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

The research team, led by Jingwen Xiao and Xiaowei Zhou of Wuhan Sports University alongside colleagues from Central China Normal University, set out to settle debates that have simmered in the sport psychology literature for decades. Previous meta-analyses had suggested that talking to oneself helps, but they disagreed on the details — on whether instructional phrases beat motivational ones, on whether saying “I” or “you” to yourself matters, on whether the words should be spoken aloud or kept silent, and on how the demands of the motor task shape which kind of self-talk works best. The new analysis, drawing on randomized controlled trials identified through searches of seven electronic databases, was designed specifically to test these emerging and previously established moderator variables and to clarify how they interact with one another.

The scale of the evidence base is meaningful if not enormous. After applying predefined inclusion and exclusion criteria following PRISMA guidelines, the team retained 33 eligible randomized controlled trials encompassing 56 separate effect sizes and a pooled sample of 1,848 participants. That these were randomized controlled trials — the gold standard for causal inference — matters enormously. Participants were assigned to self-talk interventions or control conditions by chance, which means the observed differences in performance cannot be easily explained by pre-existing differences in motivation, skill, or confidence. The analysis was conducted with a three-level random-effects model using the metafor package in R, a statistically sophisticated approach that acknowledges a crucial structural reality of meta-analysis: multiple effect sizes drawn from the same study are not independent of one another, and ignoring that nesting inflates the apparent certainty of the findings.

The headline result is striking in its clarity. Across all included trials, self-talk improved sport performance with a pooled effect size of Hedges’ g equal to 0.52, with a 95 percent confidence interval spanning 0.35 to 0.68 and a p-value below 0.001. In practical terms, an effect of this magnitude means that the average athlete receiving a self-talk intervention performed better than roughly 70 percent of athletes in control conditions — a difference comparable to many physical training interventions that athletes and teams spend considerable money pursuing. Hedges’ g is a corrected version of the standardized mean difference, adjusting for small-sample bias, which makes this estimate more trustworthy than a raw Cohen’s d would be in a literature populated by modestly sized experiments.

But the true novelty of this analysis lies in what it found — and did not find — among the moderators. The most consequential moderator to survive statistical scrutiny was self-talk content itself, which significantly influenced the size of the performance benefit at p below 0.05. This distinction maps onto a classic framework in the field: instructional self-talk, consisting of cue words that direct attention to technique and movement execution, such as “elbow up” or “follow through,” versus motivational self-talk, consisting of phrases designed to energize, build confidence, and sustain effort, such as “I can do this” or “give it everything.” The significant moderation effect confirms that what you say to yourself is not interchangeable — the words must be matched to the psychological function they are meant to serve, and some content strategies reliably outperform others.

Perhaps more surprising is the list of variables that failed to reach significance. The moderating effects of motor skill level, motor demands, overtness of the self-talk, personal pronoun use, application situation, and intervention duration were all statistically non-significant at p above 0.05. This is a genuine challenge to received wisdom. The influential matching hypothesis in sport psychology has long held that instructional self-talk works better for fine, precision-based tasks while motivational self-talk works better for gross, strength- and endurance-based tasks. The new analysis tested the interaction between motor demand and content directly and found it did not reach significance. Similarly, the interaction between motor demand and overtness — whether the self-talk was spoken aloud or performed silently — also failed to achieve statistical significance. The message is that the broad strokes of self-talk may be more forgiving than theorists have assumed: athletes appear to benefit across skill levels, task types, and delivery formats, so long as the content is appropriately constructed.

Methodological rigor was a central selling point of the update. Study quality was assessed using the Risk of Bias 2.0 tool, the current standard from the Cochrane collaboration for randomized trials, which evaluates bias arising from the randomization process, deviations from intended interventions, missing outcome data, measurement of outcomes, and selective reporting. The certainty of the evidence was further evaluated using GRADEpro, the grading framework that rates confidence in effect estimates across a body of evidence. The authors explicitly frame their contribution as providing improved methodological rigor relative to earlier syntheses, and the three-level modeling with restricted maximum likelihood estimation represents a technical step forward in handling the dependence among effect sizes. A likelihood ratio test framework was employed in the moderator analyses, ensuring that comparisons between subgroups accounted for the hierarchical structure of the data.

Yet the analysis is not without an important caveat, and the authors are commendably upfront about it. Publication bias diagnostics revealed a possible small-study effect, with Egger’s test returning a p-value below 0.001. In plain language, this pattern suggests that smaller studies in the self-talk literature may be more likely to report larger effects — a signature consistent with publication bias, in which null or weak findings languish in file drawers while positive results reach print. If that is the case, the pooled estimate of 0.52 may represent something of an optimistic ceiling, and the true effect could be somewhat smaller. The authors explicitly caution that the results should be interpreted with this in mind, a level of transparency that strengthens rather than weakens confidence in the overall enterprise.

The practical implications ripple outward from the laboratory to the field, the gym, and the starting line. Because the moderators that many practitioners obsess over — pronoun choice, overtness, duration, skill level — showed no significant effects, the barrier to entry for self-talk training appears low. An athlete does not need a precisely calibrated script or weeks of graduated practice to benefit; what matters most is choosing content that fits the intended function, whether that is refining technique with instructional cues or summoning effort and confidence with motivational phrases. For coaches, this suggests that embedding brief self-talk cues into technical instruction and pre-performance routines is an evidence-supported practice, not a superstitious flourish. For recreational athletes, the finding that benefits emerge across skill levels is quietly democratizing: the mental tool is not reserved for elites.

The authors close with a call that is both standard and necessary: future research should include more high-quality studies to further examine how relevant moderators shape the effect size of self-talk interventions. The unresolved questions are real — the non-significant interaction tests may reflect genuine null effects or simply insufficient statistical power within the available trial pool, and the small-study effect flagged by Egger’s test demands larger, preregistered, rigorously blinded trials to confirm the true magnitude of the benefit. Still, the core conclusion stands on firm ground. With 33 randomized controlled trials and nearly two thousand participants pointing the same direction, the science now says what athletes have intuited all along: the voice in your head, given the right words, is a genuine performance enhancer — free, portable, and always available on demand.

Subject of Research: Effects of self-talk interventions on sports performance and their moderator variables

Article Title: Emerging moderators in effects of self-talk on sports performance: an updated meta-analysis of randomized controlled trials

Article References: Xiao, J., Zhou, X., Wang, N., Li, J., Zhu, J., Li, J., & Hong, X. (2026). Emerging moderators in effects of self-talk on sports performance: an updated meta-analysis of randomized controlled trials. BMC Psychology. https://doi.org/10.1186/s40359-026-05674-0

Image Credits: AI Generated

DOI: 10.1186/s40359-026-05674-0

Keywords: self-talk, sports performance, meta-analysis, sport psychology, randomized controlled trials, instructional self-talk, motivational self-talk, moderators, publication bias, motor skill, Hedges g, BMC Psychology