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Figure 3.3 Serial position function for letter‐in‐string visibility with central fixations (i.e., fixation on the middle letter) explained by the combined influence of acuity (linear drop from fixated letter to outer letters) and crowding (greatest for inner letters that are flanked by two letters, least for outer letters that are flanked by one letter). The figure also illustrates the advantage for initial position, which can be much greater depending on the testing conditions.

      Grainger et al., 2016/With permission of Elsevier.

Evidence supporting the role of acuity and crowding in letter identification comes from serial position functions for letter‐in‐string visibility. These are typically obtained by presenting a string of letters to participants and post‐cueing one of the locations in the string for identification. To limit contributions from phonological processing and whole‐word representations, the letter strings in these experiments usually comprise random consonants. Such experiments typically reveal a W‐shaped function for visibility, with letter identification accuracy best in the beginning, middle, and end positions. This W‐function is thought to reflect a combination of the decrease in acuity from the central letter to the outer letters plus reduced crowding for the outer letters (Figure 3.3). While digits show a similar pattern, other kinds of simple visual stimuli, such as symbols and simple shapes do not (Tydgat & Grainger, 2009).

      It has been hypothesized that spatial attention contributes to the first‐letter advantage, whereby identification of initial letters is higher than either the letter on fixation or the final letter due to a word‐beginning bias in the deployment of attention (e.g., Aschenbrenner et al., 2017). However, given findings that suggest that attention cannot be manipulated within a word (e.g., Ducrot & Grainger, 2007), alternative accounts of the first‐letter advantage have been proposed (Chanceaux et al., 2013).

       Letter positions

Beyond letter identity, information about the positions of letters within a word is crucial for accurate word recognition. Grainger and van Heuven (2004) proposed a model of orthographic processing that provides an account of how location‐specific visual information is used to generate a location‐invariant code for letter‐in‐word order (see Dehaene et al., 2005, for a similar proposal). In this account, the first level of orthographic processing involves the location‐specific encoding of letter identities via a horizontally aligned bank of letter detectors (for written languages with horizontally aligned scripts; see Figure 3.4). These function in the same way as the isolated letter detectors previously described and are thought to be case‐specific (e.g., separate detectors for lowercase a and uppercase A). Such location‐specific and case‐specific letter detectors provide information about where different letter identities are located along a line of text relative to where readers’ eyes are looking. These location‐specific letters are analogous to the location‐specific complex features involved in visual object identification. Therefore, one of the major transitions that has to be achieved during reading acquisition is the change from letters as individual objects to letters as object parts. In other words, during the very first stages of learning to read, after learning the alphabet, letters take on the role of the complex features that subtend visual object identification.

Orthographic Processing and Word Recognition

      Having established the bases of letter‐level processing, we now turn to consider the visual and orthographic factors that determine ease of single‐word recognition.