Probing intrahemispheric PMd – M1 interactions with a novel dual-site TMS setup

Abstract

The net output of the primary motor cortex (M1) is shaped by several (non-)motor brain regions including the dorsal premotor cortex (PMd) which is playing an important role in sensorimotor integration, response selection, bimanual motor control, and motor learning. Previous dual-site transcranial magnetic stimulation (dsTMS) setups mainly investigated the interhemispheric PMd – M1 interactions, as testing the intrahemispheric PMd – M1 interactions yield technical difficulties due to the vicinity of both regions.

A novel dsTMS setup was used in a sample of 23 young healthy right-handed adults to probe intrahemispheric left PMd – M1 interactions at rest. Biphasic stimuli were applied to M1 to elicit an MEP of at least 1 mV peak-to-peak amplitude (testing stimulus, TS) in the resting first dorsal interosseus (FDI). The conditioning stimulus (CS) was applied ∼2 cm anterior to M1 at an interstimulus interval (ISI) of 6 ms and an intensity of 75% of the resting motor threshold (rMT).

To physiologically test this setup, short-interval intra-cortical inhibition (SICI) was measured with the same coil arrangement, using two coils targeted at M1 (ISI = 3 ms, CS intensity = 75% rMT, TS intensity = 1 mV).

First, SICI could robustly be elicited using the novel coil setup. Second, conditioning left PMd lead to a robust modulation of left M1 output when using a CS intensity of 75% rMT. This interaction was mostly found to be inhibitory. However, in some subjects (∼15%) a facilitatory PMd – M1 interaction was seen.

This novel coil setup opens new opportunities to measure intrahemispheric PMd – M1 interactions at rest and in different task-related contexts without facing technical difficulties such as large stimulation distances due to coil size or coil heating (particularly in small coils).