Читать книгу The Science of Reading - Группа авторов - Страница 61

The studies of letter‐in‐string processing described in the preceding section typically presented stimuli centered on fixation. However, varying fixation location within a letter string completely modifies the distribution of visual acuity across the string. This has been identified as the main factor determining what has been referred to as the “optimal viewing position” effect in words (O’Regan & Jacobs, 1992). The location of initial fixation within a word is varied by first presenting a fixation cross and then adjusting the position of the to‐be‐presented word, relative to the fixation cross. Word recognition is facilitated by fixations toward the beginning rather than the end of a word, with an optimal location just off‐center and a subsequent cost for fixations further toward the extremities. That is, given a seven‐letter word in English, word identification is best with an initial fixation approximately on the third letter, and drops monotonically from this position to the first and last letters, with an overall advantage for beginning letters. This viewing position function takes the form of a J‐shaped function for response times and the inverse of this for accuracy (e.g., O’Regan et al., 1984; O’Regan & Jacobs, 1992). Visual acuity accounts for the decrease in performance as initial fixation moves from the center out, since the summed acuity across letter positions follows this simple linear function. What remains to be explained is the asymmetric form of the viewing position function, with an advantage for initial fixations toward word beginnings, at least in languages read from left‐to‐right.

Three factors have been proposed to account for this asymmetry (see Brysbaert & Nazir, 2005, for a detailed overview). The first is perceptual learning. When reading text, the eye lands preferentially at certain positions in a word, referred to as the preferred landing position (Rayner, 1979). For long words in languages read from left‐to‐right, this position is somewhat to the left of the middle of the word (Ducrot & Pynte, 2002). Noting the similarity between the preferred landing position seen in text reading and the optimal viewing position observed for isolated words, Nazir (2000) proposed that the processing of isolated words is determined by how these words are typically fixated in normal (i.e., text) reading and that this is achieved via a form of perceptual learning. That is, a frequency‐sensitive learning mechanism operating on visuo‐orthographic representations of words. According to this account, the preferred landing position is not a consequence of readers aiming to optimize word identification processes (e.g., Legge et al., 1997). Rather, it is due to low‐level oculomotor constraints, and it is these same constraints that subsequently determine the optimal viewing position function that characterizes the processing of isolated words.

A rather different account of the asymmetric nature of the optimal viewing position function is hemispheric specialization. On this account, part of the observed asymmetry may be caused by the location of brain structures involved in processing printed words. Information falling on the right visual field of both eyes is initially projected onto the left hemisphere of the brain, whereas information falling on the left visual field is projected onto the right hemisphere. Lexical processing typically takes place in the left hemisphere. This means that information about letters left of fixation and initially sent to the right hemisphere must be transmitted through the corpus callosum to the brain regions dedicated to lexical processing and located in left ventral occipital cortex (Cohen et al., 2000). This detour takes time, and could therefore produce the observed asymmetry in the viewing position function since with initial fixations toward word beginnings more letters will be directly processed by the left hemisphere (Brysbaert, 1994).

Finally, the distribution of information within a word might result in the asymmetric function. O’Regan et al. (1984) suggested that the optimal viewing position might arise simply from a combination of changes in visual acuity and the distribution of information across a word, with word beginnings being more informative with respect to word identity than are word endings (see Clark & O’Regan, 1999, for a more detailed exploration of this hypothesis).

Brysbaert and Nazir (2005) argued that all three factors play a role in determining the shape of the viewing position function, along with visual acuity. Key evidence in favor of this stance is that the asymmetry does not completely reverse in languages read from right‐to‐left such as Arabic (Farid & Grainger, 1996) and Hebrew (Deutsch & Rayner, 1999). Instead, the viewing position function is more symmetrical in these languages. This suggests that hemispheric specialization is relevant, in combination with other factors. There is good evidence that the distribution of information within words is key and might even remove the need to appeal to a perceptual learning account. It is possible to shift the viewing position function by varying the way information is distributed across a word, for example, by comparing prefixed versus suffixed words (Farid & Grainger, 1996; Deutsch & Rayner, 1999) or considering the location of the most informative letters with respect to word identity (O’Regan et al., 1984) as well as the visibility of these letters (Stevens & Grainger, 2003). The fact that the most informative letters tend to be located toward word beginnings might explain why readers’ initial fixations tend to be located toward word beginnings (Legge et al., 1997).

Other visual factors are known to have an impact on reading fluency. Two such factors are print size and inter‐letter spacing. There is a critical print size for maximal reading speed beyond which there is no further gain (Chung et al., 1998): increasing print size in peripheral vision beyond this does not counteract the drop‐off in acuity. This is partly because increasing the size of text is accompanied by an increase in eccentricity. Similarly, there is evidence that small increases in inter‐letter spacing are beneficial for reading (e.g., Perea & Gomez, 2012; Zorzi et al., 2012), but larger increases in inter‐letter spacing eventually interfere with reading (e.g., Chung, 2002; Legge et al., 1985), as does smaller than normal inter‐letter spacing (Montani et al., 2015).

In sum, the position in a word where readers first fixate that word has a strong impact on ease of word identification, along with the effects of other more obvious visual factors such as print size and inter‐letter spacing.