Dissociable pupil-linked arousal during overt and inner speech

Abstract

Inner (covert) speech, the silent production of language in ones mind, plays a central role in human cognition and is known to activate speech-related cortical regions while also requiring executive and attentional resources. However, the conventional focus on cortical similarity or dissimilarity between inner and overt speech leaves unaddressed the question of whether arousal systems sustain cortical language processing. Here, using pupillometry across three controlled counting experiments, we show a dissociation between inner and overt speech. Overt speech consistently led to pupil dilation across all conditions. Inner speech, by contrast, produced a strikingly different profile: pupil constriction during simple counting, no deviation from baseline during noun-phrase counting, and clear dilation only under a high cognitive load, but with dilation remaining significantly less than that for overt speech in every condition. Covert verbal production thus proceeds at a reduced arousal cost, yet this cost scales flexibly with task demands rather than being fixed. This pupillometric dissociation reveals that inner and overt speech occupy fundamentally distinct arousal states, even when their cortical substrates partially overlap. These findings open a non-invasive window for tracking inner speech physiology across development, individual differences, and clinical populations in whom inner speech is altered.