“Where Would You Stimulate?” Beliefs About Anatomical Relevance for Enhancing Motor Performance With Non-Invasive Electrical Stimulation

Introduction

Non-invasive electrical stimulation has been used in experimental and clinical settings (eg, stroke rehabilitation) to enhance motor performance. Transcranial direct current stimulation (tDCS), transcutaneous vagus nerve stimulation (tVNS), and transcutaneous electrical nerve stimulation (TENS) are among the most common modalities due to their ease of use and affordability. However, these techniques have had mixed results for enhancing motor performance.^1,2 Recent work has demonstrated that psychological factors like treatment expectation can significantly affect motor outcomes post-stimulation, even in sham-controlled studies, which could contribute to such variability in results.^3,4 However, human participants bring a wide range of pre-existing beliefs about a given treatment and knowledge of their bodies. Since people outside of clinical or research fields may have limited knowledge of stimulation modalities and their mechanisms of action, they may instead rely on the “closeness is strength of effect” metaphor,^5,6 where the spatial (eg, physical contact or treatment application) proximity is required for an outcome. Under this framework, individuals tend to judge the efficacy of a treatment from the spatial proximity of the cause (ie, the treatment) relative to the effect (ie, the specific improvement).^5,7

Given that different stimulation modalities with different stimulation locations are used in neurorehabilitation to improve a given motor function, such as the spatial proximity between the stimulation site and the targeted region of improvement needs to be considered. For example, to improve hand function post-stroke, electrodes would be placed on the scalp for tDCS, near the ear or on the neck for tVNS, and on the arm close to the hand for TENS. However, based on the above framework, people may perceive the electrode location physically closer to the hand as a more optimal location than further away (ie, the head). This exploratory study investigated beliefs about the optimal target region of stimulation for maximizing the hypothetical effect of stimulation itself.

Methods

Eighty participants completed an Arizona State University Institutional Review Board-approved online study on Qualtrics. Participant demographics are reported in Table 1. A spatial mapping task was employed in which participants were asked where they would approximately place the electrodes of an electrical stimulator to enhance each of the following functions: hand movement, arm movement, leg movement, and attention/concentration. Participants indicated their optimal electrode placement for each function via a mouse click on a blank human figure; mouse click locations were considered during analysis as head, neck, torso/back, arm, hand, leg, feet, or other (Figure 1(A)). By asking participants to identify the optimal placement, our task served as a behavioral proxy for existing beliefs about potential treatment effectiveness. These locations were considered as physical evidence of the “closeness is strength” association, where the optimal target region for stimulation is determined by the spatial proximity to the specific body function. Mouse-clicks were collapsed across hemispheres for analysis and visualization (Figure 1(B)). Participants’ prior knowledge of electrical stimulation was recorded; no medical history or information was collected.

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Table 1.

Demographic Characteristics of Participants (n = 80).

Characteristics
Age	46.93 ± 12.64
Biological sex
Female	39 (48.75)
Male	41 (51.25)
Education
Highschool	10 (12.5)
Some college	14 (17.5)
2-years degree	7 (8.75)
4-years degree	26 (32.5)
Masters	18 (22.5)
Doctorate	5 (6.25)
Ethnicity
Hispanic	4 (5)
Non-Hispanic	76 (95)
Race
Asian	3 (3.75)
Black or African American	16 (20)
White or Caucasian	57 (71.25)
Other or mixed	4 (5)
Prior knowledge
Yes	9 (11.25)
No	71 (88.75)

Data are presented as mean ± standard deviation and n (%).

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Figure 1.

(A) Human figure in anatomical position with overlay-colored areas that represented the defined target regions used for the analysis. Participants indicated their optimal electrode placement for each function by clicking on a blank human figure (without the colored areas shown) with their mouse or cursor. Click locations were categorized into the following areas: head (red), neck (yellow), torso/back (brown), arm (light green), hand (blue), leg (purple), feet (dark green), or other (gray). (B) Density maps illustrating the participant’s selected optimal target anatomical region for enhancing each specific function in the blank anatomical human figure. The density level, indicated by color intensity (ie, hotter colors), denotes the concentration of participants’ clicks. Any responses indicated on the left side of the body (36% of the total responses) were superimposed on the corresponding location on the right side for data visualization and analysis.

Fisher’s exact tests with Monte Carlo simulations were used to evaluate the association between selected target regions and body functions. Separately, the target regions selected were compared by sex and prior knowledge with Fisher’s exact tests. Chi-square tests were performed to assess potential differences in match rates, anatomically aligned responses (eg, “Leg” for Leg Movement), across the functions. More details on the study’s protocol and statistical analysis are provided in the Supplemental Material.

Results

Results showed a significant association between the perceived optimal target region of stimulation and the body function to be enhanced (Fisher’s exact test with Monte Carlo simulation, P < .001). Responses did not depend on prior electrical stimulation knowledge (Fisher’s exact test, P = .242) nor participant sex (Fisher’s exact test, P = .923). Matches between the perceived optimal target region of stimulation and the body function in question were common: >87% for both the arm and the leg for enhancing arm and leg movement, respectively. Furthermore, 83% selected the head for attention/concentration. While 47.5% selected the hand for enhancing hand movement, 35% selected the arm, and 7.5% chose the head as a relevant region for stimulation. When comparing the proportion of matches across the 4 functions, there was significantly higher mismatching when selecting the body region for enhancing hand movement compared to the other 3 functions (χ² > 21.721, df = 1, P < .001). However, there were no significant differences in body regions selected for the other functions (χ² < 0.877, df = 1, P > .349).

Discussion

This study showed that, overall, people matched the perceived optimal target region of stimulation to the body function to be enhanced. Although beliefs about the optimal stimulation location did not depend on prior knowledge of electrical stimulation, the general awareness of stimulation tended to be low (11.5% in this study and 18.7% in prior work ⁸ ). However, we cannot rule out that the lack of differences regarding prior knowledge may be influenced by the unbalanced subgroup sizes. It remains possible that extensive knowledge or experience could modulate this spatial proximity association, ⁵ which should be explored further in populations with greater knowledge or exposure to stimulation (eg, those with spinal cord stimulators). Nevertheless, it is plausible these beliefs about where stimulation would be most effective for a given function likely contribute to (or interact with) the perception of treatment efficacy, independent of any information provided by the clinician or experimenter. ⁹

Regarding limitations, this study did not directly compare different stimulation modalities, as these interventions typically involve interactions among verbal information, physical sensations, and technical parameters. However, by isolating these pre-existing beliefs here, we can demonstrate the potential implications of these phenomena when stimulation is applied to certain regions. It is therefore plausible that applying stimulation closer to (or on) the perceived optimal target region for a given function may be perceived as more relevant or effective than distal applications (eg, TENS on the arm vs tDCS on the scalp for enhancing hand movement). Further, while this pilot study focused on non-invasive stimulation modalities, these results may also apply to other physical manipulations. Finally, since participants’ neurological status (eg, stroke and spinal cord injury) was not collected, it is unclear how pre-existing beliefs about the optimal stimulation sites persist or change following neurological impairment. Future research should consider the extent to which this phenomenon varies across clinical and non-clinical cohorts. Additionally, future studies will also explore how these pre-existing beliefs influence and interact with treatment expectation, a premise supported by evidence of site-specific placebo effects in which responses occur only in the treated body region and not beyond. ¹⁰ We encourage future clinical trials to evaluate and, when feasible, control for participants’ pre-existing beliefs regarding spatial proximity and treatment expectations regarding electrical stimulation to better attribute any observed benefits (or lack thereof) to the appropriate mechanisms.

Supplemental Material