Silent Speech Runs the Brain’s Arousal System on Low Power, Pupil Study Reveals

There is an old saying that the eyes speak as much as the mouth. A new study suggests that when it comes to arousal, they may be speaking entirely different languages depending on whether the words leave your lips or stay locked inside your head. Researchers at the University of Tsukuba and Kyoto University in Japan have discovered that talking aloud and talking silently to yourself produce strikingly different patterns of pupil-linked brain arousal, with inner speech sometimes causing the pupil to constrict rather than dilate. The finding, published in the journal iScience, offers a fresh window into one of the most private of human experiences: the voice inside our heads.

Inner speech, the act of speaking silently in one’s mind, is a cornerstone of human cognition. We use it to plan our day, rehearse conversations, regulate our emotions, and wrestle with difficult decisions. For decades, neuroimaging studies have shown that inner speech draws on a fronto-temporal speech-production network that partly overlaps with, but is not identical to, the network engaged during overt speech, including regions such as the left inferior frontal cortex, the supplementary motor area, the insula, and superior temporal areas. Depending on the region and task examined, inner speech can produce weaker, comparable, or even greater cortical activation than spoken speech. Yet nearly all of this work has focused on the cortex itself, treating speech-production networks as if they operated in isolation from the systems that regulate their functional state.

The Japanese team, led by Ryuta Kuwamizu, took a different approach. Rather than asking which cortical regions light up during inner versus overt speech, they asked whether those regions operate in the same neuromodulatory state during the two forms of speech. To answer this, they turned to pupillometry, the continuous, non-invasive measurement of pupil diameter. The pupil is a remarkably faithful proxy for brain arousal: it tracks the coordinated activity of the locus coeruleus-noradrenergic system, the cholinergic basal forebrain, the hypothalamic orexin system, and the large-scale cortical network states these systems govern. Crucially, unlike EEG, fMRI, or facial electromyography, pupillometry remains robust during overt articulation and requires no head-mounted apparatus, making it uniquely suited for directly comparing silent and spoken speech within a single paradigm.

The researchers designed an elegant counting task. Across three experiments, a total of 80 healthy, native Japanese-speaking participants sat before a screen-based eye tracker and completed blocks of three conditions: counting aloud, counting silently in their heads, and an instructed no-thought control in which they attempted to think of nothing at all. Each block consisted of a 30-second rest period followed by a 45-second task period, with pupil diameter recorded continuously at 60 Hz. In the first experiment, participants simply counted upward from one. In the second, they counted using grammatically structured numeral-noun phrases in Japanese, such as one cat, two cats, adding phonological and morphological complexity. In the third, they performed step counting in increments of two, three, or four, sharply increasing demands on working memory and cognitive control.

The results upended the team’s initial hypothesis. They had expected inner speech to produce pupil dilation comparable to overt speech, reflecting the cognitive effort of covert verbal production. Instead, in the simple counting experiment, inner speech produced significant pupil constriction relative to baseline, while overt speech produced significant dilation. Inner speech even drove the pupil smaller than the no-thought condition did. In the second experiment, the constriction vanished: inner speech no longer differed from baseline, while overt speech again produced robust dilation. Only in the third experiment, under the highest cognitive load, did inner speech finally produce clear pupil dilation, converging toward, but never reaching, the level seen during spoken counting.

The most robust finding across all three experiments was the consistent dissociation between the two speech modes. Overt speech reliably elicited pupil dilation in every condition, whereas inner speech was always accompanied by lower pupil-linked arousal, even as its direction relative to baseline shifted from constriction to no change to dilation as task demands grew. The researchers verified that participants genuinely engaged in inner counting by comparing final counts across conditions: counting speed correlated strongly between overt and inner speech in every experiment, with Spearman correlations ranging from 0.85 to 0.92. In the noun-phrase experiment, participants actually counted to higher numbers during inner speech, consistent with evidence that silent speech can proceed faster than spoken speech.

The magnitude of the overt-speech effect is itself remarkable. In the step-counting experiment, the sustained pupil dilation reached approximately 0.4 millimeters for more than 10 seconds, a response comparable in size to those reported during high-intensity cycling at roughly 80 percent of maximal oxygen uptake. In other words, speaking aloud under cognitive load drives pupil-linked arousal on a scale similar to vigorous physical exertion. This likely reflects the integrated demands of motor execution, respiratory variation, sensorimotor feedback, and attention that vocalization entails, each of which engages brainstem arousal systems and cortical network states.

The authors propose a state-dependent account of inner speech. On this view, inner and overt speech share much of the same linguistic machinery, but they run that machinery in different neuromodulatory states. Overt speech requires transforming an internal linguistic representation into an externally executable sensorimotor format, complete with articulatory execution, respiratory coordination, and auditory feedback monitoring against an efference copy of the motor command. These demands may recruit stronger noradrenergic gain modulation of speech-related auditory-motor networks, whereas inner speech can often remain in a compressed representational format and operate at lower gain. Notably, inner speech arousal scaled with cognitive-control demand rather than linguistic complexity per se: enriching the verbal content alone did not raise arousal, but forcing participants to update and maintain running sums did. This may explain why neuroimaging studies have reported such mixed cortical differences between covert and overt speech; the shared networks may simply be operating at different arousal states across studies and tasks.

The implications stretch well beyond the laboratory. Because pupillometry is contactless and silent, it could be used to study how inner speech becomes internalized over childhood development, and in populations such as young children or certain clinical groups for whom overt reporting or sensor-based recording is difficult. The capacity of inner speech to sustain verbal processing at reduced arousal may be advantageous where mental calm and efficient cognition must coexist, as in cognitive control, sport performance, or the routinized inner counting many people use as a sleep aid. Yet the same low-arousal efficiency might also enable maladaptive cognitive loops that contribute to rumination. Conversely, the elevated arousal accompanying overt speech may carry functional value; prior work has found that pupil dilation during reading aloud better tracks the memory advantage for spoken words than for silent ones.

The researchers are careful to note the limits of their work. The study operationalized inner speech as silent numerical counting, an externally instructed, repetitive task that cannot capture the full diversity of naturally occurring inner speech, from dialogic self-conversation to spontaneous verbal thought. The no-thought condition was an instructed control rather than verified mental silence, and pupillometry provides only an indirect index of arousal, not a direct readout of inner speech itself. The sex ratios also differed across the three experiments, warranting caution in cross-experiment comparisons. Still, the authors frame the work as a focused first step toward a neuromodulatory science of verbal thought. If the pattern holds in more naturalistic settings, the little voice in our heads may turn out to be defined not just by what it says, but by the quiet, energy-efficient state in which it says it, a whisper that runs the brain’s arousal machinery on a fraction of the power demanded by the spoken word.

Subject of Research: Pupil-linked arousal differences between inner speech and overt speech

Article Title: Dissociable pupil-linked arousal during overt and inner speech

Article References: Kuwamizu, R., Watanabe, R., Yamamoto, N., Otani, K., & Moriguchi, Y. (2026). Dissociable pupil-linked arousal during overt and inner speech. iScience, 29(11), Article 117825. https://doi.org/10.1016/j.isci.2026.117825

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.117825

Keywords: inner speech, overt speech, pupillometry, arousal, locus coeruleus, noradrenergic system, cognitive load, counting task, neuromodulation, working memory, self-talk, brain state