A. Participants
Ten healthy controls (HC) (6 females, 4 males, mean age ± standard deviation (SD) 63.7 ± 9.9 years old) and 8 subjects diagnosed as Mild Neurocognitive Disorder (MND) (7 females, 1 male, mean age ± SD 75.7 ± 5.5 years old) were recruited at the Nice Research Memory Center (CMRR) & Cognition Behaviour Technology laboratory (CoBTeK). All the subjects were recruited in the context of Marco-Sense multi-centric research protocol. The study was performed in accordance with the Declaration of Helsinki and approved by the ethical committee CPP Ile de France (N° IDRCB: 2019-A00342-55). All participants received detailed written explanations on the study and signed written informed consent. Participants were not included in the study if they had a score at the Mini-Mental State Examination (MMSE) < 22 [25] and a Frontal Assessment Battery (FAB) score < 11 [26]. All the participants were right-handed. Eventually, based on the Bank National Alzheimer Diagnosis, all the MND patients have been classified according to the predominant deficit. Particularly, MND subjects which present dysexecutive MND impairments (e.g. action planning and motor programming deficits) have been named as d-MND.
B. Instruments
A novel ring-shaped device, called SensRing (Fig. 1), has been developed at the BioRobotics Institute of Scuola Superiore Sant’Anna (Pisa, Italy) to fully track the orientation and movement of the finger where it is worn [23]. The device, based on an ARM®Cortex™-M3 32-bit STM32-F103 microcontroller (STMicroelectronics, Italy), can acquire and store data with 50 Hz sampling frequency. SensRing mount 9–axes inertial measurement unit (IMU) LSM9DS1 (STMicroelectronics, Italy), including a 3D digital linear acceleration sensor (selectable full scale: ±2/±4/±8/±16 g), 3D digital angular rate sensor (selectable full scale: ±245/±500/±2000 dps), and 3D digital magnetic sensor (selectable full scale: ±4/±8/±12/±16 gauss). SensRing selected 2 g, 2000 dps, and 4 gauss as full scales. An integrated Bluetooth module (Rigado BMD-350, Nordic Semiconductor, Norway) allows wireless communication for data transmission towards a generic control station. A dedicated interface, developed in Visual Studio 2019 (Microsoft Corporation, USA) and based on C# language, ensures managing the connection and the acquisition of sensors data. A small, rechargeable PoLi battery, externally fixed to the wrist with an elastic band, supplies the SensRing. Integrated solutions for the battery are currently under development.
C. Experimental Protocol
Before starting the trial, the experimenters required the subjects to wear SensRing on the proximal phalanx of the dominant index finger. Participants sat in front of a table, laying their dominant hand on the starting position (3 cm from the edge of the table midsagittal position, and 15 cm away from the midsection). Experimenters instructed the participants to perform shorts reach-to-grasp (RG) and after-grasp (AG) sequences with three different end-goals, adapted from a previous study [7]. For each task, a can (ø = 5 cm, h = 8.5 cm) has been positioned in front of the participant, at 21 cm from the hand starting position along the midsagittal plane. The initial position was acquired as a baseline for 5 seconds. Afterwards, a tone was indicated the beginning of the task. For each condition, 10 repetitions have been performed. During the drinking condition (DRINK), subjects had to reach the can, grasp it, and lift it up simulating a drinking action. On the other hand, during the placing condition (PLACE), subjects had to reach the can, grasp it and place it inside a cup (ø = 7 cm), located 28 cm at the right side with respect to the initial position of the can. Finally, the passing condition (PASS) required the subjects to reach the can, grasp it and pass it to a partner. The partner sat to the right side of the table with the hand resting (on the same position of the cup in the previous condition) ready to take the can. Both for PLACE and PASS, the can was repositioned on the initial position after each repetition. The order of conditions was randomized across participants.
D. Signal Processing and Feature Extraction
Inertial data acquired with SensRing have been stored and offline processed by using MATLAB R2018a (The MathWorks, Inc., Natick, MA, USA). Accelerations and angular velocities, acquired from the accelerometer and gyroscope integrated into the IMU, were pre-processed with a fourth-order low-pass digital Butterworth filter, using a 5 Hz cut-off frequency to delete high-frequency noise.
Two characteristic phases have been identified in each repetition: the RG phase from the beginning of the action to the grasping of the object, and the AG phase from the object grasping to the end of the task. Accordingly, custom algorithms have been implemented to segment the signal into these phases, across each exercise. The dominant axis of the angular velocity has been used as the reference signal for the segmentation, and three characteristic times have been calculated for each repetition: the starting time, the grasping time, and the end time (Fig. 2). Then, a set of kinematic parameters was computed from accelerations and angular velocities as detailed in Table 1, to investigate different aspects of the motor performances including, for instance, energy, duration, velocity [27]–[29].
All the parameters have been measured for each repetition both in the RG and the AG phases, except for the reaction time, the amplitude and time of maximum hand opening that have been calculated during the RG phase only. Totally, 21 parameters composed the dataset of each exercise (i.e., 12 features for the RG and 9 for the AG phase).
E. Data Analysis
Qualitative variables, such as gender and education level, have been compared using Chi2 test, whereas the remaining clinical data measured, as quantitative variables, through the Mann-Whitney U-test. The Kolmogorov-Smirnov test has been preliminarily applied to verify the data distribution of each extracted parameter (see Table 1). Since all parameters resulted as not normally distributed, non-parametric statistical tests have been adopted for data analysis.
Specifically, two macro-analyses were carried out:
1) Inter-group analysis: to evaluate if kinematic parameters may be able to differentiate HC and d-MND in terms of action planning and performance. The non-parametric Mann-Whitney U-test was applied to investigate significant differences (p < 0.05) between the two groups.
2) Intra-group analysis: to investigate, within each group, if motor patterns are modulated based on the action planning and the execution of three conditions with different end-goal. Non-parametric Wilcoxon test was used to find significant differences (significant level set at p-value < 0.05) between pairs of conditions (i.e., DRINK vs PLACE, DRINK vs PASS, PLACE vs PASS).
Additionally, a correlation analysis of the motor performance to the MMSE score has been executed for each parameter, calculating the Spearman’s correlation coefficients. This analysis investigates whether the motor parameters correlate to the clinical score of a standard neuropsychological test, typically used as a screening tool for cognitive assessment. d-MND and HC are considered as a unique group for this analysis.