2.1 Participants
Participants were recruited using convenience sampling by posters placed on social media. Children with DCD were included in the study if they satisfied four specific criteria from the Diagnostics and Statistical Manual of Mental Disorders, Fifth Edition: (a) scored below the 5th percentile in the Hong Kong Fine Motor Test for the School-aged (see Supplemental material A for details); (b) scored below age-specific cut off in the Developmental Coordination Disorder Questionnaire (criterion B); (c) parents and teachers reported an early onset of symptoms (criterion C); and (d) no report of intellectual or visual impairment, or other physical and neurological conditions (criterion D). Children with left-hand dominance were excluded from the study.
The control group consisted of children with typical development (TD) who met the following inclusion criteria: (a) age- and gender-matched with the DCD participants; (b) right-handed; and (c) no reported intellectual or visual impairments, or any other physical or neurological conditions. Children in the TD group were also assessed using the Hong Kong Fine Motor Test for the School-aged and Developmental Coordination Disorder Questionnaire, achieving scores within the normal range. Table 1 provides a summary of the demographics and details of the children involved in the study.
(Table 1 General information of the participants)
2.2 Study design
Prior to the experiment, they were seated upright on a chair with adjustable seat height, seat depth, and footrest height in front of a table and a computer monitor. A pressure button, used as the starting point, was located on the table. Three pencils to be reached were aligned vertically at angles of −30°, 0°, and 30° relative to the pressure button. The center of the pressure button was set 30 cm from each pencil and 50 cm from the monitor.
At the onset of the experiment, participants were directed to press the button with their right palm. A white cross then appeared on the monitor serving as a preparatory signal. Following a random interval of 1 to 2 s, the white cross transitioned to a slide featuring three images depicting the positions (left, middle, and right) of three pencils, coupled with an auditory cue that signaling the start of the reaching movement. One of the three images was highlighted to prompt participants about the target pencil. Initially, the middle image was always highlighted. However, once the participant's palm left the button, this image could either stay highlighted (non-jump trial) or switch off, with one of the lateral images (left or right) lighting up (jump trial), each with an equal probability of occurring. Participants were informed about these possibilities beforehand and instructed to execute their movements swiftly and accurately. They were then required to return their hand to the starting point and wait for the next trial. Each trial lasted about 5 seconds and was followed by a 20-second rest period. All stimuli were presented using E-Prime (version 2.10, Psychology Software Tool, USA).
The experimental setup consisted of two runs, each containing 24 trials—16 non-jump and 8 jump trials, randomized throughout. Each run lasted around 10 minutes, separated by a 3-minute rest interval. The entire session was recorded using a synchronized camera system. In addition, preceding the formal testing phase (before participants were fitted with the fNIRS cap and probes), a practice session with 12 trials (8 non-jump and 4 jump) was given to acquaint participants with the task.
(Fig. 1 Illustration of the task paradigm
The task was initiated with participants placing their right palm on the start button. After a random interval of 1 to 2 s, they were signaled to reach the target. During their ongoing reaching, there is a 67% probability that the target remained unchanged and a 33% chance that it shifted to a different target, as depicted in the monitor. The time limit for this movement was 5 s. Upon reaching the target, the participants automatically returned their hand to the starting position and then rested for a 20-second period before proceeding with the next trial.)
2.3 fNIRS measurement
The brain activity during the task was recorded using a multichannel fNIRS optical topography system (ETG-4000, Hitachi Medical Co., Japan) with near-infrared light at wavelengths of 695 nm and 830 nm; its sampling rate was 10.0 Hz. Two 3×5 probe sets with eight sources and seven detectors were fastened in a customized EEG cap and placed on the participants’ head, resulting in 2 × 22 channels in total (Fig. 2). Specifically, the two probes were symmetrically positioned with channel 21/25 aligned to the 'C1/C2' point of the international 10/20 system, and the alignment of channels 3-12-21/25-36-43 corresponded to the 'C5-C1/C6-C2' line. The distance between each probe was 3 cm. The 'Cz' point on the cap served as a reference marker to ensure consistent positioning across participants. Additionally, a 3D-magnetic space digitizer (EZT-DM401, Hitachi Medical Corporation, Japan) was employed to document the 3D location of each probe on the participant's head. These coordinates were subsequently registered into Montreal Neurological Institute coordinates using the AtlasViewer toolbox for meticulous probe-set placement verification (32). Table 2 depicts the cortical regions (i.e., regions of interest; ROIs) covered by the probe arrangement in the present study, according to the estimation of probabilistic anatomical locations of channels based on the Brodmann area atlas.
(Fig. 2 Arrangement of fNIRS channels)
(Table 2 Allocation of the fNIRS channels to cortical regions)
2.4 Data analysis
2.4.1 Behavioral data analysis
To evaluate the behavioral performance of the task, movement time difference (MTdiff) was computed by subtracting the movement time of the jump condition from the non-jump condition. Movement time was defined as the duration between reaching initiation and completion. Reaching initiation was identified as the point when the participants’ hand left the press button, while completion was marked by the moment when the participants touched the pencil. The timing of all these events was determined by analyzing the videos recorded by the synchronized camera during the experiment.
2.4.2 fNIRS data processing and analysis
The recorded original fNIRS signals were analyzed using HOMER2 (Huppert et al., 2009). First, noisy channels were identified and pruned. Subsequently, all normal fNIRS signals were converted into optical density. Motion artifacts were then corrected using wavelet filtering (inter-quartile range = 0.1), a method suggested to yield promising outcomes in the pediatric population (33). Next, a bandpass filter ranging from 0.01 Hz to 0.1 Hz was applied to remove the slow drifts and cardiac pulsation signal. After that, optical density was converted into the concentration change of oxyhemoglobin concentration (ΔHbO) and deoxyhemoglobin concentration (ΔHbR) using the modified Beer-Lambert law with a partial pathlength factor of 6. Finally, the resulting data were block averaged within a period from 3 s before the task to 12 s after the task as the time-series concentration changed.
ΔHbO was utilized for further analysis due to its higher contrast-to-noise ratio compared to ΔHbR (34). ΔHbO was averaged over a time window ranging from 5 to 10 s after task onset as the brain activation amplitude for each trial. This is based on the understanding that ΔHbO requires approximately 5 s to achieve a stable change and typically peaks within 5 to 10 s after movement onset during motor tasks (35, 36). Notably, in the current study, ΔHbO peaked at approximately 7 to 8 s post task onset according to the time-series data.
2.6 Statistical analysis
An independent sample t test was first used to investigate the difference in behavioral performance by comparing the MTdiff between children with DCD and TD. To visualize the cortical response for the task at the group level, we calculated the between-condition difference in brain activation amplitude using a paired t test and generated t-maps for both groups using the BrainNet Viewer toolbox, according to the previously registered MNI coordinates (37). The t values are reported in Supplemental material B and for visualization purposes only (i.e., not for interpretation). Furthermore, the average brain activation amplitude of each ROI was analyzed to explore the effects of group and condition on cortical activities. A 2×2 mixed ANOVA was performed on the brain activation amplitude in each ROI, involving group (DCD v.s. TD) as a between-subjects factor and condition (non-jump v.s. jump) as a within-subjects factor. Additionally, Pearson’s r correlation coefficients of the between-condition difference in brain activation amplitude in those ROIs with identified significant differences in the former test and MTdiff were calculated to further examine the association between cortical responses and behavioral performance. All statistical analyses will be executed using SPSS (version 22, IBM, USA) with a significance level set at 0.05 and a confidence interval of 95%.