Position sensitivity in the visual word form area

Abstract

Seeing words involves the activity of neural circuitry within a small region in human ventral temporal cortex known as the visual word form area (VWFA). It is widely asserted that VWFA responses, which are essential for skilled reading, do not depend on the visual field position of the writing (position invariant). Such position invariance supports the hypothesis that the VWFA analyzes word forms at an abstract level, far removed from specific stimulus features. Using functional MRI pattern-classification techniques, we show that position information is encoded in the spatial pattern of VWFA responses. A right-hemisphere homolog (rVWFA) shows similarly position-sensitive responses. Furthermore, electrophysiological recordings in the human brain show position-sensitive VWFA response latencies. These findings show that position-sensitive information is present in the neural circuitry that conveys visual word form information to language areas. The presence of position sensitivity in the VWFA has implications for how word forms might be learned and stored within the reading circuitry.

Position coding in the visual word form area

“…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.…”

“…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.…”

“…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)(2)(3)(4)(5).…”

“…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.…”

“…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).…”