiterates. Consistent evidence was also reported when comparing 9-year-old normal readers and dyslexic children. Not only did responses to written words showed a greater left lateralization in normal readers, but responses to faces were also more strongly right lateralized (Monzalvo et al., 2012; cf. also Monzalvo, 2011). Further developmental studies also reveal a tight link between reading acquisition and changes in face processing. Cantlon et al. (2011) demonstrated that 4-year-old-children show decreasing responses to faces in the left fusiform gyrus with increasing knowledge of letter and number symbols. Li et al. (2013) found for Chinese preschooler’s a facilitative effect of early exposure to reading in neural responses to visual words. Such a facilitative effect had a temporary cost in neural response to faces, which did not show the mature pattern seen in adults. Dundas et al. (2012) studied the development of hemispheric specialization for written words and faces in children, adolescents and adults, and found that the left-hemispheric specialization for words develops prior to the right-hemispheric specialization for faces, with face lateralization related to reading comprehension ability (cf. also Pinel et al., 2014). The competition between different categories seems to rely on enhanced neuronal specificity, namely on decreased responses to non-preferred stimuli as opposed to an increased response to the preferred category. Indeed, Cantlon et al. (2011) found a decrease for non-preferred stimuli in VWFA and FFA during child development. Joseph et al. (2011) reported both progressive changes, i.e., increasing face-specialization in a brain region with age, and regressive changes, i.e., decreasing face-specialization with age. Indeed, many brain regions recruited in children for face processing showed reduced specialization for faces by adulthood. Such regressive changes support the idea that some areas of the face network may lose out, at a cellular and a functional level, to promote specialization. Consistent with this scenario, in Pinel et al. (2014)’s study stronger leftward asymmetry for the VWFA and rightward asymmetry for the FFA were characterized by reduced activation in homolog areas of the contralateral hemisphere. All these observations support the existence of competition for cortical space between the VWFA and the preexisting neural coding of faces (Dehaene, 2005; Dehaene and Cohen, 2007; Plaut and Behrmann, 2011), with some displacement of fusiform face-sensitive areas toward the right hemisphere. A similar theoretical perspective (Nestor et al., 2012; Behrmann and Plaut, 2013) espouses a competitive interaction between distributed circuits for faces and word recognition for foveally-biased cortex, constrained by the need to integrate reading with the language system primarily left-lateralized. Recently, we investigated the behavioral consequences of this brain reorganization (Ventura et al., 2013). Using the face composite task, we found that literates are consistently less holistic than illiterate