Abstract
The decoding of visually presented line segments into letters, and letters into words, is critical to fluent reading abilities. Here we investigate the temporal dynamics of visual orthographic processes, focusing specifically on right hemisphere contributions and interactions between the hemispheres involved in the implicit processing of visually presented words, consonants, false fonts, and symbolic strings. High-density EEG was recorded while participants detected infrequent, simple, perceptual targets (dot strings) embedded amongst a of character strings. Beginning at 130 ms, orthographic and non-orthographic stimuli were distinguished by a sequence of ERP effects over occipital recording sites. These early latency occipital effects were dominated by enhanced right-sided negative-polarity activation for non-orthographic stimuli that peaked at around 180 ms. This right-sided effect was followed by bilateral positive occipital activity for false-fonts, but not symbol strings. Moreover the size of components of this later positive occipital wave was inversely correlated with the right-sided ROcc180 wave, suggesting that subjects who had larger early right-sided activation for non-orthographic stimuli had less need for more extended bilateral (e.g., interhemispheric) processing of those stimuli shortly later. Additional early (130–150 ms) negative-polarity activity over left occipital cortex and longer-latency centrally distributed responses (>300 ms) were present, likely reflecting implicit activation of the previously reported ‘visual-word-form’ area and N400-related responses, respectively. Collectively, these results provide a close look at some relatively unexplored portions of the temporal flow of information processing in the brain related to the implicit processing of potentially linguistic information and provide valuable information about the interactions between hemispheres supporting visual orthographic processing.
Highlights
Written words are among the most pervasive visual stimuli that modern humans encounter over the course of their lives
Evoked responses in the present design most notably identify strong right-occipital enhancement for non-orthographic stimuli (170–190 ms) that was followed by a bilateral occipital modulation (240–300 ms). These activations appear to work in a interactive fashion, such that the size of components the later positive occipital wave was inversely correlated with the right-sided ROcc180 wave, suggesting that subjects who had larger early right-sided activation for non-orthographic stimuli may have had less need of a more extended bilateral processing shortly later
Regardless, the large and dominant size of the right-sided effect is clear and robust in the present study, which we believe is best explained as being due to our choice of a highly orthogonal, implicit-visual-processing task. Due to their high temporal resolution, ERPs provide an excellent tool for studying the temporal dynamics of the functional selectivity related to linguistic processing in the brain
Summary
Written words are among the most pervasive visual stimuli that modern humans encounter over the course of their lives. Previous studies of the processing of visually presented word and word-like stimuli using positron emission tomography (PET) (Petersen et al, 1988, 1990; Petersen and Fiez, 1993; Liotti et al, 1994; Price et al, 1996) and functional magnetic resonance imaging (fMRI) (Puce et al, 1996; Indefrey et al, 1997; Cohen et al, 2000, 2002; Tagamets et al, 2000; Baker et al, 2007), have emphasized the left-lateralized nature of word-form processing in occipital cortex (reviewed in Bastiaansen and Hagoort, 2006; Dien, 2009b) These studies have found that visually presented real words and pseudowords (pronounceable non-words), relative to nonword control stimuli (e.g., consonant strings and digits), selectively elicit enhanced activity in portions of the left medial occipital cortex. While these hemodynamically based neuroimaging studies have been highly valuable in identifying the anatomical substrates of visual orthographic processing, they are, notably limited in their ability to reveal much about the temporal dynamics of the underlying processing
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