Assistant Professor, Department of Human Physiology
Ph.D. University of California, San Diego
B.A Tufts University
Ian Greenhouse’s research examines how humans initiate and cancel movement. His lab combines behavioral testing with electrophysiology, neuroimaging, and brain stimulation in healthy and clinical populations. His current research explores the relationship between the inhibitory neurochemical gamma-aminobutyric acid (GABA) and motor performance.
Dr. Greenhouse earned his BA in Psychology at Tufts University and his Ph.D. at the University of California, San Diego. He completed postdoctoral training at the University of California, Berkeley. He joined the Department of Human Physiology at the University of Oregon in the Fall of 2017.
A Single Mechanism for Global and Selective Response Inhibition under the Influence of Motor Preparation.
J Neurosci. 2020 10 07;40(41):7921-7935
Authors: Raud L, Huster RJ, Ivry RB, Labruna L, Messel MS, Greenhouse I
In our everyday behavior, we frequently cancel one movement while continuing others. Two competing models have been suggested for the cancellation of such specific actions: (1) the abrupt engagement of a unitary global inhibitory mechanism followed by reinitiation of the continuing actions; or (2) a balance between distinct global and selective inhibitory mechanisms. To evaluate these models, we examined behavioral and physiological markers of proactive control, motor preparation, and response inhibition using a combination of behavioral task performance measures, electromyography, electroencephalography, and motor evoked potentials elicited with transcranial magnetic stimulation. Healthy human participants of either sex performed two versions of a stop signal task with cues incorporating proactive control: a unimanual task involving the initiation and inhibition of a single response, and a bimanual task involving the selective stopping of one of two prepared responses. Stopping latencies, motor evoked potentials, and frontal β power (13-20 Hz) did not differ between the unimanual and bimanual tasks. However, evidence for selective proactive control before stopping was manifest in the bimanual condition as changes in corticomotor excitability, μ (9-14 Hz), and β (15-25 Hz) oscillations over sensorimotor cortex. Together, our results favor the recruitment of a single inhibitory stopping mechanism with the net behavioral output depending on the levels of action-specific motor preparation.SIGNIFICANCE STATEMENT Response inhibition is a core function of cognitive flexibility and movement control. Previous research has suggested separate mechanisms for selective and global inhibition, yet the evidence is inconclusive. Another line of research has examined the influence of preparation for action stopping, or what is called proactive control, on stopping performance, yet the neural mechanisms underlying this interaction are unknown. We combined transcranial magnetic stimulation, electroencephalography, electromyography, and behavioral measures to compare selective and global inhibition models and to investigate markers of proactive control. The results favor a single inhibitory mechanism over separate selective and global mechanisms but indicate a vital role for preceding motor activity in determining whether and which actions will be stopped.
PMID: 32928884 [PubMed - indexed for MEDLINE]
Response preparation involves a release of intracortical inhibition in task-irrelevant muscles.
J Neurophysiol. 2020 Dec 23;:
Authors: Gomez IN, Ormiston K, Greenhouse I
Action preparation involves widespread modulation of motor system excitability, but the precise mechanisms are unknown. In this study, we investigated whether intracortical inhibition changes in task-irrelevant muscle representations during action preparation. We used transcranial magnetic stimulation (TMS) combined with electromyography in healthy human adults to measure motor evoked potentials (MEPs) and cortical silent periods (CSPs) in task-irrelevant muscles during the preparatory period of simple delayed response tasks. In Experiment 1, participants responded with the left-index finger in one task condition and the right-index finger in another task condition, while MEPs and CSPs were measured from the contralateral non-responding and tonically contracted index finger. During Experiment 2, participants responded with the right pinky finger while MEPs and CSPs were measured from the tonically contracted left-index finger. In both experiments, MEPs and CSPs were compared between the task preparatory period and a resting intertrial baseline. The CSP duration during response preparation decreased from baseline in every case. A laterality difference was also observed in Experiment 1, with a greater CSP reduction during the preparation of left finger responses compared to right finger responses. Despite reductions in CSP duration, consistent with a release of intracortical inhibition, MEP amplitudes were smaller during action preparation when accounting for background levels of muscle activity, consistent with earlier studies that reported decreased corticospinal excitability. These findings indicate intracortical inhibition associated with task-irrelevant muscles is transiently released during action preparation and implicate a novel mechanism for the controlled and coordinated release of motor cortex inhibition.
PMID: 33356901 [PubMed - as supplied by publisher]