Читать книгу The Science of Reading - Группа авторов - Страница 58

The preceding section examined the identification of letters in isolation, but the vast majority of written words are composed of more than one letter. We move now to consider the factors that govern the processing of letter identities in letter strings under conditions that minimize any higher‐level phonological, morphological, or lexical influences.

Grainger et al. (2016) defined three factors that determine the visibility, and therefore the identifiability, of individual letters presented in letter strings: visual acuity, crowding, and spatial attention. Acuity and crowding are thought to be the principle factors involved in determining letter‐in‐string visibility (see Figure 3.3). Visual acuity is determined by the density of retinal receptors and drops sharply and linearly from the point of focus of the eyes within foveal vision – which is the region of vision that generally encompasses single word reading. Crowding is another general visual constraint that determines the identifiability of visual objects as a function of the proximity of surrounding objects (Pelli & Tillman, 2008). It can be thought of as a form of visual clutter, with the deleterious impact of surrounding objects increasing the closer they are to the target object. This interference in object identification is determined by the spatial extent of the crowding zone that surrounds a to‐be‐identified object. Experiments have shown a linear increase in the extent of the crowding zone as the eccentricity of the center of the zone increases. This function is known as Bouma’s law (Pelli & Tillman, 2008), named after Herman Bouma, a pioneer in letter perception and crowding research.

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).