Final answer:
Deep brain stimulation (DBS) is reserved for the most resistant cases of severe depression when usual approaches of therapy and medication don't work due to its invasive nature and potential risks.
Step-by-step explanation:
Deep brain stimulation (DBS) is reserved for the most resistant cases of severe depression when usual approaches of therapy and medication don't work due to its invasive nature and potential risks. DBS involves surgically implanting small electrodes in specific brain regions to stimulate neural activity using electrical impulses. It is typically considered as a last resort treatment option when all other methods have failed to provide relief for the patient.