The electrophysiological reality of parafoveal processing: On the validity of language-related ERPs in natural reading

A central question in psycholinguistics is how the human brain processes language in real time. To answer this question, the differences between auditory and visual processing have to be considered. The present dissertation examines the extent to which event-related potentials (ERPs) in the human electroencephalogram (EEG) interact with different modes of presentation during sentence comprehension. Besides the two classical modalities, auditory and rapid serial visual presentation (RSVP), the monitoring of readers’ eye movements was chosen as a new mode of presentation. Here, the temporal paradox between neuronal ERP effects and behavioral effects in the eye movement record were of particular interest. Specifically, by concurrently measuring ERPs and eye movements in natural reading, the dissertation aimed to shed light on the counterintuitive fact that difficulties in sentence comprehension arise earlier in eye movement measures than in the corresponding neuronal ERP effects. In contrast to RSVP and the auditory modality, reading offers a parafoveal preview of upcoming words (Rayner 1998), which enables the brain to process information of words before these are fixated for the first time (in foveal vision). When the word Gegenteil in example (1) below is fixated and processed, the brain concurrently processes some information of the upcoming parafoveal words von and weiß. (1) Schwarz ist das Gegenteil von weiß. (2) Schwarz […] blau. (3) Schwarz […] nett. The parafoveal preview mostly provides orthographic (word form) information, while semantic information is not conveyed (Inhoff & Starr 2004; White 2008). Whereas word form and lexical meaning are processed simultaneously with RSVP and auditory presentation, the parafoveal preview in natural reading allows for a temporal decoupling such that word forms are processed before meaning. This is one reason for the faster information uptake in reading. The present dissertation is the first to systematically investigate the influence of the parafoveal preview in sentence processing. Participants read sentences such as in (1)-(3), in which two adjectives were either antonyms (1), semantically related non-antonyms (2), or semantically unrelated non-antonyms (3). ERPs were computed for the last fixation before the target word (the sentence-final word in 1-3), which was assumed to capture parafoveal processing, and for the first fixation on the target, that should reflect foveal processing. The results were compared to two experiments using identical stimuli with auditory and RSVP presentation, and the parafoveal preview clearly led to different ERP results. While the RSVP and auditory presentations replicated the finding of a P300 to the second antonym in (1) (Kutas & Iragui 1998; Roehm et al. 2007), there was no P300 in response to antonyms at any fixation position in natural reading. However, the dissociation of parafoveal and foveal processing in reading also made it possible to disentangle different processes underlying the N400. There was a reduced parafoveal N400 for (1,2) compared with (3), which could be attributed to the preactivation of the word forms of the expected antonyms and of semantically related non-antonyms. In foveal vision, all non-antonyms (2,3) showed an enhanced N400 compared with (1) because they were unexpected and implausible in the sentence context. This dissociation between the preactivation of a word-form and the contextual fit of a word’s meaning is impossible with the other two modes of presentation, because orthographic and semantic information become available almost at the same time and are thus processed simultaneously. Furthermore, the parafoveal N400 effect was not accompanied by changes in the duration of the corresponding fixation, whereas the foveal N400 was. Similarly, with the concurrent measurement of ERPs and eye movements, the temporal paradox described above remained, as effects in the eye movement record preceded the neuronal ERP effects. Further support for these central findings came from two additional experiments that investigated different stimuli with concurrent ERP-eye tracking measures. Altogether, the experiments revealed that the previous findings on the language-related N400 can be replicated with natural reading, but they can also be differentiated qualitatively by virtue of the characteristics of natural reading. Although the behavioral and neuronal effects mirrored one another, not every neuronal effect necessarily translates into a behavioral output. Finally, even concurrent ERP-eye tracking measures cannot resolve the temporal paradox