This is a prospective, randomized, double-blind, parallel-group controlled trial. The aim of this research project is to compare the clinical benefits achieved in patients with major depressive disorder (MDD) following two types of intervention: iTBS active alone or iTBS active combined with olfactory stimulations.

Major depressive disorder (MDD) is the most widespread psychiatric disorder, affecting 5% of adults according to the World Health Organization. Anhedonia, defined as the loss of the ability to experience pleasure, is one of the key symptoms of depression, possibly due to a dysfunction of the reward system. Intermittent theta-burst stimulation (iTBS) targeting the dorsolateral prefrontal cortex (DLPFC) has been demonstrated as an emerging treatment option for treatment-resistant depression. One explanation is that iTBS could work through modulating the reward system (increasing dopamine release). One way to improve the therapeutic benefits of non-invasive brain stimulation is to combine it with other therapeutic strategies. Interestingly, olfactory training -daily short-term exposure to pleasant odors- improves significantly depressive symptoms in MDD patients. Indeed, the olfactory system and the reward system are closely related through the olfactory tubercle, which is in the ventral striatum and directly connected to the ventral tegmental area.

The general aim of this research project is to test whether a combination of iTBS targeting the left DLPFC with an olfactory training can improve treatment outcome in MDD, compared to iTBS targeting the left DLPFC alone.

The investigators hypothesize that combining iTBS treatment on DLPFC with hedonic olfactory stimulation potentiates the effect of iTBS treatment administered alone on depressive symptoms, especially anhedonia (physical, social or olfactory). Moreover, the investigators also hypothesize that the superiority of the combined approach is underpinned by greater modulation of connectivity activity and strength between brain regions involved in dopaminergic transmission, compared with iTBS alone.

Inclusion Criteria:

• had a primary diagnosis of single-episode or recurrent non-psychotic major depressive disorder according to Diagnostic and Statistical Manual of Mental Disorders, fifth edition (DSM-5) criteria;
• scored 20 or over on the MADRS (Montgomery-Åsberg Depression Rating Scale) (Montgomery and Åsberg, 1979) and scored higher than 2 on the MADRS item 8 anhedonia factor score;
• a stable antidepressant medication for 4 weeks prior to inclusion; be able to speak and read French;
• sign a consent form before intervention.

Exclusion Criteria:

• a pre-existing condition that affects olfaction including congenital anosmia, upper respiratory tract infection, nasal and/or sinus disease, brain injury or nasal surgery; a neurological disease;
• other comorbid psychiatric disorders or substance abuse (except tobacco);
• contraindications to TMS (medical devices implanted or metallic foreign body in the head);
• pregnant or lactating mothers (controlled by urine pregnancy tests);
• measure of protection or guardianship of justice.

Describe the IPD sharing plan, including what IPD are to be shared with other researchers.

Patients will be block randomized (1:1) to receive either iTBS active combined with an olfactory stimulation or TBS active alone. Computer generated random numbers will be used to generate the allocation sequence. The allocation sequence will be handled by an independent person from the Administrative Department of the Hospital and will be unavailable to those who enroll and assign patients. Patients will be blind to their treatment assignment. Moreover, the medical doctors who will administer the clinical scales and the researchers that will analyze the data will be blinded to the allocation.