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Pulling Illusion in Individuals with Neurological Disorders
Takeshi Tanabe, Satoshi Yamamoto, Toru Yamada, Daisuke Ishii, Yutaka Kohno
TL;DR
The study asks whether peripheral vibration sensitivity alone explains the pulling illusion or whether processing beyond basic detection is required. It evaluates directional discrimination and fingertip vibration thresholds in 25 people with diverse neurological disorders. Directional discrimination was not robustly associated with vibration threshold but was negatively associated with motor-related signs, supporting involvement of perceptual processing beyond basic vibration detection.
Problem
The mechanism of the pulling illusion is unclear, including whether peripheral vibrotactile sensitivity alone is sufficient for it to emerge.
Method
The study evaluated pulling-illusion directional discrimination and fingertip vibration detection thresholds in 25 participants with neurological disorders spanning peripheral to central impairments.
Results
Motor-related signs showed a robust negative association with directional discrimination, whereas vibration detection threshold was not robustly associated with performance.
Takeaways & Limitations
The findings suggest that the pulling illusion involves perceptual processing beyond basic vibration detection.
Takeaways & Limitations
The heterogeneous cohort provided limited sample sizes for individual neurological disorders, constraining disorder-specific interpretations.
Abstract
from arXiv · showhide
The pulling illusion induced by asymmetric vibration stimuli has attracted attention for its potential applications in rehabilitation and sensory assessment. However, the underlying mechanism of the pulling illusion remains unclear. This study addressed the central question of whether peripheral vibrotactile sensitivity alone is sufficient for the illusion to emerge or whether processing beyond basic vibration detection is also required. Neurological disorders can involve impairments at different levels of the nervous system, providing an opportunity to examine this question. Accordingly, we evaluated directional discrimination performance for the pulling illusion and fingertip vibration detection thresholds in 25 participants with diverse neurological disorders affecting different levels of the nervous system, from peripheral to central. Clustering analysis identified contrasting profiles, with high directional discrimination performance despite elevated vibration detection thresholds and chance-level performance despite relatively low-to-intermediate thresholds. In the generalized linear mixed model, motor-related signs, including hemiplegia and tremor, showed a robust negative association with directional discrimination performance, whereas vibration detection threshold was not robustly associated with performance. Furthermore, in participants with hemiplegia, directional discrimination performance was around chance level on the affected side and close to 100% on the unaffected side, despite stimulus amplitudes well above the measured vibration detection thresholds on both sides. Collectively, these findings suggest that the pulling illusion depends on perceptual processing beyond basic vibration detection, through which asymmetric vibration is experienced as directional pulling.
I. INTRODUCTION
The pulling illusion has potential rehabilitation and sensory-assessment applications, but its mechanism remains unclear. This study tests whether peripheral vibration sensitivity alone explains the illusion or whether additional neurological processing is involved.
- I. INTRODUCTION: The pulling illusion is a directional pulling sensation typically induced by asymmetric vibration waveforms.The waveform combines brief, rapid acceleration in one direction with longer, slower acceleration in the opposite direction.
- I. INTRODUCTION: Its clinical application is limited because the mechanism underlying the illusion remains unknown.The illusion has been explored for rehabilitation and sensory assessment, but its unresolved mechanism constrains those applications.
- I. INTRODUCTION: The conventional account predicts that directional discrimination should depend on peripheral detection of vibration-induced fingertip skin deformation.Previous studies reportedly found that vibration detection threshold was not consistently associated with directional discrimination, while delayed contralateral parietal activity was associated with the illusion.
- I. INTRODUCTION: The study examines whether directional discrimination can remain preserved despite impaired vibration detection or weaken despite relatively preserved detection.Participants with neurological disorders provide variability across peripheral sensory pathways and central nervous-system processing.
II. METHODS
The study enrolled 25 people with neurological disorders spanning peripheral to central impairments. Participants completed the procedures under approved ethical protocols, with all enrolled participants completing the experiments.
- II. METHODS: 25 participants with neurological disorders were recruited, spanning impairments from the peripheral to the central nervous system.The cohort included seven participants with carpal tunnel syndrome, while the supplied passage truncates the remainder of the condition breakdown.
- II. METHODS: The cohort was intentionally heterogeneous to provide variability in functionally relevant neurological impairments.This design supported evaluation across different levels of nervous-system involvement.
- II. METHODS: All participants completed the experimental procedures under ethics approval and informed consent.The study was approved by the Ibaraki Prefectural University of Health Sciences Ethics Committee under approval number 1107.
B. Study Design
The study modeled directional discrimination performance as a function of vibration threshold, motor-related signs, age, and gender. Bilateral side-level measurements enabled participant-specific and tested-side analyses.
- B. Study Design: Directional discrimination performance was modeled using vibration threshold, motor-related signs, age, gender, and participant-specific variability.Motor-related signs represented hemiplegia or tremor on the tested side.
- B. Study Design: The pulling illusion was measured with a directional discrimination task, while fingertip vibration detection thresholds quantified basic sensory function.Both measures were obtained bilaterally, with each tested side treated as an analysis unit.
- B. Study Design: Each participant contributed up to two observations corresponding to the left and right tested sides.Motor-related signs were coded separately for each tested side.
C. Experimental setup
A compact, suspended two-actuator device delivered the vibration stimuli while participants held it with three fingers. The setup was adapted to reduce load and stabilize grasp for participants with motor impairments.
- C. Experimental setup: Two parallel voice-coil actuators mounted on a custom fixture generated opposing translational forces for clockwise and counterclockwise pulling illusions.Control signals were generated in MATLAB and converted into audio signals using Psychtoolbox.
- C. Experimental setup: Participants held the device with their thumb, index, and middle fingers, with a soft polyurethane band securing the fingers when needed.The stabilization measure accommodated participants who might be unable to grasp the device independently.
- C. Experimental setup: The vibratory device measured 53(w)×28(h)×27(d) mm and weighed 67.4 g.It was suspended using a constant-load spring adjusted to 14.0 gf to reduce participant load.
D. Procedure
Vibration thresholds were measured bilaterally before testing the pulling illusion, using 75-Hz sinusoidal stimuli and accelerometer-based analysis.
- Procedure: Thresholds were measured bilaterally before the directional illusion task, with hemiplegic participants tested unaffected-side first.Testing order was randomized for other participants.
- Procedure: 75-Hz sinusoidal vibration, matching the illusion stimuli’s fundamental frequency, was used to measure absolute detection thresholds.The method of limits efficiently estimated thresholds after an initial method-of-adjustment range estimate.
- Procedure: The asymmetric vibration waveforms were designed to induce clockwise or counterclockwise pulling illusions, with measured signals compared against target profiles.Solid lines represented measured values and broken lines represented targets.
- Procedure: Accelerometer-recorded time-series data were analyzed with fast Fourier transform, and thresholds were averaged across four ascending and descending trials.Stimulus intensity was adjusted in 10% steps until response changes were detected.
2) Directional discrimination of the pulling illusion:
Directional discrimination was assessed with randomized clockwise and counterclockwise asymmetric vibrations, then analyzed alongside vibration thresholds using clustering and a binomial GLMM.
- 2) Directional discrimination of the pulling illusion:: Directional discrimination performance was defined as the correct-response rate for the pulling illusion.Opposing translational illusory forces between parallel actuators induced clockwise or counterclockwise directions.
- 2) Directional discrimination of the pulling illusion:: Participants verbally reported the perceived direction of one-second clockwise or counterclockwise stimuli in a 2AFC task with 50% chance performance.Stimuli were randomly presented while participants held the device as during threshold measurement.
- 2) Directional discrimination of the pulling illusion:: Two participants with markedly below-chance performance on one side were excluded before statistical analyses at the participant level.P9 and P20 showed 27.5% and 22.5% correct responses, respectively, on one tested side.
- 2) Directional discrimination of the pulling illusion:: Hierarchical clustering summarized side-level combinations of log-transformed vibration thresholds and standardized directional discrimination performance into four response profiles.Euclidean distances and Ward’s linkage were used to generate the clusters.
- 2) Directional discrimination of the pulling illusion:: A binomial GLMM modeled correct responses using vibration threshold, age, gender, and motor-related signs as fixed effects.The model used the total number of trials per condition as the denominator.
III. RESULTS
Among 23 participants contributing 46 side-level observations, directional discrimination clustered near 100% or chance, while motor-related signs—not vibration threshold—showed the clearest modeled association.
- III. RESULTS: Directional discrimination showed a bimodal-like distribution, with many side-level observations near 100% and another group around chance level at 50%.The clustering and GLMM included 46 side-level observations from 23 participants after exclusion.
- III. RESULTS: Four clusters captured contrasting profiles, including high performance with elevated thresholds and chance-level performance with low-to-intermediate thresholds.Other clusters combined high performance with low thresholds or chance performance with elevated thresholds.
- III. RESULTS: The GLMM’s fixed-effect robustness was evaluated with bootstrap 95% confidence intervals on coefficient estimates shown on the log-odds scale.The figure uses a zero reference line and standardized continuous predictors.
- III. RESULTS: Motor-related signs showed a robust negative association with directional discrimination (βMS = −1.95, 95% CI [-2.48, -1.62]), whereas vibration threshold was not robustly associated (βVT = 0.08, 95% CI [-0.28, 0.64]).Age also showed a negative association, while gender’s confidence interval included zero.
- III. RESULTS: In hemiplegic participants, affected-side directional discrimination was around chance, whereas unaffected-side performance was close to 100%.This side difference was clearer and more consistent for directional discrimination than for vibration threshold.
A. Motor-related signs and directional discrimination performance for the pulling illusion
Motor-related signs were associated with poorer pulling-illusion discrimination beyond what fingertip vibration thresholds explained, including a marked affected–unaffected side contrast in hemiplegia.
- A. Motor-related signs and directional discrimination performance for the pulling illusion: Clusters with high thresholds and high performance, or low-to-intermediate thresholds and chance performance, showed that peripheral sensitivity alone was insufficient.These profiles contrasted with the expected low-threshold/high-performance and high-threshold/chance patterns.
- A. Motor-related signs and directional discrimination performance for the pulling illusion: Vibration threshold showed no robust association with performance, whereas motor-related signs showed a robust negative association in the GLMM.This contrast supports involvement of neurological functions beyond basic vibration detection.
- A. Motor-related signs and directional discrimination performance for the pulling illusion: In hemiplegia, affected-side performance remained around chance while unaffected-side performance was close to 100%, despite no significant side difference in vibration thresholds.The comparison was performed within participants with unilateral motor-related signs.
- A. Motor-related signs and directional discrimination performance for the pulling illusion: Stimulus acceleration was approximately 10–100 times higher than measured detection thresholds, including on affected sides, making detection failure an insufficient explanation for reduced discrimination.The finding supports a distinction between detecting vibration and perceiving directional pulling.
- A. Motor-related signs and directional discrimination performance for the pulling illusion: The pulling illusion is characterized as asymmetric vibration perceived as directional pulling rather than as a mere fingertip vibrotactile sensation.The authors suggest that processing beyond basic vibration detection contributes to illusory pulling perception.
B. Implications for investigating the mechanisms of the pulling illusion
The pulling illusion cannot be fully explained by fingertip vibrotactile sensation alone. Its emergence may involve processing across ascending somatosensory pathways and cortical perceptual systems.
- B. Implications for investigating the mechanisms of the pulling illusion: The pulling illusion requires consideration of perceptual processing beyond basic fingertip vibration detection.Somatosensory signals travel through ascending pathways before cortical processing contributes to directional pulling perception.
C. Limitations and future work
The study’s heterogeneous sample and functional grouping limit disorder-specific interpretation. Direct peripheral-function measures and further investigation of unusual response patterns are needed.
- C. Limitations and future work: The heterogeneous sample limited disorder-specific interpretations, although it enabled comparison across diverse functional profiles.Future studies should target conditions such as hemiplegia and Parkinson’s disease with tremor.
- C. Limitations and future work: Motor-related grouping may not capture disorder-specific symptoms, individual variability, or other factors influencing directional discrimination.The model excluded potentially relevant variables.
- C. Limitations and future work: Vibration detection threshold did not directly measure peripheral nerve function, so its lack of robust association with performance is not definitive.Nerve conduction studies are needed to clarify this relationship.
- C. Limitations and future work: Two participants with markedly below-chance performance were excluded from clustering and regression analyses, leaving the origin of these patterns unresolved.Possible explanations included procedural factors, reversed perceived direction, or individual neurological characteristics.