Distinct cortical excitability and connectivity profiles within the human SMA complex.

Area of Science:

• Neuroscience
• Cognitive Neuroscience
• Systems Neuroscience

Background:

• Brain function increasingly relies on network interactions, particularly for higher-order cognitive processes.
• The supplementary motor area (SMA) complex is a key hub linking cognitive and sensorimotor functions.
• Mapping SMA complex dynamics in humans is challenging but crucial for understanding cognition.

Purpose of the Study:

• To establish normative neurophysiological signatures of the SMA complex (pre-SMA and SMA).
• To investigate causal, time-resolved cortical excitability and connectivity using TMS-EEG.
• To differentiate the neurophysiological profiles of pre-SMA and SMA subregions.

Main Methods:

• Utilized navigated transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) in 21 healthy subjects.
• Recorded cortical excitability (GMFP) and oscillatory properties (α, β2 power, NF) across six SMA complex targets.
• Assessed corticothalamic structural connectivity using diffusion-weighted imaging metrics (AFD, FBC) to thalamic nuclei.

Main Results:

• Significant variations in cortical excitability and oscillatory properties were found across SMA complex stimulation targets.
• A rostro-caudal gradient of cortical excitability was observed, with differences diminishing over time.
• Distinct structural connectivity patterns were identified between SMA subregions and thalamic nuclei.

Conclusions:

• Provided direct human in vivo evidence for distinct neurophysiological and structural connectivity profiles of pre-SMA and SMA.
• These differences suggest functional specialization, with pre-SMA involved in cognitive control and SMA in motor networks.
• TMS-EEG and diffusion-weighted connectomics effectively measure fine-grained neurophysiological differences for potential individualized neuromodulation.