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Oscillations; Lexical decision; EEG
Abstract:
Auditory word recognition is often described as a linear process of lexical access. Commonly, however, the perceptual evidence is ambiguous (mishearings, slip of the tounges, etc.), and requires additional compensatory and re-evaluatory processing. We used a lexical decision task in an electroencephalography (EEG) study and manipulated lexico-semantic access by parametrically varying the factor “wordness”: Stimuli were three-syllabic German words (“real”), pseudowords derived by only exchanging the core vowel in the second syllable (“recoverable” pseudowords), and pseudowords derived by scrambling entire syllables (“opaque” pseudowords). While listeners performed the lexical decision very accurately (accuracy for all conditions > 95 %), time-locked potentials (ERPs) at the third syllable revealed successive peak delay and amplitude height of the following negative deflection (N400) dependent on wordness; the N400 response to opaque pseudowords was strongest and peaked latest. At and after stimulus offset, alpha power (9 -- 13 Hz) was suppressed as a monotonic function of wordness (real < recoverable < opaque) with stronger suppression indicating less effort in lexico-semantic access. Simultaneously, however, theta power (3 -- 5 Hz) was most enhanced for recoverable pseudowords; they closely resemble a word, but also provide contradicting perceptual evidence (the altered vowel). This led to response conflict and increase in cognitive control. Accordingly, source localisation indicated that theta power changes emerged from the dorsal Anterior Cingulum (dACC). The results show that auditory word comprehension requires a cascade of evoked (N400) and induced (alpha, theta) brain processes, which reflect success, effort, and conflict in lexical access.
