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.
Gamma Oscillations in the Hyperkinetic State Detected with Chronic Human Brain Recordings in Parkinson’s Disease
In this study we used a cool new device that lets us record electrical brain signals in Parkinson’s disease patients, and also delivers therapeutic electrical stimulation to help their symptoms (DBS, deep brain stimulation)! We recorded basal ganglia local field potentials (electrical brain signals from deep in the brain) and motor cortex electrocorticography (electrical brain signals from the surface of the brain – a part that controls movements). People have been able to record these signals before, but it was always in special circumstances, like in the hospital or operating room. Here the device is fully implanted so we can even record signals in the patient’s home! Pretty neat! Using this device we discovered brain signal that occurred when patients were experiencing dyskinesia, or involuntary movements, that occur because of their medications. Now that we know the signal, the next step is seeing if we can use this to customize stimulation, so that the stimulation can be adjusted to try to avoid these side effects. This is called “close loop DBS” and you can read more about it in another here: https://kids.frontiersin.org/article/10.3389/frym.2016.00010. We are currently testing this in our new experiment!
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.
Check out Nicki’s latest paper!
In this paper we showed that some of the markers of excessive synchrony in Parkinson’s disease that we have observed using invasive recordings in humans (electrocorticography, or electrodes right on the surface of the brain), can also be detected non-invasively with electrodes on the scalp! Pretty cool!
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