Dissociable pupil-linked arousal during overt and inner speech

Using pupillometry across three experiments, this study reveals that while overt speech consistently triggers pupil dilation, inner speech exhibits a dissociable, load-dependent arousal profile characterized by constriction or minimal dilation, indicating that covert verbal production operates at a reduced and flexibly scaled arousal cost compared to overt speech despite shared cortical substrates.

The Gist

The Big Question: Does the "Silent Voice" Work the Same Way as the "Loud Voice"?

We all know the feeling of talking out loud versus talking to ourselves in our heads. We often assume that when we "speak" silently, our brain is doing the exact same job as when we speak out loud, just with the volume turned down.

The researchers in this paper asked a different question: Does the body's "engine" rev up the same way for both?

To find out, they didn't look at brain scans (which can be noisy and expensive). Instead, they looked at something much simpler and more revealing: your pupils.

The Tool: Pupils as "Arousal Meters"

Think of your pupils (the black dots in the middle of your eyes) like the gas pedal of a car.

• When you need to do something hard, exciting, or physically demanding, your brain hits the gas, and your pupils get big (dilate).
• When you are relaxed or doing something very routine, your pupils stay small or even get tiny (constrict).

The researchers used a camera to watch these "gas pedals" while people did three different counting tasks.

The Experiment: Three Ways to Count

They asked participants to count numbers in three different ways:

1. The "No-Thought" Baseline: Just sit there and try to think of nothing.
2. Overt Speech (The Loud Voice): Count out loud (e.g., "One, two, three...").
3. Inner Speech (The Silent Voice): Count silently in your head.

They tested this in three different "difficulty levels":

• Level 1 (Easy): Simple counting (1, 2, 3...).
• Level 2 (Medium): Counting with words (e.g., "One cat, two cats...").
• Level 3 (Hard): Step counting (1, 3, 5, 7... or 1, 4, 7, 10...).

The Surprising Results

Here is where the story gets interesting. The researchers expected the "Silent Voice" to look like a quieter version of the "Loud Voice." Instead, they found two completely different engines running.

1. The Loud Voice (Overt Speech)

The Analogy: Imagine a sports car revving its engine.
Every time people counted out loud, their pupils got bigger. This happened even when the counting was easy.

• Why? Speaking out loud is a full-body event. It involves moving your mouth, controlling your breath, and hearing your own voice. Your brain says, "Okay, we are doing something physical! Let's pump up the energy!"

2. The Silent Voice (Inner Speech)

The Analogy: Imagine a hybrid car in "Eco Mode."
The reaction to silent counting was weird and changed depending on how hard the task was:

• Easy Counting: When people just counted "1, 2, 3" in their heads, their pupils actually got smaller than when they were just sitting still! It was like the brain was saying, "This is so easy, I can actually relax even more."
• Medium Counting: When they added words ("One cat, two cats"), the pupils stayed the same size as when sitting still. No extra energy needed.
• Hard Counting: When the math got tricky (counting by 3s or 4s), the pupils did get bigger, but they never got as big as they did when speaking out loud.

The Big Takeaway

The study proves that talking to yourself is not just "quiet talking." It is a fundamentally different state of mind.

• Speaking out loud is a high-energy, high-arousal activity. It wakes up your whole brain and body.
• Thinking silently is a low-energy, efficient activity. Your brain can do it with a fraction of the "fuel."

Why does this matter?
Think of it like this: If you are trying to solve a complex problem, you might want to use your "Silent Voice" because it's fuel-efficient and keeps you calm. But if you are trying to memorize something important, "Speaking out loud" might be better because that extra burst of energy (the pupil dilation) helps your brain lock the memory in.

The researchers also noted that this "Silent Voice" mode is flexible. It can be super relaxed (pupils shrinking) when the task is easy, but it can ramp up (pupils growing) when the task gets hard—just never quite as hard as speaking out loud.

In a Nutshell

Your eyes tell a secret story: When you speak out loud, your brain is in "Full Throttle" mode. When you talk to yourself, your brain is in "Eco Mode," saving energy and staying calm, unless the task gets really tough, at which point it gently presses the gas—but never as hard as it does when you actually make a sound.

Technical Summary

1. Problem Statement

Inner (covert) speech is a fundamental cognitive process for human thought and self-regulation. While neuroimaging studies have established that inner speech shares neural substrates with overt (aloud) speech—particularly in left-lateralized language networks like Broca's area—the prevailing "motor-attenuation" view suggests inner speech is merely overt speech with motor commands withheld.

However, this focus on cortical similarity overlooks a critical dimension: the arousal systems (specifically the locus coeruleus–noradrenergic and cholinergic systems) that sustain cortical language processing. It remains unknown whether inner and overt speech differentially engage these subcortical arousal systems. Existing methods (like fMRI) are often susceptible to speech-related noise, making it difficult to compare these modalities directly. The authors hypothesized that while cortical substrates may overlap, the physiological arousal states supporting them might be fundamentally distinct.

2. Methodology

The study employed pupillometry (measuring pupil diameter) as a non-invasive, real-time index of arousal dynamics. The researchers conducted three controlled experiments using a numerical counting paradigm to systematically vary verbal content and cognitive load.

• Participants: Healthy native Japanese-speaking university students (Expt 1: $N=23$; Expt 2: $N=27$; Expt 3: $N=30$).
• Experimental Design: A block-design paradigm where participants performed three conditions in randomized order:
  1. No-thought: Attempting to think of nothing.
  2. Overt speech: Counting aloud.
  3. Inner speech: Counting silently.
• The Three Experiments:
  • Experiment 1 (Simple Counting): Counting consecutive integers (1, 2, 3...) to establish a baseline for simple verbal production.
  • Experiment 2 (Numeral-Noun Phrase Counting): Counting using grammatically structured phrases with classifiers (e.g., "one cat, two cats...") to increase phonological and morphological complexity.
  • Experiment 3 (Step Counting): Counting in increments (e.g., 1, 3, 5... or 1, 4, 7...) to impose a higher cognitive load on working memory and executive function.
• Data Collection: Pupil diameter was recorded at 60 Hz using a screen-based eye tracker (Tobii Pro Spectrum). Data were baseline-adjusted (comparing the 45s task period against the 15s pre-task baseline) and analyzed using repeated-measures ANOVA and paired t-tests.
• Compliance Check: Validity of inner speech was verified by correlating the final numbers counted in overt vs. inner conditions; strong positive correlations ($\rho > 0.85$) confirmed participants were engaging in the task.

3. Key Contributions

• First Direct Arousal Comparison: This is the first study to directly compare the pupil-linked arousal profiles of inner speech, overt speech, and a no-thought baseline within a matched task structure.
• Dissociation of Arousal: It challenges the assumption that inner speech is simply "overt speech minus vocalization" by demonstrating a fundamental dissociation in the arousal systems engaged.
• Dynamic Scaling of Inner Speech: It reveals that the arousal cost of inner speech is not fixed; it scales flexibly with task demands (cognitive load and linguistic complexity), whereas overt speech consistently recruits high arousal.
• Methodological Validation: It validates pupillometry as a robust, non-invasive tool for studying covert verbal processes without the artifacts associated with speech movements in other imaging modalities.

4. Key Results

The results showed a consistent dissociation between overt and inner speech across all three experiments:

• Overt Speech: Consistently elicited significant pupil dilation (increased arousal) relative to baseline in all three experiments. This suggests vocalization recruits high levels of brainstem and cortical arousal.
• Inner Speech: Produced a variable profile that was significantly attenuated compared to overt speech in every condition:
  • Experiment 1 (Simple Counting): Inner speech caused pupil constriction (decreased arousal) relative to baseline, significantly lower than both the no-thought and overt conditions.
  • Experiment 2 (Complex Phrases): Inner speech showed no significant deviation from baseline (neutral arousal), though it remained significantly lower than overt speech.
  • Experiment 3 (High Load): Inner speech elicited pupil dilation, indicating engagement of arousal systems under high cognitive load. However, the magnitude of dilation remained significantly lower than that of overt speech.

Summary of the Pattern:

• Overt Speech: High, consistent arousal (Dilation).
• Inner Speech: Low, flexible arousal (Constriction $\to$ Neutral $\to$ Dilation, but always < Overt).

5. Significance and Implications

• Physiological Efficiency: The findings suggest that inner speech allows for verbal processing at a reduced physiological cost. While it shares cortical networks with overt speech, it operates in a distinct, lower-arousal state. This efficiency may explain why inner speech can be faster and more fluid than overt speech.
• Cognitive Control & Clinical Applications: The ability to sustain verbal processing with low arousal may be advantageous for tasks requiring mental calm (e.g., sports performance, sleep aids) but could also contribute to maladaptive loops like rumination. Conversely, the high arousal of overt speech may enhance memory encoding.
• Developmental & Clinical Tracking: Because pupillometry is contactless and robust against movement, these findings open new avenues for tracking the development of inner speech in children and investigating inner speech deficits in clinical populations (e.g., schizophrenia, aphasia) where the balance between covert and overt processing is altered.
• Theoretical Shift: The study moves the field beyond a binary "shared vs. distinct" cortical view, proposing that inner and overt speech are fundamentally distinct in their arousal states, even when their cortical substrates partially overlap.