Ian was a guest on the first neurofriends podcast at Stanford University put together by Sarah Hillenbrand. He was interviewed by Sarah and Lea, a Stanford undergraduate, about a paper that he thinks is particularly interesting.
In this study we aimed to understand the relationship between stable individual differences in motor system excitability and reaction time by combining brain stimulation, neurochemical imaging, and behavioral testing. We used transcranial magnetic stimulation (TMS) to measure the intrinsic (resting) excitability of the pathway between the brain and muscles in the hand. We also measured reaction time by recording activity from the same hand muscle when it was used to respond during the performance of a simple computer task. We used magnetic resonance spectroscopy (MRS) to measure concentrations of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in four different regions of the cortex. We found that individuals with a more excitable motor output pathway also had faster reaction times. Surprisingly, individuals with a more excitable motor output pathway also had higher concentrations of GABA in the motor cortex, but not in the other three brain regions we measured. These results suggest that people with a more excitable motor output pathway are faster to execute planned movements and also have more GABA available in the motor cortex. Larger amounts of GABA may support a greater capacity to inhibit a more excitable motor output pathway to maintain homeostasis within the motor system.
Individual differences in GABA content are reliable but are not uniform across the human cortex
In this study we used magnetic resonance spectroscopy (MRS) to measure the content of the inhibitory neurotransmitter GABA in four different cortical regions. We discovered that individuals with higher GABA content in one cortical area don’t necessarily have higher GABA content in other cortical areas. We also show that our measurements are reliable across weeks. Our findings are important because they suggest that GABA levels are regionally specific, and they support earlier studies that suggest local individual differences in GABA content relate to behavior. Those earlier studies didn’t look at measurement reliability and regional specificity in the same individuals. We did that in our study.
Check out the recent Journal Club commentary published in the Journal of Neuroscience by and Gerard Derosiere from Université catholique de Louvain along with Ian and coauthors’ response.
In this set of experiments we found the surprising result that the motor system is broadly inhibited during the preparation of responses. For example, your left index finger is inhibited when you prepare to move your right pinky finger. We suggest a new framework to account for this unexpected result and propose that an inhibitory brain mechanism suppresses noise in the motor system to facilitate the recruitment of a selected action.
Here is a link to a blog by our resident physicist at UC Berkeley who has been helping to conduct fMRI in dogs on our scanner: Dog fMRI at UC Berkeley