Input-output curves demonstrate motor cortex hyperexcitability associated with cognitive impairment in Alzheimer's disease.
Recent studies suggest that cortical hyperexcitability in Alzheimer's disease (AD) may accelerate disease progression. However, the field lacks validated non-invasive methods to assay excitability in humans. In this study we used transcranial magnetic stimulation with electromyography to investigate mechanisms of motor cortical excitability in 63 biomarker-positive early-AD (ranging from mild cognitive impairment to mild dementia) participants and 72 cognitively unimpaired age-matched adults (CU). Single-pulse stimulation was applied to motor cortex to record resting motor thresholds (rMT) and motor-evoked potentials. An Input-Output curve was generated by delivering 10 pulses each at eight different intensities of maximum stimulator output (%MSO). Linear models or equivalent non-parametric tests 1) compared excitability metrics between groups, 2) tested correlations with cognition (mini-mental state exam, MMSE; Alzheimer's disease assessment scale-cognitive, ADAS-Cog), and 3) tested the impact of APOE4. Results show that early-AD participants have increased motor cortical excitability than CU, with lower rMT (p < 0.001), lower Input-Output curve Inflection Point (p = 0.007), and higher Dynamic Range (p = 0.035). An analysis of the Input-Output curve adjusting for rMT showed larger responses in AD specifically in the 135-150% rMT range (p = 0.023). In AD, higher excitability was related to worse cognition (rMT: MMSE p = 0.008, Inflection Point: MMSE p = 0.030 and ADAS-Cog p = 0.041). There was no relationship between APOE4 and excitability. In conclusion, AD participants have increased motor cortical excitability, related to cognition. This is evident both at lower and higher stimulation intensities. TMS may provide a useful measure of target engagement for therapies aimed at preserving cognition or slowing decline in early-AD.