Long-latency modulation of motor cortex excitability by ipsilateral posterior inferior frontal gyrus and pre-supplementary motor area.

Francesca Fiori,Alessio Avenanti,Emilio Chiappini,Vincenzo Romei,Riccardo Paracampo,Marco Soriano,Sara Borgomaneri

doi:10.1038/srep38396

Abstract

The primary motor cortex (M1) is strongly influenced by several frontal regions. Dual-site transcranial magnetic stimulation (dsTMS) has highlighted the timing of early (<40 ms) prefrontal/premotor influences over M1. Here we used dsTMS to investigate, for the first time, longer-latency causal interactions of the posterior inferior frontal gyrus (pIFG) and pre-supplementary motor area (pre-SMA) with M1 at rest. A suprathreshold test stimulus (TS) was applied over M1 producing a motor-evoked potential (MEP) in the relaxed hand. Either a subthreshold or a suprathreshold conditioning stimulus (CS) was administered over ipsilateral pIFG/pre-SMA sites before the TS at different CS-TS inter-stimulus intervals (ISIs: 40–150 ms). Independently of intensity, CS over pIFG and pre-SMA (but not over a control site) inhibited MEPs at an ISI of 40 ms. The CS over pIFG produced a second peak of inhibition at an ISI of 150 ms. Additionally, facilitatory modulations were found at an ISI of 60 ms, with supra- but not subthreshold CS intensities. These findings suggest differential modulatory roles of pIFG and pre-SMA in M1 excitability. In particular, the pIFG –but not the pre-SMA– exerts intensity-dependent modulatory influences over M1 within the explored time window of 40-150 ms, evidencing fine-tuned control of M1 output.

Highlights

(through direct or indirect connections) from the site of stimulation to M1
Our findings show that long-latency functional connections do exist between rostral premotor and motor areas, and that specific time intervals and intensities are crucial for observing causal influences of pIFG and pre-SMA over M1 excitability during a resting state
Twelve right-handed healthy participants took part in an experimental session and a control session performed on two separate days, during which motor-evoked potentials (MEPs) induced by a test stimulus (TS) delivered over the left M1 were collected from the right first dorsal interosseous (FDI)

Summary

Introduction

(through direct or indirect connections) from the site of stimulation to M1. The CS is followed by a test stimulus (TS) that is administered over M1 to induce motor-evoked potentials (MEPs) in contralateral muscles. Using dsTMS, studies have focused on how M1 excitability is influenced by a CS administered over posterior inferior frontal cortices[20,31,32,33,34] and the supplementary motor complex[15,20,21,35] These studies have focused on short-latency connectivity using various ISIs of

Objectives

Methods

Results

Conclusion