Age-specific neural responses to SMA and M1 stimulation during implicit motor sequence learning: Insights from a concurrent tDCS-fNIRS approach

Implicit Motor Sequence Learning (IMSL) is critical for automating motor skills, yet it declines with age, impacting independence and quality of life. This study investigated the age-specific roles of the primary motor cortex (M1) and supplementary motor area (SMA) in IMSL using a novel concurrent transcranial direct current stimulation (tDCS) and functional near-infrared spectroscopy (fNIRS) approach. Twenty younger adults (21-32 years old [M = 24.3 ± 3.26]) and twenty older adults (60-76 years old [M = 67.5 ± 4.88]) performed a Serial Reaction Time Task (SRTT) in three cross-over, counterbalanced sessions with anodal tDCS targeting: i) SMALeft, ii) M1Left, or iii) sham stimulation during task. Neural activity and connectivity were measured pre-, during-, and post-stimulation using a 20-channel fNIRS, covering prefrontal, premotor, and motor regions. Results revealed significant age-related differences in SRTT performance, with older adults consistently performing poorer despite practice-driven improvements in both groups. While tDCS did not enhance motor learning behaviourally, distinct neural effects emerged, demonstrating age-specific modulation. In older adults, M1 stimulation elicited task- dependent, asymmetric O₂Hb changes in the premotor area (PMA): decreased PMARight activity during sequence blocks and increased activity during random blocks, with opposite patterns in PMALeft. SMA stimulation normalised weak PMALeft-prefrontal connectivity in older adults, whereas younger adults showed strengthened connectivity between PMARight- PFCRight and PFCLeft-PFCRight, reflecting its role in spatial integration and motor planning. These findings advance the understanding of IMSL-related neuroplasticity, highlighting the importance of optimising tDCS protocols to support motor learning in ageing population.