What makes written words so special to the brain?

Reading is an integral part of life in today’s information-driven societies. How the human brain sees and processes words is the focus of this ebook. It includes a collection of 22 papers that illustrate current issues in the neurobiology and psychophysics of word processing. Using varieties of behavioral tests and neuroimaging techniques, they investigated word processing mechanisms across different alphabetic and logographic writing systems, such as English, Chinese, Arabic, Japanese, French or German. Each paper provides useful literature reviews, methodological developments and a host of novel findings that will inspire future investigations into the neural systems that support reading. Several behavioral, fMRI and ERP studies investigated how word-likeness modulated the underlying cognitive and neural processing. The fMRI studies focused on many regions of the reading system, in particular a region in the left ventral occipitotemporal cortex known as the visual word form area (VWFA), whereas the ERP studies featured prominently the N1/N170 component around 200ms post-stimulus. Participants were mainly healthy skilled readers, although a few studies have also looked at reading in dyslexic, autistic, or congenitally deaf subjects. Ludersdorfer et al. (2013) investigated how the VWFA responds to visual and auditory stimuli that differed in their word-likeness. VWFA activation decreased for visual stimuli from false-fonts over pseudowords to words, presumably reflecting more efficient processing of familiar words. In contrast, auditory stimuli lead to a general deactivation in visual areas for all stimuli, except for the VWFA where deactivation was spared for word and pseudoword stimuli, presumably due to modulation by linguistic information. Deng et al. (2013) used two cross-modal tasks, phonological retrieval of visual words and orthographic retrieval of auditory words, to examine the unimodal and multimodal regions for logographic language processing. The VWFA responded exclusively to visual inputs, whereas an adjacent region in the left inferior temporal gyrus showed comparable activation for both visual and auditory inputs. A role of the VWFA in integrating visual and auditory processes is also suggested by McNorgan et al. (2013) who reported correlations between a behavioral phonemic awareness task (phoneme elision) and neural activation in an audiovisual condition in typically developing children. No such correlations were found in children with reading disability, nor in any of the groups for unimodal stimuli. Using ERP, an inverse word-like effect was also found for single letters by Herdman and Takai (2013), who reported an increased and delayed N1 for pseudoletters than letters, which was not modulated by attention. As studies using entire words typically report a reversed effect, this may suggest that processing single letters differs from processing letter strings and that early orthographic processing of letters is largely automatic. Hasko et al. (2013) not only showed a positive word-like effect in the N1 component, as the N1 was larger for letter than false font strings in normally reading children, but they also revealed that this effect was reduced in dyslexic children, presumably reflecting deficient orthographic processing in dyslexics. While Hasko et al. (2013) did not find N1 differences between pseudohomophones and words in children, Taha and Khateb (2013) found a larger N1 for pseudohomophones than words in Arabic, suggesting that such effect may depend on reading development, task, or properties of the writing system. Orthographic analysis in the later part of the N1 also seems to be sensitive to stimulus repetition, as shown by Du et al. (2013). However, such N200 repetition effects appear to be delayed if word form configuration is changed, which can be achieved in Chinese by switching characters in two-morphemic words. Regarding the impact of orthographic depth on reading routes, Buetler et al. (2014) recorded electrical brain activity in highly proficient bilinguals who read the same pseudowords either in German or French. The topography of the ERPs to identical pseudowords differed 300–360ms post-stimulus onset when the pseudowords were read in different orthographic depth context. Their findings suggest that reading in a shallow context relies more on non-lexical pathways with greater engagement of frontal phonological areas, whereas reading in a deep orthographic context recruits less non-lexical pathways with greater engagement of visuo-attentional parietal areas. We note that many of these fMRI and ERP studies used conditions which differed in word-likeness, and reported either positive or negative relations in the observed neural activation. The reason for this divergence is still poorly understood, but probably reflects that orthographic processing includes both visual processing and modulation by linguistic information. Regarding the elusive role of the VWFA, a review by Vogel et al. (2014) challenges current models that posit a functional specialization of the VWFA solely for words. They argue that the VWFA is not used specifically or even predominantly for reading. In their model, the VWFA is used in processing visually complex stimuli in “groups,” and it is strongly connected to the dorsal attention network so that attention can be directed to familiar