This trial aims to assess the efficacy and tolerability of Magnetic Seizure Therapy (MST) and two different forms of electroconvulsive therapy (ECT) in sustaining response during and after a course of continuation treatment.

The study will involve a parallel-group clinical trial with three treatment arms conducted at the Centre for Addiction and Mental Health (CAMH) in Toronto, ON, Ontario Shores Centre for Mental Health Sciences in Whitby, ON and University of British Columbia (UBC) Hospital in Vancouver, BC. It will include participants who are classified as responders or remitters in the CREST-MST (NCT03191058) trial, the CORRECT-BD (NCT03641300) trial and individuals responding to bitemporal ECT after participating in either of the above trials, who qualify for a continuation course of convulsive therapy to prevent relapse of depression. Individuals blinded to their acute course of treatment will continue to receive the convulsive therapy to which they responded in in a blinded fashion. Continuation treatments will be scheduled according to a modified version of the Symptom-Titrated Algorithm-based Longitudinal ECT (STABLE) algorithm. STABLE includes an initial four week period of ECT delivered on a fixed schedule (once or twice per week), and then transitions to a symptom-driven schedule whereby ECT is delivered between zero and two times per week based on the patient's weekly Hamilton Rating Scale for Depression (HRSD-24) outcomes during weeks five to 24.

Further, this trial will also include non-responders to MST or right unilateral ultrabrief pulse ECT (RUL-UB ECT) from CREST-MST or CORRECT-BD, who are switched to bitemporal ECT based on their clinical indication. They will receive bitemporal ECT on an acute basis, i.e. 2 - 3 times per week. If their symptoms respond or remit with bitemporal ECT, they will also be offered a continuation course of bitemporal ECT as part of this trial and will be followed in the same manner as those receiving MST or RUL-UB ECT.

Participants in the Acute Phase (bitemporal ECT) will have already met the diagnostic criteria and severity eligibility criteria specified in the protocols of CREST-MST and CORRECT-BD.

At the time of recruitment, participants in the Continuation Phase (MST, RUL-UB ECT, Bitemporal ECT) will meet the following eligibility criteria:

Inclusion Criteria:

1. are inpatients or outpatients;
2. are voluntary and competent to consent to treatment and research procedures according to ECT/MST attending psychiatrist;
3. have met diagnostic criteria as assessed by MINI V6.0 in CREST-MST or CORRECT-BD
4. are 18 years of age or older
5. achieve remission defined as HRSD-24 < 10 and a > 60% decrease in scores from baseline on two consecutive ratings OR achieve response on HRSD-24 defined as a 50% reduction in symptoms from baseline;
6. are considered to be appropriate to receive convulsive therapy as assessed by an ECT attending psychiatrist and a consultant anaesthesiologist
7. are agreeable to keeping their current antidepressant treatment constant during the intervention;
8. are likely able to adhere to the intervention schedule;
9. meet the MST safety criteria;
10. If a woman of child-bearing potential: is willing to provide a negative pregnancy test and agrees not to become pregnant during trial participation.

Exclusion Criteria:

1. have a concomitant major unstable medical illness;
2. are pregnant or intend to get pregnant during the study;
3. have probable dementia based on study investigator assessment;
4. have any significant neurological disorder or condition likely to be associated with increased intracranial pressure or a space occupying brain lesion, e.g., cerebral aneurysm;
5. present with a medical condition, a medication, or a laboratory abnormality that could cause a major depressive episode or significant cognitive impairment in the opinion of the investigator (e.g., hypothyroidism with low TSH, rheumatoid arthritis requiring high dose prednisone, or Cushing's disease);
6. have an intracranial implant (e.g., aneurysm clips, shunts, stimulators, cochlear implants, or electrodes) or any other metal object within or near the head, excluding the mouth, that cannot be safely removed;
7. require a benzodiazepine with dose greater than lorazepam 2 mg/day or equivalent or any anticonvulsant due to the potential of these medications to limit the efficacy of both MST and ECT;
8. are unable to communicate in English fluently enough to complete the neuropsychological tests;
9. have a non-correctable clinically significant sensory impairment (i.e., cannot hear or see well enough to complete the neuropsychological tests)

Individuals blinded to their acute course of treatment will continue to receive the convulsive therapy to which they responded in in a blinded fashion.