Before testing, each participant underwent an initial screening that focused on the inclusion and exclusion criteria. Individuals completed several intake measures and assessments, including a standard demographic form, handedness assessment using the Edinburgh Handedness Inventory [
21], and a medical history form. A member of the study team evaluated disease progression and severity. PwPD completed the Movement Disorder Society’s (MDS-UPDRS) [
22], whereas PwMCI completed the MOCA [
18].
Measuring proprioceptive sensitivity
The height of the chair and apparatus were adjusted based on each participant’s seated height. Each participant placed their forearm on the splint, aligning their elbow with the pivot point on the testing apparatus. Participants were instructed to relax their arm on the rectangular splint while their limb was placed in the starting position with their shoulder slightly abducted and their elbow flexed to 90º. A handheld goniometer was used to ensure consistency of the starting position. Participants wore goggles and headphones with pink noise to obscure visual and auditory cues and distractors during testing. Each trial began with the researcher tapping the participant’s shoulder to indicate the beginning of a new trial. The tactile cue was accompanied by the verbal command “concentrate now,” which repeatedly reminded the individual to focus on the task.
Each participant completed two proprioceptive testing procedures, including passive motion sense (PMS) and limb position sense (LPS). Each trial of the PMS task involved the participant comparing two angular velocities. For the PMS task, the comparison velocities ranged between 1.58º/s and 2.70º/s, with step increments of 0.15º/s. Eight different comparison velocities were used: 1.58º/s, 1.73º/s, 1.88º/s, 2.03º/s, 2.18º/s, 2.33º/s, 2.58º/s, and 2.70º/s. Each trial comprised a standard velocity of 1.50º/s paired with one of the eight comparison velocities. Each velocity was presented for 2 s before switching to the other velocity. The interval between the two velocities was 500 ms. Within a single trial, the two velocities to be discriminated always moved in the same direction (flexion or extension). After each trial, the participant verbally indicated which velocity was faster.
LPS trials followed a similar pattern. The comparison displacements for LPS ranged between 8.8º and 9.92º, with step increments of 0.16º. Eight different comparison displacements were used: 8.8º, 8.96º, 9.12º, 9.28º, 9.44º, 9.60º, 9.76º, and 9.92º. Each trial comprised a standard displacement of 10º paired with one of the eight comparison displacements. The movement direction for each pair of stimuli was always away from the body. The inter-stimulus interval between the standard and comparison angular displacements was approximately 1 s. For the first stimulus, each participant's arm was passively moved to the desired angular displacement at a constant velocity of 2º/s and then returned to the starting position. After a 1-s interval, the participant's arm moved again to the second desired angular displacement with the same constant velocity. At the end of each trial, participants had to indicate which angular displacement was further away from their body.
In both proprioceptive testing procedures, the standard and comparison stimuli were presented randomly. If a participant reported losing focus or needing to repeat a trial, the trial would be repeated only once. A standard forced-choice paradigm was used throughout the testing procedure; therefore, participants could not respond with “I do not know” or “They were the same,” rather, the individual was asked to select the trial that met the specific task instructions. The experimenter recorded each participant’s verbal response at the end of each trial.
LPS testing was always conducted first, as our pilot testing indicated that participants had more difficulty with the LPS task than the PMS task. Before beginning each testing paradigm, three practice trials were administered to confirm that participants understood the experimental procedure. Standard surface EMG with a sampling rate of 1000 Hz was used to monitor myoelectric activities of the biceps to ensure participants remained passive and did not generate any movement during the test. Trials exhibiting visible electromyography (EMG) activity were excluded and repeated once without being counted towards the 72 LPS and 72 PMS trials. The more affected arm was examined in PwPD, whereas the dominant arm was examined in PwMCI and the control group.
To gain further insights into the participants' strategies and explore their approaches during the trials, we conducted an "exit interview" after the task.
Olfactory function
The traditional Chinese version of the University of Pennsylvania smell identification test (UPSIT-TC), a modified version of the original UPSIT [
23], was used to measure each participant’s odor identification function (Sensonics, Inc. Hadden Heights, NJ). The validity and reliability of UPSIT-TC have been established [
24]. The UPSIT-TC comprises four 10-odor booklets. Each “scratch-and-sniff” odor is embedded in a microcapsule and covered in a brown rectangular area at the bottom of each page. Participants were instructed to scratch the brown area with a pencil tip in a standardized manner and choose the correct answer from four choices. The total score for the UPSIT-TC ranges from 0 to 40, with 40 indicating no errors made.
The smell threshold test (STT) (Sensonics, Inc., Hadden Heights, NJ) was used to assess smell discrimination. The STT has been examined for its correlation with other olfactory tests and test–retest reliability [
23,
25]. Two sniff bottles in a quasi-random sequence were provided on the threshold response forms. The participants had to judge which odorant in a given pair was stronger. The total score on the STT ranges from − 10 to − 2, with higher negative values indicating better performance.