Parkinson's disease (PD) is associated with abnormal beta oscillations (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) in the basal ganglia and motor cortex (M1). Recent reports show that M1 beta-high gamma Hz) phase-amplitude coupling (PAC) is exaggerated in PD and is reduced following acute deep brain stimulation (DBS). Here we analyze invasive M1 electrocorticography recordings in PD patients on and off DBS, and in isolated cervical dystonia patients, and show that M1 beta oscillations are nonsinusoidal, having sharp and asymmetric features. These sharp oscillatory beta features underlie the previously reported PAC, providing an alternative to the standard interpretation of PAC as an interaction between two distinct frequency components. Specifically, the ratio between peak and trough sharpness is nearly perfectly correlated with beta-high gamma PAC (r = 0.96) and predicts PD-related motor deficit. Using a simulation of the local field potential, we demonstrate that sharp oscillatory waves can arise from synchronous synaptic activity. We propose that exaggerated beta-high gamma PAC may actually reflect such synchronous synaptic activity, manifesting as sharp beta oscillations that are "smoothed out" with DBS. These results support the "desynchronization" hypothesis of DBS wherein DBS counteracts pathological synchronization throughout the basal ganglia-thalamocortical loop. We argue that PAC can be influenced by more than one mechanism. In this case synaptic synchrony, rather than the often assumed spike-field coherence, may underlie exaggerated PAC. These often overlooked temporal features of the oscillatory waveform carry critical physiological information about neural processes and dynamics that may lead to better understanding of underlying neuropathology. IntroductionParkinson's disease (PD) is characterized by neuronal degeneration in multiple systems, including midbrain dopaminergic neurons. Though beta (13-30 Hz) oscillations are a normal feature of the basal ganglia-thalamocortical loop, PD is associated with excessive neuronal synchronization in the beta band (1, 2). Despite an established relationship between beta band neuronal synchronization and PD, the physiological mechanism causing motor dysfunction has been unclear. Excessive phase-amplitude coupling (PAC) between beta phase and high gamma amplitude (50-200 Hz) may offer an explanation (3)(4)(5). PAC between distant neural populations has been linked to enhanced neural information flow (6-8), long-term potentiation (9), and improved behavioral performance (10). However PAC strength is greater in M1 of PD patients compared to patients with isolated cervical dystonia or epilepsy (3), leading to the hypothesis that beyond its facilitative role, PAC may play a role in neural pathology (3,4,11).Analyses of PAC implicitly presuppose two separate, interacting physiological processes: a low-frequency component associated with an oscillation in the synaptic currents and a separate high-frequency component associated with local...