The temporal dynamics of implicit processing of non-letter, letter, and word-forms in the human visual cortex

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.

Keywords: ERPs; visual cortex; visual orthography; word reading.

Figure 1

Stimulus schematic showing example character strings for the four stimulus types and the infrequent dot-string targets.

Figure 2

‘LOcc140’ results. Topographic distributions, shown from a posterior perspective, of the mean activity from 130–150 ms for Words (top left), Consonants (top right), False Fonts (middle), and their Differences (Words minus False Fonts on bottom left and Consonants minus False Fonts on bottom right).

Figure 3

‘ROcc180’ results are displays for false fonts (cyan) versus consonants (magenta) on top and symbols (orange) versus consonants (magenta) on the bottom. Left panels: Grand average ERPs at a pair of left and right occipital sensors [Note that slightly different sensor locations are used for the two different contrasts.] Middle panels: Difference maps computed between conditions from 170–190 ms are shown from a posterior perspective. Right panels: BESA-computed equivalent current dipole sources for these effects (single-source solutions) and the associated source waveforms are shown on the right, along with the residual variance values for these solutions.

Figure 4

Sequential topographic maps (posterior view) across time of the grand average voltage distributions for the different contrasts, as labeled.

Figure 5

Late-latency effects. ERPs elicited by each stimulus type are shown at three central-parietal sites. Top: The amplitude of the negative-going wave in the 300–500 ms range showed a hierarchical pattern, with more word-like stimuli eliciting greater negative deflections (i.e., words > consonants > false fonts > symbols), activity differences likely corresponding to modulations of the linguistically related N400 component. Bottom: The topographic distributions of the difference waves, relative to symbol strings, for each stimulus type, shown as spline-interpolated flat maps.