🧠 neuroscience

Vestibular control of posture deviates from Weber's law in response to electrical vestibular stimulation

This study demonstrates that the vestibular control of posture deviates from Weber's law, as balance discrimination thresholds increase sublinearly with electrical stimulation amplitude due to specific, frequency-independent changes in response gain and signal-dependent variability.

Original authors: Omerani, A., Mackrous, I., Carriot, J., Simoneau, M.

Published 2026-10-01

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CC BY 4.0

Original authors: Omerani, A., Mackrous, I., Carriot, J., Simoneau, M.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Every time you stand still, your body is actually in a constant, subtle state of motion. To keep you upright, your brain relies on a delicate feedback loop: it senses tiny shifts in your balance, calculates the necessary correction, and sends signals to your muscles to adjust your stance. This system must work across a vast range of movements, from the faintest sway to more vigorous rocking. For over a century, scientists have operated under a general rule of thumb known as Weber's law, which suggests that as a stimulus gets stronger, your ability to detect small changes in it gets worse in direct proportion. In simple terms, if you are holding a heavy weight, you need a much larger addition to notice a difference than if you were holding a light one. This rule applies to many senses, but recent research has shown that the inner ear, which tells your brain how you are moving, does not always follow this pattern when it comes to perception. The big question remained: does this exception also apply to the physical act of balancing itself, or does the body's motor control system still obey the old rules?

A team of researchers set out to answer this by testing how well people could detect changes in their balance when their inner ears were artificially stimulated. They recruited ten healthy young adults and had them stand on a sensitive platform with their eyes closed. To create a sense of motion without actually moving the participants' heads, the researchers applied a gentle electrical current to the skin behind the ears. This current, known as electrical vestibular stimulation, tricks the brain into thinking the head is tilting, prompting the body to sway in the opposite direction to correct the perceived error. The researchers increased the strength of this electrical signal gradually, from zero up to 2 milliamperes, while the participants stood still. They tested this at four different speeds of signal change, ranging from very slow to moderately fast, to see if the body's ability to sense and correct its position changed as the stimulus became stronger.

The results showed that the body's balance system does not follow the expected rule. As the electrical signal grew stronger, the participants' ability to detect changes in their balance did not degrade as quickly as Weber's law would predict. Instead of getting significantly worse with stronger signals, the system remained surprisingly precise. The researchers found that this high level of accuracy was maintained because of two specific adjustments the body made. First, the strength of the body's corrective response did not drop off as fast as it theoretically should have when the signal got stronger. Second, the natural "jitter" or inconsistency in how the body moved from one moment to the next did not keep growing in a straight line. Instead, this variability increased at first but then leveled off, stabilizing even as the electrical signal continued to rise. It is this leveling off of the body's internal noise, combined with the sustained strength of the corrective response, that allowed the participants to keep detecting subtle changes in their balance even when the stimulus was intense.

Interestingly, the speed at which the electrical signal changed did not alter the overall ability of the participants to detect these changes. Whether the signal cycled slowly or more quickly, the precision of the balance control remained consistent, and the ratio of the useful signal to the background noise stayed the same. This suggests that the body's balance system has a robust, built-in mechanism for maintaining sensitivity across different conditions. The study confirms that the inner ear's control over posture operates with a unique set of rules that differ from simple perception, allowing humans to maintain stability and detect motion with greater accuracy than previously thought possible. These findings offer a new way to look at how the brain manages the complex task of staying upright, suggesting that the system is far more adaptable and precise than standard models of sensory processing would indicate.

Problem Statement
Human upright stance relies on the continuous detection of body motion across a wide range of amplitudes and frequencies. According to Weber's law, a sensory system's ability to detect changes in a stimulus deteriorates proportionally as the stimulus amplitude increases; consequently, the discrimination threshold (DT) should increase linearly with stimulus magnitude. While vestibular self-motion perception has been shown to deviate from this prediction (Mallery et al., 2010), it remains unknown whether this deviation extends to the vestibular control of posture. Given that perception and postural control rely on distinct neural mechanisms, this study investigates whether the vestibular control of balance follows the same scaling rules as perception or if it also deviates from Weber's law at large stimulus amplitudes. Furthermore, the specific contributions of response gain and inter-trial variability to these DT patterns, and the influence of stimulus frequency on these dynamics, were previously uncharacterized in the context of postural control.

Methodology
The study involved 10 healthy young adults (mean age 28 ± 5 years) standing upright with eyes closed on a force platform. Whole-body motion was evoked using amplitude-increasing electrical vestibular stimulation (EVS) delivered in a bipolar binaural configuration (anode right/cathode left) to the mastoid processes. The EVS amplitude increased linearly from 0 to 2.5 mA, though DTs were computed over the 0–2.0 mA range. Stimulation was applied at four frequencies: 0.1, 0.2, 0.5, and 1 Hz.

Lateral ground reaction forces were recorded at 2048 Hz. The methodology for quantifying balance control DTs involved:

Signal Processing: Band-pass filtering lateral forces at the EVS frequency and aligning them with the EVS signal via cross-correlation to remove delays.
Gain and Variability Calculation: Using a sliding window approach (one cycle length), the gain of the balance-correcting response was estimated as the slope of the linear fit between the EVS signal and the lateral force. Inter-trial variability was calculated as the mean standard deviation of the lateral force across 15 trials within each window.
DT Computation: Discrimination thresholds were calculated using signal detection theory (Eq. 1), derived from the ratio of variability to gain, assuming a $d'$ of 1.16 (79.4% probability of correct detection).
Signal-to-Noise Ratio (SNR): SNR was quantified in the frequency domain by computing the ratio of the power spectral density (PSD) of the trial-averaged signal to the mean PSD of the residual noise.
Statistical Analysis: Nonlinear mixed-effects (NLME) models were fitted to the data to determine power-law relationships ( $DT = a \times EVS^b$ ). Weber's law holds if the scaling exponent $b = 1$ . Deviations were assessed via 95% confidence intervals. The contributions of gain and variability were isolated by modeling inverse gain and fitting quadratic models to variability profiles.

Key Results

Deviation from Weber's Law: Across all tested frequencies (0.1, 0.2, 0.5, and 1 Hz), balance control DTs increased sublinearly with EVS amplitude. The scaling exponents ( $b$ ) were significantly less than 1 (ranging from 0.64 to 0.86), indicating that the postural system's ability to detect changes in vestibular input does not degrade as rapidly as predicted by Weber's law.
Contributions of Gain and Variability:
- Gain: The balance-correcting response gain decreased with increasing EVS amplitude, but more slowly than the strictly inverse relationship required to produce strict adherence to Weber's law.
- Variability: Inter-trial variability initially increased with EVS amplitude but then saturated (leveled off) at higher amplitudes. This non-linear structure was confirmed by significant negative quadratic coefficients in the variability models.
- Combined Effect: Simulations demonstrated that assuming linear variability (extrapolated from low amplitudes) resulted in predicted DTs that diverged from observed data. Only by incorporating the observed non-linear saturation of variability did the predicted DTs match the computed DTs. Thus, both the sublinear decrease in gain and the saturation of variability are necessary to explain the deviation from Weber's law.
Frequency Effects:
- Absolute DT: The absolute discrimination thresholds did not differ significantly across the four EVS frequencies.
- SNR: The signal-to-noise ratio of the balance-correcting response remained similar across all frequencies.
- Scaling Exponent: The shape of the DT-amplitude relationship (the scaling exponent) was frequency-dependent. The exponent was significantly smaller at 0.1 Hz compared to 0.2 Hz and 0.5 Hz, but did not differ significantly from the 1 Hz condition.

Significance and Claims
The authors claim that this study extends previous psychophysical findings regarding vestibular perception (Mallery et al., 2010) to the domain of postural control. The primary significance lies in demonstrating that the vestibular control of posture possesses a specific property: it discriminates changes in body sway with greater accuracy than predicted by Weber's law across a broad range of amplitudes and frequencies.

The paper posits that this enhanced precision arises from a specific neural and motor strategy where the system adjusts both the gain of the corrective response and the structure of its variability (saturation at high amplitudes) to maintain discrimination performance. Unlike previous studies that focused on constant-amplitude stimuli, this work highlights how the system regulates intrinsic noise within the sensorimotor loop when faced with time-varying, increasing-amplitude virtual head-motion errors.

The authors conclude that these findings provide a better understanding of the balance control discrimination process and offer potential additional assessment criteria for vestibular impairments. They suggest that this methodology could advance the understanding of balance control in populations with increased postural instability, such as older adults and individuals with vestibular disorders, by providing a framework to investigate how intrinsic noise is regulated within sensorimotor control loops. The study modestly notes that while the deviation from Weber's law is robust, it is smaller than that observed in conscious perception of whole-body rotation, likely due to the additional noise introduced by active motor corrections and proprioceptive feedback in a standing posture.

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