Position coding in the visual word form area.

H ow does our brain achieve its impressive expertise to effortlessly read written words? It has been proposed that a key element of the capability to recognize visually presented words (as well as other objects) is a normalization process that is assumed to operate along the visual hierarchy. This normalization process would remove varying aspects of presented words from the evoked retinal representations in the eye and early visual cortex, including spatial position, size, and rotation, resulting in standardized (invariant) representations in higher-level visual areas (Fig. 1 A and B). Numerous studies (1–5) have identified a region in a small strip of the fusiform gyrus in the left hemisphere that seems to be specialized for orthographic processing of words called the visual word form area (VWFA). Because this area is at a rather high level within the visual hierarchy and in close proximity to areas specialized for object recognition (e.g., faces), dominant reading theories assume that the representations within the VWFA contain invariant representations of words (5). The study by Rauschecker et al. in PNAS (6) challenges this traditional view, showing, with a series of elegant experiments, that the representations in VWFA are actually position-dependent (i.e., they reflect the original position of words at the retina) (Fig. 1 C and D), despite the rather highlevel location of this area in the visual hierarchy. How does the study by Rauschecker et al. (6) reveal position-dependent representations in VWFA? Previous brain imaging studies using centrally presented word stimuli reported only one VWFA in the left hemisphere (1–5). This finding had been used as additional evidence that this area contains invariant word representations, because it is generally assumed that, at a level of invariance across the whole visual field, a single specialized brain area in only one hemisphere would be sufficient. This reasoning parallels findings for the fusiform face area, a specialized area for face processing (7) that is located in close proximity to the VWFA and reported to exist (mainly) in the right hemisphere. The study by Rauschecker et al. (6) investigates this assumption systematically by measuring brain activity with functional MRI while words are presented at different positions in the visual field (Fig. 1). When words are presented at the right side of the point of gaze (marked by the blue fixation cross in Fig. 1), activation is observed primarily in the VWFA of the left hemisphere (Fig. 1C), whereas words presented at the left side result in activity primarily in the right hemisphere in a region at a corresponding hierarchical level (Fig. 1D). This observation not only shows position dependence, but it also reveals that the area specialized for visual word form is located in both hemispheres. Because the traditional view locates this area only in the left hemisphere, the study by Rauschecker et al. (6) labels the VWFA in the right hemisphere as rVWFA. Although a hemispheric bias of activity for words presented in the left vs. right visual field is sufficient to show positiondependent coding in VWFA, the study by Rauschecker et al. (6) also reveals that words displaced vertically with respect to the point of gaze evoke different activity patterns in VWFA. Because the presentation of words at different vertical positions in the visual field leads to strongly overlapping responses in VWFA in both hemispheres, the study by Rauschecker et al. (6) relies on the results of multivoxel pattern classification to infer position-sensitive coding. By analyzing differences in distributed spatial patterns, such multivariate classifiers are often able to separate activation from different experimental conditions, even if they evoke similar levels of activity in the same brain region. To show that they are able to separate conditions from each other, classifiers first learn to distinguish condition effects based on one part of the data Fig. 1. Schematic drawing of position coding in VWFA. In the traditional view, it is assumed that VWFA is not sensitive to the position of words in the visual field, i.e., the representation within VWFA for a word is assumed to be identical no matter whether the word is presented at the right (A) or left (B) side of the point of gaze (indicated by the blue cross in the depicted screen). In the view described in the work by Rauschecker et al. (6), VWFA is not invariant to the position of a word, i.e., the representation within VWFA codes at which position in the visual field a word is presented (C and D). According to this view, the left and right VWFA together comprise a complete retinotopically organized map of the visual field.