Individual Differences in Resting Corticospinal Excitability Are Correlated with Reaction Time and GABA Content in Motor Cortex

Abstract

Individuals differ in the intrinsic excitability of their corticospinal pathways and, perhaps more generally, their entire nervous system. At present, we have little understanding of the mechanisms underlying these differences and how variation in intrinsic excitability relates to behavior. Here, we examined the relationship between individual differences in intrinsic corticospinal excitability, local cortical GABA levels, and reaction time (RT) in a group of 20 healthy human adults. We measured corticospinal excitability at rest with transcranial magnetic stimulation, local concentrations of basal GABA with magnetic resonance spectroscopy, and RT with a behavioral task. All measurements were repeated in two separate sessions, and tests of reliability confirmed the presence of stable individual differences. There was a negative correlation between corticospinal excitability and RT, such that larger motor-evoked potentials (MEPs) measured at rest were associated with faster RTs. Interestingly, larger MEPs were associated with higher levels of GABA in M1, but not in three other cortical regions. Together, these results suggest that individuals with more excitable corticospinal pathways are faster to initiate planned responses and have higher levels of GABA within M1, possibly to compensate for a more excitable motor system.

Dissociable roles of cortical excitation-inhibition balance during patch-leaving versus value-guided decisions

“…As reported above, task-specific effects were found for patch-leaving and value-guided choice in dACC and vmPFC, respectively. In contrast to these specific effects, overall response speed in the patch decision stage was related to E/I balance in primary motor cortex: higher E/I balance was associated with slower responding, in agreement with other studies showing a positive relationship between motor cortical GABA levels and corticospinal excitability 39,40 . In the value-guided choice phase however, overall response speed was correlated with E/I balance in dACC, not in primary motor cortex.…”

Maturation changes the excitability and effective connectivity of the frontal lobe: A developmental TMS–EEG study

“…The functional significance of the suggested increase in inhibition in children remains uncertain. In a recent TMS and magnetic resonance spectroscopy study in adults (Greenhouse & King, ), more excitable corticospinal pathways were associated with higher GABA concentrations in the motor cortex. These higher GABA concentrations were suggested to indicate a homeostatic mechanism, since more excitable pathways may require a larger inhibitory neurotransmitter reservoir (Greenhouse & King, ).…”

“…In a recent TMS and magnetic resonance spectroscopy study in adults (Greenhouse & King, ), more excitable corticospinal pathways were associated with higher GABA concentrations in the motor cortex. These higher GABA concentrations were suggested to indicate a homeostatic mechanism, since more excitable pathways may require a larger inhibitory neurotransmitter reservoir (Greenhouse & King, ). Similarly, in a recent TMS–EEG study in healthy adults, poorer performance in the attention task was related to larger N100 amplitudes, indicating more pronounced GABA B ergic inhibition.…”

Big GABA: Edited MR spectroscopy at 24 research sites

“…Healthy brain function relies on GABAergic inhibitory processes, and understanding GABAergic mechanisms in both healthy and pathological brain function has been one core focus of neuroscience. MRS measurements of GABA have been associated with individual differences in hemodynamic and electrophysiological signals (Donahue et al, 2010; Hu et al, 2013; Kapogiannis et al, 2013; Muthukumaraswamy et al, 2009) and a number of measures of cognition (Fujihara et al, 2015; Shibata et al, 2017; Yoon et al, 2016) and behavior (Boy et al, 2011; Greenhouse et al, 2017; Puts et al, 2011; Silveri et al, 2013). Differential levels of GABA have been observed in a number of neuropsychiatric disorders, such as schizophrenia (Kegeles et al, 2012; Öngür et al, 2010; Rowland et al, 2016; Yoon et al, 2010) and depression (Bhagwagar et al, 2008; Hasler et al, 2007; Price et al, 2009), neurodevelopmental disorders such as autism spectrum disorder (Drenthen et al, 2016; Gaetz et al, 2014; Puts et al, 2016) and attention deficit hyperactivity disorder (Bollmann et al, 2015; Edden et al, 2012a), and neurological diseases, such as Parkinson’s disease (Emir et al, 2012), amyotrophic lateral sclerosis (Foerster et al, 2012; Foerster et al, 2013) and diabetic neuropathy (Petrou et al, 2012).…”