Reading and dyslexia
2.5 How written words are coded?
Some models of word recognition have been expressed in the form of computer programs which aim to simulate aspects of human word recognition. The most recent models (Plaut et al., (1996), Coltheart et al. (2001), Zorzi et al. (1998) here described were able to simulate the data reported from cognitive psychol-ogy in terms of RTs, accuracy and effects of lexicality, frequency and regularity, although they adopted different assumptions. However, as we know, the fact that models are adequate does not guarantee that they are correct.
Although computational models have played a key role in the study of read-ing, most of them have focused on the interactions between orthography, phonol-ogy and semantics, ignoring the more peripheral stage of visual word recog-nition. Most current computational models (e.g. Coltheart et al., 2001; Mc-Clelland and Rumelhart, 1981) assumes that abstract letter representations are directly connected to word form representations, ignoring the possible role of sublexical units such as syllables. A further limit is that these models of-ten presuppose a case- and location- invariant representation (Dehaene, et al., 2005). The main problem is that the processing from a single letter to the word identification has been underestimated. Therefore since most of the models did not focus on the early stage of word recognition, they do not make predictions about the reading patterns which result from a deficit at the different levels of this early stage.
Therefore, we can hypothesize which stages we are likely to carry out when we see a word. When a word such as ’albero’ is presented, it is first analysed in terms of contours, shape and single letters by the visual areas (ranging from V1 to V4) which compute increasingly abstract representations. The first stage specific to orthographic material we experience is of letter processing. Then letters are integrated into subunits like syllables and morphemes. Finally the word unit is carried out.
Different types of evidence suggest the existence of these stages. The exis-tence of the letter-level processing is provided by various studies. First, it is unlikely that reading is based directly on visual features or shape information:
any time we see a word written in a new font we should not be able to read it. Second, the mixed effect, revealed more than 25 years ago (McClelland, 1976), where a word like ’aLbErO’ can be read with relative ease (although more slowly than the normal presentation). An analysis at the single abstract
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letter identity level must be carried out in order to read the word. Third, migra-tion errors that can occur in attenmigra-tional dyslexia as well as with normal sub-jects under certain conditions of brief presentation (Davis and Coltheart, 2002), indicating that letters are coded separately. Fourth, experimental psychology studies using priming methods have consistently shown that information con-cerning the identity and the position of individual letters is already coded in the early phase of processing (Peressotti and Grainger, 1995). Humphreys, Evett and Quinlan (1990), for example have shown that letter-string primes facilitate the identification of the target word when prime and target share common letters. This priming effect can occur with only 1 letter and it varies with the position of that letter. For instance, the target report was facilitated when primes and targets had the first letter in common, but not when the let-ter was the second one or the third one. In fact the effect is more robust for end letters than middle letters. Fifth, an imaging study with normal subjects showed that the most posterior part of the visual word form area (VWFA), a brain region particularly responsive to visual word recognition, was bilaterally activated by letter-form processing and insensitive to the word-level (Dehaene, Jobert, Naccache, Ciuciu, Poline, Le Bihan and Cohen, 2004).
As mentioned before, the integrative process of letters into sublexical units has been largely underestimated. However there is evidence indicating an in-termediate stage comprising bigrams, syllables and morphemes before access-ing the word form. First, Graaccess-inger and Whitney described a scheme called
’open bigrams’ (Schoonbaert and Grainger, 2004; Grainger and Whitney, 2004;
Whitney, 2001). This scheme, that has not been implemented in a computa-tional model, comprises 5 layers: a retinal level, a feature level, a letter level and then a bigram level before the final, word level. Therefore letter and word levels are not directly connected. On this account, word coding is based on or-dered letter pairs, the bigrams, which do not contain precise information about letter position: for example the word ’take’ is represented by activation of units
representing TA TK, TE, AK, AE and KE1.
This model can account for effects of similarity and of letter transposition shown in experimental studies of priming. For example in studies of masked-priming, ’garden’ is identified more rapidly when preceded by a masked prime that respects the relative positions as ’grdn’, compared to the control condition
’pmts’ or to the condition where the order is changed as ’gdrn’. Although this scheme has some problems (for instance it fails to assign a unique code to each word) it takes into account the issue of how integration of letters can occur.
Second, there is evidence in favour of syllabic processing in reading words at least in languages with clear syllabic boundaries such as Italian, French or Spanish (Ferrand, Segui and Grainger, 1996; Carreiras, Alvarez and de Vega, 1993). In a study with ERPs, Carreiras et al. (in press) found that when in bisyllabic words and pseudowords there was a match between syllable bound-aries and colour boundbound-aries, different evoked responses emerged compared to when there was a mismatch between the syllable boundaries and the colour boundaries. In particular, the ERP effect of colour-syllable congruency for both words and pseudowords was very early, namely in the P200 time window. Lex-icality effects showed up at the N400 component. This suggests that: 1. at least in languages with clear syllabic boundaries, the syllable has a role in word processing, not only for its phonological but also for its visual nature; 2.
generally, we might not process bisyllabic words as a whole.
Finally, in a recent model put forward by Dehaene et al. (2005), it was attempted to explain how words are coded by solving problems of location and case invariance (see Fig. 2.10). Their model is inspired from neurophysiological models of invariant object recognition and proposes a hierarchy of local com-bination detectors sensitive to increasingly larger fragments of words. More
1It is noteworthy that in these calculations open bigrams are limited to a maximum of two intervening letters, therefore the word ’garden’ is represented only by GA, GR, GD, AR, AD, AE, RD, RE, RN, DE, DN and EN.
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Figure 2.10: Model of invariant word recognition by a hierarchy of local combi-nation detectors (Dehaene, Cohen, Sigman and Vinckier, 2005). This model is inspired from neuropsysiological models of invariant object recognition. Each neuron is assumed to pool activity from a subset of neurons at the immediately lower level, thus lead to an increasing complexity, invariance and size of the receptive field at each stage.
specifically, by assuming to pool activity from a subset of neurons at the imme-diate lower level, they propose that at first there are neurons which respond to local contrasts, then to oriented bars and then to local contours (fragments).
At the next stage, combinations of fragments can be used to recognize a letter shape in a specific case; abstract letter identities is then recognized at the next stage by pooling activity from letter shapes detectors. The subsequent stage comprises neurons sensitive to local combination of bigrams, as it is hypothe-sized by the model of Grainger and Whitney, (2004); finally there are neurons sensitive to ordered combinations of bigrams, morphemes and small words.
Therefore as occurs in object recognition, an integrative process is postulated for word recognition through a hierarchy of local combination detectors.
In the present work we attempted to study the early stage of word coding by focusing both on the letter level and on the integrative process that brings to the identification of syllables and words.