Reliability of a TMS-derived threshold matrix of corticomotor function

Area of Science:

• Neuroscience
• Motor Control
• Aging

Background:

• Transcranial magnetic stimulation (TMS) typically analyzes suprathreshold motor evoked potentials (MEPs), potentially missing insights from subthreshold corticomotor pathway activation.
• Subthreshold excitability assessment is crucial for understanding corticomotor pathway integrity, especially in neurological conditions like stroke.
• Older adults represent a key demographic for studying neuroplasticity and recovery, yet their subthreshold corticomotor responses require further investigation.

Purpose of the Study:

• To evaluate the test-retest reliability of a new compositional analysis method for motor evoked potential (MEP) data in healthy older adults.
• To compare the compositional characteristics of MEPs between the dominant and non-dominant upper limbs.
• To investigate the relationship between subthreshold corticomotor responses and the resting motor threshold.

Main Methods:

• Twenty-three healthy older adults underwent two identical TMS sessions.
• Stimulus-response (S-R) curves and threshold matrices were generated using single-pulse TMS across varying intensities for four upper limb muscles.
• Compositional analysis metrics, including subliminal, subthreshold, and suprathreshold MEP components, were calculated and their reliability assessed using Intraclass Correlation Coefficients (ICCs).

Main Results:

• Stimulus-response curve slopes demonstrated reliable test-retest properties across all muscles (ICCs ranging from 0.58 to 0.88).
• Subliminal and suprathreshold MEP components of the threshold matrix exhibited good to excellent reliability (ICCs 0.79–0.94).
• Subthreshold MEP components showed poor reliability (ICCs 0.05–0.39), likely due to a floor effect, with no significant compositional differences between dominant and non-dominant limbs.

Conclusions:

• The threshold matrix, using compositional analysis, reliably assesses subliminal and suprathreshold motor evoked potentials in older adults.
• The poor reliability of subthreshold MEPs warrants further methodological refinement to overcome potential floor effects.
• Future research should explore the clinical utility of this compositional analysis for tracking corticomotor pathway recovery following neurological injury.