**2.2 The effect of literacy on the processing of non-linguistic face materials**

Many studies have demonstrated that literacy acquisition modulates the ability to process non-linguistic objects [11], especially faces. We here review a tremendous amount of work on children, adults, and special populations.

One way of understanding the associations between literacy acquisition and face processing is to track the development of face processing ability with the size of literacy or to the relations between lexical and face processing in children. Evidence from the functional magnetic resonance imaging (fMRI), a high-spatial-resolution technique, has suggested that, during literacy acquisition in children, there may be coordination between word and face processing in the left fusiform gyrus (FFA), also termed visual word form area (VWFA) [12, 13]. For instance, neural responses to faces in VWFA declined gradually with the increase in children's letter knowledge [14]. Consistently, event-related potentials (ERPs) studies have identified a stable electrophysiological hallmark, N170 response, of viewing words, which was elicited at electrodes over the left occipitotemporal areas (roughly corresponding to the VWFA). The N170 peaks about 150–200 ms after word onset, and may index the visual processing of words at an expert level. Using the color-matching task, Li et al. found that both the left-lateralization indexed by the N170 of words and the vocabulary was associated positively with the right-lateralization of faces in Chinese preschool children [15]. Similarly, employing the "half-field" paradigm, Dundas et al. showed that the emergence of face lateralization was related positively to reading competence with the control of age, reasoning scores, and face discrimination accuracy [16]. Furthermore, they also reported that the N170 evoked by faces in the right hemisphere was positively related to that by words in the left hemisphere in American children [17]. In a word, there is a tight link between visual face processing and word lateralization or between this processing and the size of literacy.

Also, empirical work from normal adults' study provided stronger evidence of the relationships between word and face processing. For example, using the adaptation paradigm, in which the first adaptor face (or word) was followed subsequently by a target face (or word) of either the same or different identity, Cao et al. found that the adaptor face led to the reduced N170 response to the target word, while the adaptor word did not result in the decreased N170 response to the target face [18]. Neural adaptation occurred because the neural response to the test stimulus was reduced when the stimulus was preceded by a physically identical or categorically identical adaptor stimulus. Therefore, the results of this study may indicate that the facial N170 function partially encompasses the N170 function of word processing, which is consistent with the neural recycling hypothesis. In another study, subjects were asked to view artificial objects (i.e., face-like Greebles) centrally presented with the concurrent lateral presentation of faces and then to judge which side each face was presented. Results showed that the N170 response to faces tremendously decreased after subjects were trained to recognize Greebles compared to before those Greeble novice [19]. Analogously, the bilateral N170 response to faces decreased in identifiable Chinese characters and faces as compared to unidentifiable conditions [20]. In Robinson et al.'s study with an attentional blink paradigm, they found that word (target 2) recognition performance was inferior at short inter-target lags when the word stimulus was preceded by faces compared to glasses and words condition. No effects were observed when words were followed by other objects. Furthermore, ERP results indicated that N170 responses to face (target 1) were associated with the reduction of N170 to words within the face–word condition in the left hemisphere, but not for other object-word conditions [21]. These findings indicated that face and word shared some overlapping neural resources, possibly associated with the same specialized processing module.

In contrast with these studies mentioned above, the special population, such as dyslexia, alexia, prosopagnosia, illiterates, and so on, can provide stronger evidence for the relationship between words and face processing. For example, dyslexic readers performed poorer on recognition of both word and face and even decreased level of hemispheric lateralization to words and faces compared with normal readers [22]. Face recognition was impaired severely with bilateral compared to unilateral temporo-occipital cortex lesion [23], and a left occipital impairment gave rise to both pure alexia and prosopagnosia [24]. Also, prosopagnosic patients showed mild but reliable words recognition deficits, and pure alexic patients showed face recognition deficits [25]. Another important avenue is to examine the differences in face processing between literates and illiterates. The ideal illiterate and literate groups differ only in whether the script system could be used to read and write by them, so they are the ideal group for researchers to understand the relationship between face and word processing. For example, Deheane et al. found that reduced responses to faces in VWFA were observed for literates compared to illiterate adults [26]. And ERP results showed that the literacy effects were observed not only in the letter strings but also in faces, revealing the impact of literacy on common early visual processing [27]. These findings have led to a proposal that face and word processing engage some overlapping neural substrates, and there are interactions between the development of visual representations for faces and words [28]. However, these studies did not weigh the potential contribution of the script system to face processing because they used the script system of one language in a particular social culture.
