Abstract
Living in a complex and dynamic environment continuously requires the rapid generation of motor responses when necessary but also the promptly inhibition of actions when they would be unsuitable. In the last years many studies have investigated the role of various cortical and subcortical brain regions in inhibitory control and converging evidences suggest a crucial role of the right Inferior Frontal Gyrus (rIFG) and of the Supplementary Motor Area (SMA). However the precise role and the exact timing of the contribution of these areas in this process remain unclear. The aim of the present study was to investigate the role of rIFG and SMA in motor planning and motor inhibition by establishing the specific time course and the causal interactions of these regions in relation to the left primary motor area (LM1). In a sample of 10 healthy subjects paired transcranial magnetic stimulation (TMS) was applied with an interstimulus interval of 6 ms over the rIFG-LM1 and over SMA-LM1 before and 50, 75, 100, 125, 150 ms after the presentation of visual stimuli in a simple GO/NOGO task. For each interval Motor Evoked Potentials (MEPs) and Reaction Times (RTs) were collected. When paired TMS was applied over the rIFG, MEPs were markedly and selectively increased for the NOGO trials at 50 ( p = 0.004), 100 ( p = 0.002) and 150 ms ( p = 0.002) after the stimulus onset. On the other hand, no differences were found at 75 and 125 ms after cue presentation for the NOGO trials and at any delay for the GO trials. A trend for a similar temporal profile of cortico-cortical activation was found for the SMA-LM1 connectivity for the NOGO trials peaking at 50, 100 and 150 ms after the cue onset. On the contrary, MEPs increased in the GO trials at 75 ( p = 0.015) and 125 ms ( p = 0.004) after the cue presentation. Task accuracy was high in all experimental conditions, but we observed that Go Reaction Times (RTs) were slowed down at 100 and 150 ms after the stimulus presentation in all experimental conditions. These findings provide evidence of a very early rIFG-M1 and SMA-M1 functional interaction to inhibit a motor response. Notably, we found that connectivity peaked repeatedly every 50 ms. It is possible that these fluctuating temporal profiles of cortical interactions could resemble the underlying reverberant oscillation of beta rhythms. Indeed, the present study suggests a rIFG specialization in response inhibition and a specialization of the SMA in response planning.
Published Version
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