Association between reliable measures of human reticular formation motor output

The reticular formation is a key hub involved in motor control, sending descending projections to the spinal cord via the reticulospinal tract and ascending projections to primary motor cortex via the thalamus. The aim of the present study was to determine the inter-day reliability of three non-invasive measures of descending and ascending reticular formation motor output and their association.

In 20 healthy young participants, we combined a loud acoustic stimulus with transcranial magnetic stimulation to measure ipsilateral motor evoked potentials (iMEPs) and contralateral motor evoked potential (cMEP) suppression, and with a visual stimulus to measure the StartReact effect. Inter-day measurements were separated by a mean of 10 days. From the repeated measurements, we determined the absolute reliability (i.e., measurement error) and relative reliability (i.e., intraclass correlation coefficient).

We observed that all three measures (iMEPs, cMEP suppression, StartReact) have low-to-moderate measurement error (ranging from 10 to 29%) between sessions. Using the measurement error, we determined smallest detectable change estimates that should be surpassed to conclude that modulation exceeds what is expected due to measurement noise. We found that all three measures have excellent relative reliability (all intraclass correlation coefficients > 0.88). Finally, we observed an association between the magnitude of iMEP presence an cMEP suppression (ρ = -0.60, p = 0.035).

iMEPs, cMEP suppression and StartReact are reliable non-invasive measures for tracking inter-day changes in reticular formation motor output in humans.

Our findings provide clear reliability guidelines for future studies assessing modulation of reticular formation motor output with intervention or disease progression.