Objective: The goal of the present study was to compare the indirect estimate of the central motor conduction time (CMCT) with direct measurement of the corticospinal conduction time between the motor cortex and cervical enlargement in macaque monkeys.
Methods: Responses to transcranial magnetic stimulation of the motor cortex were recorded from intrinsic hand muscles in adult macaque monkeys. The CMCT was calculated by subtracting the peripheral conduction time, measured with the F-wave method, from the latency of the motor evoked potentials (MEPs). In two monkeys, the actual conduction time between the motor cortex and cervical enlargement was measured directly by different invasive techniques.
Results: We found that the indirect calculation of CMCT overestimates the corticospinal conduction time to a significant extent.
Conclusions: One possible source of error is an underestimate of the MEP peripheral conduction time. A collision test confirmed this hypothesis and showed that only a marginal proportion of the motoneurones that respond to a weak corticospinal input also participate in the F-wave. A more accurate estimate of the CMCT could be obtained by using the longest F-wave latency, rather than the shortest, to calculate the peripheral conduction time.