The purpose of this study is to evaluate and compare sensory discrimination ability for electrotactile stimulation among young adults, older adults, and patients with stroke. Participants will receive non-invasive electrotactile stimulation through electrodes attached to the skin of the upper limb, such as the fingertip, palm, elbow, or shoulder, depending on the stimulation condition. The study will assess discrimination thresholds for changes in stimulation frequency, stimulation amplitude, and temporal gaps in stimulation, together with task accuracy and reaction time. The results are expected to provide foundational data for sensory assessment, electrotactile feedback design, and sensory-based rehabilitation technologies for older adults and stroke patients.

Accurate perception of sensory information is essential for movement control and interaction with the external environment. It is also important for providing precise feedback during motor rehabilitation. Aging and neurological injury, such as stroke, can affect tactile and proprioceptive information processing. These changes are important considerations when designing sensory-feedback-based rehabilitation approaches and assistive devices.

Electrotactile stimulation can be delivered non-invasively through the skin surface. By controlling stimulation frequency, amplitude, and temporal features, electrotactile stimulation can convey different types of tactile information. Because perception of electrotactile stimulation may vary according to individual sensory sensitivity, age, and neurological status, quantitative assessment of electrotactile information processing across participant groups is needed.

The just noticeable difference (JND) is a quantitative measure of the minimum detectable difference between stimuli. In this study, JND will be used to evaluate discrimination ability for electrotactile stimulation frequency, amplitude, and temporal gaps. Task accuracy and reaction time will also be analyzed to characterize sensory sensitivity, information processing, and response characteristics.

The study will include three participant groups: young adults, older adults, and stroke patients. Young and older adult participants will visit the laboratory on two consecutive days and perform electrotactile sensory discrimination tasks. Stroke patients will participate in a single study visit. Before the main experiment, stroke patients will undergo preliminary assessments of handedness, cognitive function, upper-limb motor function, proprioception, cutaneous sensation, and hand function.

For each participant, the stimulation range will be individualized based on sensory thresholds. The perception threshold (PT), defined as the voltage at which the participant first perceives electrotactile stimulation, and the discomfort threshold (DT), defined as the voltage at which the participant first begins to feel discomfort, will be measured. Stimulation amplitude used during the experimental tasks will be set within the range between PT and DT and will remain below DT. If a participant reports discomfort or pain during the experiment, stimulation will be stopped immediately.

Electrotactile stimulation will be provided using a voltage-limited and current-limited stimulator. The maximum applied voltage will be limited to 30 V, and the maximum allowable current will be limited to 10 mA as an additional safety limit. In practice, the expected current during stimulation is approximately 2 to 4 mA when electrodes are attached to the skin. The stimulation frequency used in this study will not exceed 100 Hz.

Participants will complete electrotactile frequency discrimination, amplitude discrimination, and temporal gap discrimination tasks. In the frequency discrimination task, a reference electrotactile stimulus and a comparison stimulus will be presented, and participants will report which stimulus is perceived as faster. In the amplitude discrimination task, two electrotactile stimuli with different amplitudes will be presented, and participants will report which stimulus is perceived as stronger. In the temporal gap discrimination task, participants will receive electrotactile stimulation containing brief temporal gaps, and their ability to detect short interruptions in stimulation will be evaluated.

For young and older adult participants, the study may also include a cognitive load condition. Participants will complete both a focused electrotactile discrimination condition and a cognitive load condition in which they perform a simple arithmetic task, such as serial subtraction by 7 from 100, while judging electrotactile stimuli. This condition will be used to evaluate the effect of cognitive load on sensory discrimination performance and to compare age-related differences. Stroke patients will not undergo the cognitive load condition in order to minimize fatigue and experimental burden and to ensure safe participation.

Before the main electrotactile discrimination experiment, stroke patients will undergo preliminary assessments to characterize baseline function. These assessments may include handedness assessment, Korean Mini-Mental State Examination (K-MMSE), Fugl-Meyer Assessment of the Upper Extremity (FMA-UE), Fugl-Meyer upper-limb position sense test, and sensory testing including sharp/blunt discrimination, temperature sensation, and monofilament sensory testing.

For stroke participants, clinical information related to stroke may be reviewed or collected only when the participant already has such materials and voluntarily provides them. These materials may include MRI or CT radiology reports, discharge summaries, medical opinion letters or diagnostic certificates, and MRI or CT image files on CD or USB. The information may be used to confirm stroke onset timing, lesion location, lesion size, lesion laterality, and radiological findings. Collected materials will be used only for research purposes. Personally identifiable information will be de-identified, securely stored for the required retention period according to relevant regulations, and then discarded after the study.

Participants will be monitored for discomfort, pain, skin redness, allergic reaction, dizziness, excessive fatigue, or other adverse symptoms during the experiment. If a participant expresses a wish to stop participation for any reason, or if severe discomfort occurs due to electrical stimulation, the experiment will be stopped immediately. After participation, participants will be monitored by questionnaire 3 days and 7 days after the experiment to assess whether skin allergic reactions, arm discomfort, or other symptoms occurred after participation.

Inclusion Criteria:

• Young Adult Group:
• Age 19 to 54 years.
• Has tactile sensation in the hands and fingers.
• Provides voluntary written informed consent personally or through a legally authorized representative, if applicable.

Older Adult Group:

• Age 55 to 70 years.
• Has tactile sensation in the hands and fingers.
• Provides voluntary written informed consent personally or through a legally authorized representative, if applicable.

Stroke Patient Group:

• Age 19 years or older.
• Diagnosis of ischemic or hemorrhagic stroke.
• Stroke confirmed by CT or MRI.
• Stroke accompanied by proprioceptive sensory impairment.
• Residual tactile sensation in the fingertip.
• Provides voluntary written informed consent personally or through a legally authorized representative,

Exclusion Criteria:

• Young and Older Adult Groups:
• Absence of tactile sensation in the hands or fingers.
• Congenital deformity of the hands or fingers.
• Presence of a major pre-existing neurogenic disease.
• Presence of a major psychiatric disorder, such as major depressive disorder, schizophrenia, bipolar disorder, or dementia.
• Implanted pacemaker.

Stroke Patient Group:

• Difficulty communicating due to K-MMSE score < 23, severe language impairment, or similar condition.
• Absence of tactile sensation in the hands or fingers.
• Congenital deformity of the hands or fingers.
• Severe pain during elbow joint or finger movement.
• Progressive or unstable stroke.
• Presence of hemispatial neglect.
• Presence of a major pre-existing neurogenic disease.
• Presence of a major psychiatric disorder, such as major depressive disorder, schizophrenia, bipolar disorder, or dementia.
• Implanted pacemaker.

Individual participant data will not be shared with other researchers because the study includes sensitive clinical and sensory assessment data, and the current protocol does not include a data sharing plan beyond the approved research team.

No masking will be used because participants are grouped according to age and clinical status, and the study procedures involve behavioral electrotactile sensory discrimination tasks.