Multimodal integration supports neural processing of grammatical negation

Negation is a fundamental aspect of human cognition and grammar, expressed across different modalities. Yet, little is known about multimodal processing of negation, especially grammatical negation (e.g., "I did not sleep"), and the contribution of gestures and prosodic markers to its neural processing remains unexplored. This study investigates the neurocognition of multimodal negation in Turkish, a verb-final language where standard verbal negation is expressed morphologically with a postverbal suffix (-mA). Notably, this negation suffix shifts stress onto the verb stem, deviating from the default final-stress pattern of Turkish. In Turkish, standard verbal negation is also frequently accompanied by conventional negation gestures (backward hand flips), occurring on the verb stem preceding the negation suffix. Since both gestures and prosodic markers precede the negation suffix, we examined whether and how their presence affects the postverbal negation processing using electroencephalography. Gestures elicited an N100 effect driven by their inherent perceptual salience. Gestures also reduced the N400 amplitude in anticipation of the postverbal negation suffix, pointing to a facilitative role of negation gestures in negation processing. Prosody likewise strengthened this N400 reduction, but only when gestures were present. These findings demonstrate that gestures-and to a lesser extent prosody-reduce the cognitive effort involved in processing the morphosyntactic marker of verbal negation. Overall, the results provide neurophysiological evidence that multimodal information supports not only semantic processing, as shown before, but also core grammatical operations in language.