Electrotactile Frequency, Amplitude, and Temporal Gap Discrimination in Young Adults, Older Adults, and Stroke Survivors

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, frequency and amplitude JNDs will be used to evaluate discrimination ability for changes in electrotactile stimulation frequency and amplitude. Temporal sensory resolution will be evaluated using a gap-in-noise (GIN) detection task, in which participants detect brief interruptions embedded within continuous electrotactile stimulation. 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 patients with stroke. Young and older adult participants will visit the laboratory on two consecutive days and perform electrotactile sensory discrimination tasks. Patients with stroke will participate in a single study visit. Before the main experiment, patients with stroke 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 continuous electrotactile stimulation containing brief temporal gaps, and their ability to detect short interruptions in stimulation will be evaluated.

In selected experimental conditions, electroencephalography (EEG) will be recorded non-invasively from the scalp during electrotactile stimulation. EEG recording will be used to characterize neural responses associated with the processing of electrotactile sensory information. EEG measures may include time-locked neural responses and changes in neural activity associated with different electrotactile stimulation parameters and sensory discrimination performance. EEG recording itself does not deliver electrical stimulation to the participant and is used only to measure electrical activity generated by the brain.

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. Patients with stroke 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, patients with stroke 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 participants with stroke, 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.