1. Introduction
Somatosensation, also known as the sense of touch, is a crucial and commonly used sense in everyday life. Somatosensory deficit is a common symptom in both peripheral and central neurological diseases and insults [
1], such as lesions of sensory receptors, lesions of the peripheral nerves, or impairments to haptic representation in the central nervous system (specifically in the primary sensory cortex, such as stroke) [
2,
3]. Therefore, tactile spatial acuity (TSA) is commonly measured to evaluate somatosensory function [
4,
5] and yields important information for diagnostic, functional, and prognostic purposes.
Tactile spatial and directional sensitivity on a body part is mainly determined by two factors, the density of peripheral mechanoreceptors and their receptive field properties [
6,
7]. Several assessments have been developed to objectively measure tactile spatial resolution in various physiological or pathological conditions. Two-point discrimination, a tactile spatial discrimination task, is the most widely used for this purpose [
6]. In recent decades, the utility of two-point discrimination has come under scrutiny because it can only measure at the threshold of a just-noticeable difference instead of the limit of spatial resolution, and most importantly, it tends to yield inconsistent results because it cannot control nonspatial cues [
5,
6]. The JVP dome, a grating dome that assesses sensory capacity in grating orientation discrimination [
2], is regarded as the standard method used to qualify the tactile threshold for the spatial resolution [
3]. Grating orientation sensitivity is considered suitable for assessing spatial acuity because it is affected by the density of innervation and varies with the somatosensory function of the fingerpad [
8]. Nevertheless, the use of the JVP dome requires considerable skill and the indentation depth on the skin cannot be accurately controlled because it is delivered by the examiner’s hand, making it necessary to develop a fully automatic method that allows the tactile orientation discrimination task to be performed.
The tactile orientation discrimination task has been performed by healthy people [
5,
6], patients with neurological disorders [
3,
9,
10], and blind Braille readers [
11,
12]. Studies have reported that the TSA of blind Braille readers is superior to that of healthy people [
11,
12], whereas the TSA of patients with neurological disorders is worse [
3,
9,
10]. Moreover, TSA can improve with training [
11,
13]. The tactile orientation discrimination task has also been performed by children [
14] and older adults [
15,
16], indicating that older age is correlated with a decline in tactile spatial resolution.
The miniature tactile stimulator (MTS) was developed by our group for performing automatic tactile stimulations that can cover a variety of movement directions and speeds; these simulations are used to characterize the human ability to perceive shape and motion by touch [
17]. MTS consists of three independently controlled micromotors, providing three degrees of freedom, which are the grating ball’s direction of movement, speed of rotation, and depth of vertical indentation on the skin [
17]. We successfully demonstrated the functionality of the MTS for measuring the performance of tactile motion discrimination in healthy participants [
17,
18,
19], and the MTS thus offers unique potential as a fully automatic and standardized measurement of tactile acuity.
In the present study, we developed a method where MTS is used for an orientation discrimination task; this method can be applied to the measurement of the severity of somatosensory deficits. Through MTS, motion stimuli were applied to the participant’s fingerpad, and the participant reported the received orientation of the moving grating ball. We describe how the measurement was performed during the training and testing phases. Specifically, the orientation discrimination task was performed at a variety of motion speeds to characterize how much motion speed affected tactile acuity. We demonstrated that MTS-based orientation discrimination tasks can be effectively applied to evaluate tactile acuity in healthy participants.
3. Results
Sixteen healthy participants (eight men and eight women, aged 48.50 ± 1.50 years) were enrolled (
Table 1). We tested their right-hand tactile accuracies, but two of them were excluded due to carpal tunnel syndrome in at least one of their hands. All of the remaining participants completed the training and formal experiments and no adverse events were reported.
In the eye-opened training session, all of the participants achieved 100% accuracy (
n = 14). Data from the training session indicated that the participants fully understood the experimental protocol. This supported the effectiveness of the instruction to the participants. In the eye-closed training session, eleven of the fourteen participants passed the threshold (threshold = 75%,
Table 2). Although participants #3, #13, and #16 initially failed the eye-closed training (accuracy = 62.5%, 45.83%, and 58.33% respectively), these participants passed after receiving an additional eye-opened training session (accuracy = 100%).
The demographic data in the testing session are shown in
Table 3. The mean accuracy was 89.36% ± 1.12% (
n = 14), which exceeded the threshold of 75%. The results also demonstrated that accuracy in the vertical (87.35 ± 1.55%) and horizontal (91.37 ± 1.49%) orientations did not significantly differ (
p = 0.08,
Figure 3A).
Subsequently, we characterized the effect of motion speed on the performance of orientation identification. The results demonstrated that tactile accuracy differed across speeds (
p < 0.001, Friedman test), and the post hoc test revealed that accuracy at the highest speed (160 mm/s) was significantly inferior to that in each of the other three speeds (5, 10, and 40 mm/s,
Figure 3B). This finding indicates that scanning speed might affect the participants’ performance, which could be applied for assessing tactile acuity under various conditions.
We examined whether the wavelength of the grating affects the accuracy of orientation discrimination, a phenomenon that could support that the percept was mainly mediated by spatial information presented by the grating rather than the shear force. To this end, 8 of the 14 participants were presented with sinusoid grating balls with wavelengths of 1, 2, or 4 mm. The results showed that accuracy was significantly modulated by wavelength (
p = 0.006) and the post hoc analysis showed that the 1mm grating had a worse accuracy compared with the 4mm grating (
Figure 4A,
p = 0.019). The performance was thus better with a wider wavelength of grating.
In addition, we examined whether the scanning speed affected the accuracy of orientation discrimination. The results showed that accuracy was significantly modulated by motion speed (whole ANOVA model here
p = 0.009) and the post hoc analysis showed that 160 mm/s had a worse accuracy compared with 5 and 160 mm/s (
Figure 4B,
p = 0.042), indicating that the performance decreased at extremely high speeds. Moreover, there was no interaction effect between the wavelength and scanning speed (
Figure 4B,
p = 0.733), indicating that the aforementioned wavelength effect was independent of the scanning speed.
Finally, we evaluated the usability of the MTS protocol using the SUS questionnaire in seven naïve examiners (
n = 7) (
Table 4). The results demonstrated that the overall SUS score was 88.57 ± 3.61, indicating that the usability of the present protocol was excellent (the overall SUS score >85) [
22]. The recorded scores in all items were ≥3 in all questions, except item Q4 (
Table 5), “I required technical assistance to use the miniature tactile stimulator”, in which the score was only 2.71 ± 0.52, suggesting that the participants might need assistance when operating the device.
In summary, our results demonstrated that the novel procedure for tactile stimulation could be applied as a psychophysical assessment for examining the tactile acuity of healthy individuals.
4. Discussion
MTS is a robotic tactile stimulator that could be applied for tactile function capability assessments [
17,
18,
23]; it is novel because a standard and feasible method has yet to be established. In the present study, we developed an MTS-based program for tactile acuity evaluation. The results demonstrated the following: (1) the examining process could be easily performed by examiners, and all of the participants passed both training sessions with accuracy above the predefined threshold, a finding that supports the utility of the pre-test training; (2) neither discomfort nor adverse events were reported by the participants, suggesting the acceptability of this designed tactile measurement protocol; (3) all participants had above-threshold accuracy (accuracy ≥ 75%) when identifying the directions of motion at lower speeds (5, 10, and 40 mm/s), indicating that most healthy participants could perform the present protocol; and (4) the accuracy of orientation identification decreased at higher speeds, suggesting that higher speed (160 mm/s) conditions could be more challenging for tactile testing. In summary, the present study developed an evaluation protocol that could apply MTS for the automatic testing of tactile acuity.
This is the first protocol designed for the assessment of tactile function using an automatic robotic system with MTS. Because we aimed to apply this program to patients with somatosensory deficits, the protocol was designed with an adjustable degree of difficulty that can discriminate between various severities of somatosensory impairment. Considering user comfort in the testing program, we modified the body postures through which the participants received the assessment. In addition, the duration of the entire examination procedure was optimized and was shown to be tolerable by all participants.
Tactile orientation discrimination tasks have been applied to distinct body parts, such as the lip, tongue, and fingers. The levels of TSA sequenced were highest to lowest in the lip, tongue, index or ring finger, and little finger, respectively [
2,
4,
24]. In the present study, we chose the index finger as our measuring target for its advantages of fine TSA and easy manipulation compared with the lip or tongue.
An important question is whether the participant relied on the stretch force induced by the scanning movement rather than the spatial–temporal information presented by the grating orientation [
19,
25]. Indeed, in the present setup, the shear force was always orthogonal to the grating orientation, so that it was possible that the participants used the direction of shear force to infer the grating orientation. Using sine-wave grating with a variety of wavelengths, we observed a strong effect of wavelength on the accuracy, a finding that is reminiscence of the effect of grid width on static orientation judgement [
5,
26]. Furthermore, speed and wavelength are two independent factors that influence accuracy, suggesting that wavelength as a spatial factor has a distinct mechanism to affect performance and this mechanism is not affected by speed. To this end, these findings support that the participants’ performance was mainly determined by spatial–temporal cues.
For usability, the results showed that most of the SUS items had high scores, indicating that a naïve examiner could operate the MTS well after brief instruction and training. However, we found the lowest score in item Q4, suggesting that non-experienced operators might require help from others. For example, the operator sometimes hesitated to select the settings on the operational interface software and needed assistance from the experienced operator. In our opinion, protocols that simplified and optimized operative steps are expected to improve the users’ satisfaction and its clinical usability.
The MTS system and present study have several limitations. First, the sample size was small because this was a pilot study for assessing the feasibility and acceptability of the MTS-based program for measuring tactile acuity. Second, it was difficult to apply this tactile function assessment instrument to some parts of the body, such as the lip and tongue, because more adjustment was required to apply the MTS on other body parts. Third, although the protocol was optimized in the present study, it still took up to 30 min for each participant to complete the whole process. Fourth, it is possible that the subjects still had minute finger movements when the fingers were placed on the finger holder during the experiment. Without continuous monitoring of finger position, it was hard to make sure the participants remained in their assigned position throughout the experiment.
In addition, participants may have still been able to perceive the direction of motion, relying on the spatiotemporal cues provided by the miniature ball’s rotation and stretch cues—the rotation exerted on the skin surface. Indeed, tactile direction discrimination is the ability to recognize the tactile motion direction of an object moving across the skin [
27,
28]. Sensing the direction of a tactile stimulus relies on spatiotemporal cues and skin stretch through the simultaneous processing of information signaled by large myelinated afferent nerve fibers and dorsal column pathways [
29,
30]. Tactile direction judgments are sensitive to the direction of skin stretch, whereas low friction stimuli with minimal skin stretch, such as the rolling ball, provide only spatiotemporal cues (successive positions cues) for the direction of motion [
31,
32,
33]. Furthermore, SA1 receptors are at least ten times more sensitive to moving than to static stimuli [
25].
The MTS system is an automatic, noninvasive, and quantitative method for assessing tactile acuity. Our study demonstrated that this novel MTS-based protocol is safe, accessible, tolerable, and feasible as a simple standardized measurement of tactile acuity. We will conduct further studies where we will apply this program to evaluate somatosensory deficits in patients with peripheral neuropathies or central neurological disorders.