Word and pseudoword superiority effects reflected in the ERP waveform

Abstract

A variant of the Reicher-Wheeler task was used to determine when in the event-related potential (ERP) waveform indices of word and pseudoword superiority effects might be present, and whether ERP measures of superiority effects correlated with standardized behavioral measures of orthographic fluency and single word reading. ERPs were recorded to briefly presented, masked letter strings that included real words (DARK/PARK), pseudowords (DARL/PARL), nonwords (RDKA/RPKA), and letter-in-xs (DXXX, PXXX) stimuli. Participants decided which of two letters occurred at a given position in the string (here, forced-choice alternatives D and P). Behaviorally, both word (more accurate choices for letters in words than in baseline nonwords or letter-in-xs) and pseudoword (more accurate choices for letters in pseudowords than in baseline conditions) superiority effects were observed. Electrophysiologically, effects of orthographic regularity and familiarity were apparent as early as the P150 time window (100-160ms), an effect of lexicality was observed as early as the N200 time window (160-200ms), and peak amplitude of the N300 and N400 also differentiated word and pseudoword as compared to baseline stimuli. Further, the size of the P150 and N400 ERP word superiority effects was related to standardized behavioral measures of fluency and reading. Results suggest that orthographic fluency is reflected in both lower-level, sublexical, perceptual processing and higher-level, lexical processing in fluently reading adults.

Figure 1

Behavioral accuracy in the ERP letter identification task across the four conditions. Both a word superiority effect and a pseudoword superiority effect were observed, both with the typical nonword baseline and with the alternate letters-in-xs baseline. Bars indicate standard error.

Figure 2

Grand average ERP waveforms elicited at posterior medial sites by words (solid black line), pseudowords (light gray line), nonwords (dark gray line), and letters-in-xs (dashed line). The P150 and N200 are identified at site O1 and the N300 and N400 are identified at site O2. Each vertical tick marks 100 ms and negative is plotted up. Refer to Figure 6 for full electrode montage. The calibration bar marks 0.5 μV.

Figure 3

Topographical voltage maps illustrating the distributions of the word (left column) and pseudoword (right column) superiority effects in each of the four time windows of interest. A spherical spline interpolation (Perrin, Pernier, Bertrand, & Echallier, 1989) was used to interpolate the potential on the surface of an idealized, spherical head based on the mean amplitude voltages measured from the difference waves (ERPs to nonwords subtracted from ERPs to words and pseudowords) at each electrode location within the specified time windows. Peak amplitude measures (reported in the text) revealed word and pseudoword superiority effects (differential processing between words and nonwords or pseudowords and nonwords, respectively) for all components except an N200 pseudoword superiority effect.

Figure 4

Grand average ERP waveforms elicited by words (solid black line), pseudowords (light gray line), nonwords (dark gray line), and letters-in-xs (dashed line) across recording sites. Peak amplitude of the N300 was measured at all sites shown while peak amplitude of the N400 was measured at the three most posterior rows (N300 and N400 are identified at site P4). More anterior sites are toward the top of the figure while more posterior sites are toward the bottom; left hemisphere sites are on the left and right hemisphere sites are on the right; lateral sites are toward the outer edges and medial sites are toward the middle of the figure; each vertical tick marks 100 ms and negative is plotted up.

Figure 5

Illustration of the stimulus presentation sequence in a single trial.

Figure 6

Schematic representation of the electrode montage and the factors used in analyses. Six levels of the anterior/posterior factor, two levels of the lateral/medial factor, and two levels of the hemisphere factor are indicated. Electrodes in gray were not used in the primary analyses reported in the text.