Event Related Potentials Reveal Early Phonological and Orthographic Processing of Single Letters in Letter-Detection and Letter-Rhyme Paradigms

Abstract

Introduction: When and where phonological processing occurs in the brain is still under some debate. Most paired-rhyme and phonological priming studies used word stimuli, which involve complex neural networks for word recognition and semantics. This study investigates early (<300 ms) and late (>300 ms) orthographic and phonological processing of letters.

Methods: Fifteen participants aged 20-35 engaged in three two-forced choice experiments, one letter-detection (LetterID) and two letter-rhyme (Paired-Rhyme and Letter-Rhyme) tasks. From the EEG recordings, event related potential (ERP) differences within and across task stimuli were found. We also calculated the global field power (GFP) for each participant. Accuracies and reaction times were also measured from their button presses for each task.

Results: Behavioral: Reaction times were 18 ms faster to letter than pseudoletter stimuli, and 27 ms faster to rhyme than nonrhyme stimuli.

Erp/gfp: In the LetterID task, grand-mean evoked potentials (EPs) showed typical P1, N1, P2, and P3 waveform morphologies to letter and pseudoletter stimuli, with GFPs to pseudoletters being greater than letters from 160-600 ms. Across both rhyme tasks, there were greater negativities for nonrhyme than for rhyme stimuli at 145 ms and 426 ms. The P2 effect for rhyme stimuli was smaller than letter stimuli when compared across tasks.

Conclusion: Differences in early processing of letters vs. pseudoletters between 130-190 ms suggest that letters are processed earlier and perhaps faster in the brain than pseudoletters. The P2 effect between letter and rhyme stimuli likely reflect sublexical phonological processing. Together, findings from our study fill in evidence for the temporal dynamics of orthographic and phonological processing of single letters.

Keywords: EEG; event related potentials; evoked potentials; global field power; language; letter processing.

Figure 1

Global Field Power (GFP) for Letter vs. Pseudoletter Effects. Grand-mean GFP time-locked to Letter (blue line), Pseudoletter (green line) stimuli, and Letter minus Pseudoletter (black line). Bars below the waveforms designate intervals of significant differences between Letter and Pseudoletter conditions at FDR corrected levels of p < 0.05 (black bars) and p < 0.01 (gray bars). Peaks within the evoked-potentials (EPs) are labeled above their likely corresponding peaks in the GFPs.

Figure 2

EPs for Letter vs. Pseudoletter Effects. Grand-mean EPs to Letter (blue lines), Pseudoletter (green lines), and Letter minus Pseudoletter (black lines) at electrodes Fz, Pz, PO7, and PO8. Bars below the waveforms designate intervals of significant differences between Letter and Pseudoletter conditions at FDR corrected levels of p < 0.05 (black bars) and p < 0.01 (gray bars). Vertical hash lines designate the latencies for the topographies shown below. The larger black dots in the topographies designate the Fz, Pz, PO7, and PO8 electrode locations. Note the scale for the difference topography (Letter minus Pseudoletter) is half the scale for the Letter and Pseudoletter topographies.

Figure 3

GFP for NonRhyme vs. Rhyme Effects. Grand-mean GFP time-locked to Rhyme (orange line), NonRhyme (pruple line) stimuli, and NonRhyme minus Rhyme (black line). Bars below the waveforms designate intervals of significant differences between NonRhyme and Rhyme conditions at FDR corrected levels of p < 0.05 (black bars) and p < 0.01 (gray bars). Peaks within the EPs are labeled above their likely corresponding peaks in the GFPs.

Figure 4

EPs for NonRhyme vs. Rhyme Effects. Grand-mean EPs to Rhyme (orange line), NonRhyme (pruple line) stimuli, and NonRhyme minus Rhyme (black line) at electrodes Fz, Pz, PO7, and PO8. Bars below the waveforms designate intervals of significant differences between NonRhyme and Rhyme conditions at FDR corrected levels of p < 0.05 (black bars) and p < 0.01 (gray bars). Vertical hash lines designate the latencies for the topographies shown below. The larger black dots in the topographies designate the Fz, Pz, PO7, and PO8 electrode locations. Note the scale for the difference topography (NonRhyme minus Rhyme) is half the scale for the Letter and Pseudoletter topographies.

Figure 5

GFP for Letter vs. Rhyme. Grand-mean GFP time-locked to Letter (blue line), Rhyme (orange line), and Letter minus Rhyme (black line). Bars below the waveforms designate intervals of significant differences between Letter and Rhyme conditions at FDR corrected levels of p < 0.05 (black bars) and p < 0.01 (gray bars). Peaks within the EPs are labeled above their likely corresponding peaks in the GFPs.

Figure 6

EPs for Letter vs. Rhyme Effects. Grand-mean EPs to Letter (blue line), Rhyme (orange line), and Letter minus Rhyme (black line) at electrodes Fz, Pz, PO7, and PO8. Bars below the waveforms designate intervals of significant differences between Letter and Rhyme conditions at FDR corrected levels of p < 0.05 (black bars) and p < 0.01 (gray bars). Vertical hash lines designate the latencies for the topographies shown below. The larger black dots in the topographies designate the Fz, Pz, PO7, and PO8 electrode locations. Note the scale for the difference topography (Letter minus Rhyme) is half the scale for the Letter and Pseudoletter topographies.