The goal of this observational and interventional study is to understand how therapeutic deep brain stimulation (DBS) affects attention, perception and cognition in participants with Parkinson's disease (PD) and essential tremor (ET). The main questions it aims to answer are: * Does impaired control of attention and eye movement in PD alter how social cues are perceived and interpreted? * Does therapeutic DBS improve or worsen attentional and perceptual deficits for social cues in PD and ET? * Can DBS be optimized to restore normal attentional control in PD while remaining an effective therapy for other aspects of the disorder. * What do parts of the brain targeted by DBS contribute to the control of attention? Using an eye tracking camera, investigators will study how participants with PD and ET look at and perceive facial expressions of emotion before and after starting DBS therapy, in comparison to a group of healthy participants without ET, PD or DBS. Participants with PD and ET will see and rate morphed facial expressions on a computer screen in three conditions: * Before starting DBS therapy (over approximately 1 hour). * In the operating room, during the standard procedure to implant DBS electrodes, while the participant is awake (for no more than 15 minutes). * After starting DBS therapy, with brief experimental changes of DBS stimulation level and frequency (over approximately 1 hour).