This study will collect EEG data at 3 time points from up to 40 adults who are enrolled in a double clinical trial investigating a device that delivers noninvasive, self-administered external Combined Occipital and Trigeminal Neurostimulation (eCOT-NS) treatment for Major Depressive Disorder (Relivion®DP). Baseline (Pre-treatment) Frontal EEG Alpha Oscillations (Power) will be compared post-stimulation power.

EEG Biomarkers may be useful for understanding abnormal brain oscillations associated with Major Depressive Disorder (MDD) and other psychiatric disorders as well as the brain changes that occur when symptoms remit. This study will invite adults with MDD who enroll and are randomized in the Neurolief Sponsored "MOOD" study to undergo collection of EEG data at pre-treatment baseline, again after completing the 8-week blinded treatment phase, and, if eligible/appropriate, again after completing an 8-week open-label treatment phase. EEG will be recorded at rest and during a computer task. This EEG "add-on" study is observational in nature, as it will not manipulate the treatment assignment in the MOOD clinical trial or otherwise impact concurrently ongoing treatments for any condition. Researchers collecting and analyzing EEG data will be blinded to treatment assignment when participants are in the double-blinded treatment phase of the MOOD clinical trial. EEG data will be processed and analyzed to evaluate changes in frontal alpha oscillations associated with 8 weeks of active Relivion®DP stimulation.

Inclusion Criteria:

• Meets all criteria for participation in the Neurolief-sponsored MOOD clinical trial (NCT04279522) and has been randomized to a treatment group for the double-blind phase.
• Is willing and able to participate in up to 3 additional research visits for EEG data collections.

Exclusion Criteria:

• none

Adults with a primary diagnosis of MDD; ages 18-70; with current episode of primary depression lasting up to 3 years; at least moderate symptom severity reflected by score on HAMD17 scale at baseline >20; history of antidepressant medication treatment resistance or intolerance and curently on stable dose of current antidepressant medication.