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Exp Brain Res. 2015 Feb;233(2):671-7. doi: 10.1007/s00221-014-4146-z. Epub 2014 Nov 16.

Neural summation in human motor cortex by subthreshold transcranial magnetic stimulations.

Author information

1
Department of Psychiatry, Maryland Psychiatric Research Center, University of Maryland School of Medicine, P.O. Box 21247, Baltimore, MD, 21228, USA.

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 on the motor cortex using two subthreshold pulses of transcranial magnetic stimulation (TMS), each with intensities ranging from 60 to 95% of the resting motor threshold (RMT) and interstimulus interval (ISI) varying from 1 to 25 ms. We found that two subthreshold TMS pulses can produce suprathreshold 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 was 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.

PMID:
25399245
PMCID:
PMC4297236
DOI:
10.1007/s00221-014-4146-z
[Indexed for MEDLINE]
Free PMC Article

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