title: Modulation of corticospinal excitability by reward depends on task framing.
abstract: Findings from previous transcranial magnetic stimulation (TMS) experiments suggest that the primary motor cortex (M1) is sensitive to reward conditions in the environment. However, the nature of this influence on M1 activity is poorly understood. The dopamine neuron response to conditioned stimuli encodes reward probability and outcome uncertainty, or the extent to which the outcome of a situation is known. Reward uncertainty and probability are related: uncertainty is maximal when probability is 0.5 and minimal when probability is 0 or 1 (i.e., certain outcome). Previous TMS-reward studies did not examine these factors independently. Here, we used single-pulse TMS to measure corticospinal excitability in 40 individuals while they performed a simple computer task, making guesses to find or avoid a hidden target. The task stimuli implied three levels of reward probability and two levels of uncertainty. We found that reward probability level interacted with the trial search condition. That is, motor evoked potential (MEP) amplitude, a measure of corticospinal neuron excitability, increased with increasing reward probability when participants were instructed to "find" a target, but not when they were instructed to "avoid" a target. There was no effect of uncertainty on MEPs. Response times varied with the number of choices. A subset of participants also received paired-pulse stimulation to evaluate changes in short-intracortical inhibition (SICI). No effects of SICI were observed. Taken together, the results suggest that the reward-contingent modulation of M1 activity reflects reward probability or a related aspect of utility, not outcome uncertainty, and that this effect is sensitive to the conceptual framing of the task.
authors.affiliation: Behavioral Neurology Unit, National Institute of Neurological Disorders and Stroke, National Institutes of Health, 10 CENTER DR, MSC 1440, Bethesda, MD 20892-1440, USA; Center for Neuroscience and Regenerative Medicine, Uniformed Services University of Health Sciences, Henry M. Jackson Foundation, Rockville, MD 20852, USA.
authors.forename: Eric
authors.initials: E
authors.lastname: Mooshagian
authors.affiliation: Behavioral Neurology Unit, National Institute of Neurological Disorders and Stroke, National Institutes of Health, 10 CENTER DR, MSC 1440, Bethesda, MD 20892-1440, USA; Center for Neuroscience and Regenerative Medicine, Uniformed Services University of Health Sciences, Henry M. Jackson Foundation, Rockville, MD 20852, USA.
authors.forename: Aysha
authors.initials: A
authors.lastname: Keisler
authors.affiliation: Behavioral Neurology Unit, National Institute of Neurological Disorders and Stroke, National Institutes of Health, 10 CENTER DR, MSC 1440, Bethesda, MD 20892-1440, USA; Center for Neuroscience and Regenerative Medicine, Uniformed Services University of Health Sciences, Henry M. Jackson Foundation, Rockville, MD 20852, USA.
authors.forename: Trelawny
authors.initials: T
authors.lastname: Zimmermann
authors.affiliation: The Wake Kendall Group, PLLC, 5247 Wisconsin Avenue NW, Suite 4, Washington, DC 20015, USA.
authors.forename: Janell M
authors.initials: JM
authors.lastname: Schweickert
authors.affiliation: Behavioral Neurology Unit, National Institute of Neurological Disorders and Stroke, National Institutes of Health, 10 CENTER DR, MSC 1440, Bethesda, MD 20892-1440, USA. Electronic address: wassermanne@ninds.nih.gov.
authors.forename: Eric M
authors.initials: EM
authors.lastname: Wassermann
mesh_terms: D000328:Adult; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D011336:Probability; D011712:Pyramidal Tracts / Q000502:physiology*; D012201:Reward; D050781:Transcranial Magnetic Stimulation; D035501:Uncertainty; D055815:Young Adult
keywords: Corticospinal excitability; Expectation; Reward probability; Transcranial magnetic stimulation; Uncertainty