Neural correlates of perceptual consciousness from within: A narrative review of human intracranial research.

Attention‐deficit/hyperactivity disorder (ADHD), the most common pediatric behavioral diagnosis, elevates risk for subsequent substance abuse, incarceration, and impulse control disorders in adulthood. Its cardinal symptoms, impulsivity, hyperactivity, and inattention, are pervasive in many complex disorders, such as epilepsies, and monogenetic disorders, such as neurofibromatosis. Developing more effective treatments has been hampered by the reductionist diagnostic process, which classifies ADHD as categorically present or absent solely based on subjective behavioral ratings of symptoms exceeding a population threshold. This approach fails to account for heterogeneity from biological processes that might respond differentially to medical or behavioral treatments. Approaches that incorporate objective, biological measures, might complement symptom‐based diagnoses and allow for identification of biologically based subtypes. To identify useful biological measures, we and others have sought to identify brain‐based, quantitative biomarkers that reflect domains of impaired function. As children with ADHD manifest impaired motor control,, we and others have focused on physiology of motor cortex (M1), using transcranial magnetic stimulation (TMS) to demonstrate that children with ADHD show reduced resting M1 short‐interval cortical inhibition (SICI)., , Moreover, SICI correlates with ADHD symptom severity,, M1 GABA levels, and responses to both stimulant and nonstimulant medications. However, there is overlap between SICI in children with ADHD and typically developing (TD) control children. This has supported finding additional TMS‐based biomarkers of ADHD that might complement SICI, reflect other domains of impaired function,, and ultimately address biological heterogeneity in ways that can help advance clinical care. Subsequent research in ADHD has thus sought to quantify M1 physiology during performance of diagnosis‐relevant actions. Based in primate studies showing that motor cortex pyramidal cells begin to discharge in anticipation of movement onset,, investigators have used TMS to study human M1 physiology prior to self‐paced and cued movements as well as during inhibition of actions. Our rationale for choosing TMS during motor tasks requiring response inhibition is based on studies showing that impulsive and hyperactive children manifest impairments in this form of cognitive control., ,