The Electrophysiology of Language Comprehension:A Neurocomputational Model

Ongeveer tien jaar geleden stuitten onderzoekers die gebruik maken vanelektro-encefalografie (EEG) op resultaten die leken te wijzen op een"Semantische Illusie" bij het begrijpen van taal: Zinnen die qua betekenisvreemd zijn, maar verder grammaticaal correct, hadden geen effect op de aanbetekenis gerelateerde "N400 component" in het EEG signaal, maar juist op deaan structuur gerelateerde "P600 component". Op basis van deze bevindingenzijn er maar liefst vijf nieuwe modellen van taalverwerking voorgesteld. Aldeze modellen claimen dat er niet een enkele verwerkingsroute, maar twee ofmeer verwerkingroutes zijn binnen het taalverwerkingsysteem, waarvan bij éénroute de structuur van een zin volstrekt niet van belang is, maar enkel debetekenis van de woorden waaruit de zin bestaat. In dit proefschrift laat ikzien dat er een veel simpelere uitleg is voor de 'problematische'bevindingen en toon ik met computersimulaties aan dat deze alternatieveuitleg alle relevante data kan verklaren.One decade ago, researchers using event-related brain potential (ERP)measurements stumbled upon what looked like a Semantic Illusion in languagecomprehension: Semantically anomalous, but otherwise well-formed sentencesdid not affect the meaning-related N400 component, but instead increased theamplitude of the structure-related P600 component. This finding spawned fivenew models of language comprehension, all of which claim that instead ofa single comprehension process, there are two or even more separateprocessing streams, one of which is not driven by structure, but by wordmeaning alone. In this thesis, I argue that there is a much simpler way toaccount for these data, and present evidence from neurocomputionalsimulations showing that this alternative explanation is able to predict allrelevant ERP patterns found in the literature

On the Proper Treatment of the N400 and P600 in Language Comprehension

Event-Related Potentials (ERPs)—stimulus-locked, scalp-recorded voltage fluctuations caused by post-synaptic neural activity—have proven invaluable to the study of language comprehension. Of interest in the ERP signal are systematic, reoccurring voltage fluctuations called components, which are taken to reflect the neural activity underlying specific computational operations carried out in given neuroanatomical networks (cf. Näätänen and Picton, 1987). For language processing, the N400 component and the P600 component are of particular salience (see Kutas et al., 2006, for a review). The typical approach to determining whether a target word in a sentence leads to differential modulation of these components, relative to a control word, is to look for effects on mean amplitude in predetermined time-windows on the respective ERP waveforms, e.g., 350–550 ms for the N400 component and 600–900 ms for the P600 component. The common mode of operation in psycholinguistics, then, is to tabulate the presence/absence of N400- and/or P600-effects across studies, and to use this categorical data to inform neurocognitive models that attribute specific functional roles to the N400 and P600 component (see Kuperberg, 2007; Bornkessel-Schlesewsky and Schlesewsky, 2008; Brouwer et al., 2012, for reviews). Here, we assert that this Waveform-based Component Structure (WCS) approach to ERPs leads to inconsistent data patterns, and hence, misinforms neurocognitive models of the electrophysiology of language processing. The reason for this is that the WCS approach ignores the latent component structure underlying ERP waveforms (cf. Luck, 2005), thereby leading to conclusions about component structure that do not factor in spatiotemporal component overlap of the N400 and the P600. This becomes particularly problematic when spatiotemporal component overlap interacts with differential P600 modulations due to task demands (cf. Kolk et al., 2003). While the problem of spatiotemporal component overlap is generally acknowledged, and occasionally invoked to account for within-study inconsistencies in the data, its implications are often overlooked in psycholinguistic theorizing that aims to integrate findings across studies. We believe WCS-centric theorizing to be the single largest reason for the lack of convergence regarding the processes underlying the N400 and the P600, thereby seriously hindering the advancement of neurocognitive theories and models of language processing