Neural summation in human motor cortex by subthreshold transcranial magnetic stimulations

Abstract

Integration of diverse synaptic inputs is a basic neuronal operation that relies on many neurocomputational principles, one of which is neural summation. However, we lack empirical understanding of neuronal summation in the human brains in vivo. Here we explored the effect of neural summation in the motor cortex using two subthreshold pulses of transcranial magnetic stimulation (TMS), each with intensities ranging from 60% - 95% of the resting motor threshold (RMT) and interstimulus intervals (ISI) varying from 1 – 25 ms. We found that two subthreshold TMS pulses can produce supra threshold motor response when ISIs were less than 10 ms, most prominent at 1, 1.5 and 3 ms. This facilitatory, above threshold response was evident when the intensity of the subthreshold pulses were above 80% of RMT but was absent as the intensity was 70% or below. Modeling of the summation data across intensity suggested that they followed an exponential function with excellent model fitting. Understanding the constraints for inducing summation of subthreshold stimulations to generate above threshold response may have implications in modeling neural operations and potential clinical applications.

Test-retest reliability of short-interval intracortical inhibition and intracortical facilitation in patients with schizophrenia

“…Surface electromyography (EMG) was recorded from right FDI. EMG was recorded with NeuroScan Synamp 2 amplifier (Charlotte, NC) amplified (gain of 10) and sampled at 1000 Hz (Du et al, 2015; Sommer et al, 2001). Peak-to-peak amplitude of the motor-evoked potentials (MEP) was measured.…”

The role of white matter microstructure in inhibitory deficits in patients with schizophrenia

“…Surface electromyography (EMG) was recorded from right FDI. EMG was recorded with NeuroScan synamp 2 amplifier (Charlotte, NC) amplified (gain of 10) and sampled at 1000 Hz [53,54]. Peak-to-peak amplitude of the motor-evoked potentials (MEP) was measured.…”

“…Resting motor threshold (RMT) was defined as the minimum intensity needed to elicit a MEP of >50 μV in at least 5 out of 10 consecutive stimuli [54]. For paired-pulse TMS (ppTMS), the intensities of the subthreshold conditioning stimulus (CS) and the testing stimulus (TS) were set to 80% and 120% RMT, respectively [55-57].…”

N100 as a generic cortical electrophysiological marker based on decomposition of TMS-evoked potentials across five anatomic locations

“…1). TMS was first applied to left M1 to determine the resting motor threshold (RMT) which was defined according to conventional criteria as the minimum intensity needed to elicit a MEP of >50 μV in at least 5 out of 10 consecutive stimuli (Rossini et al 1994; Du et al 2015). Thereafter, two TMS intensities were tested for each of the five TMS site: suprathreshold (120 % RMT) and subthreshold (80 % RMT).…”