Combined studies of N170/M170 responses to single letters and pseudoletters

To read efficiently, individuals must be able to rapidly identify letters within their visual networks, which occurs through forming line segments into letters and then letters into words. The temporal processes and utilized brain areas that engage in this process are widely thought to be left-lateralized within the brain. However, a range of studies demonstrate that the processing of unfamiliar stimuli, such as pseudoletters, is temporally delayed and bilaterally processed when compared to letters. The present study investigated the contributions of both hemispheres and how these interactions impact the temporal dynamics of implicit visual processing of single-letters as compared to unfamiliar pseudoletters (false fonts). The results of 5 "in-house" studies are presented within a meta-analysis (synthesis analysis), 3 High-density EEG studies and 2 MEG studies. Of the 3 EEG studies, 2 focused on measuring event related potentials (ERPs) while the participants performed an orthographic discrimination task (letters vs pseudoletters) and the other was a target detection task in which the participants detected infrequent, simple perceptual targets within a series of pseudoletter and letter strings. Of the 2 MEG studies, one was a discrimination task and the other a target detection task. Delayed N170 waveforms to pseudoletters as compared to letters were exhibited across all studies. Lateralization of the ERP differences between letter-evoked and pseudoletter-evoked responses were bilaterally distributed, whereas lateralization measure separately for letters and pseudoletters were primarily left-lateralized. As a whole, these in-house studies indicate that ERPs to letters occur earlier than to pseudoletters, and that interpretation of hemispheric laterality depends on whether the researcher is assessing ERP differences between letters and pseudoletters or the ERP waveforms of the separate letter and pseudoletter conditions.