Introduction
depression, TRD, Treatment-resistant depression
LITTLE
Treatment-resistant depression (TRD) is commonly defined as two or more failed antidepressant treatments of adequate dose and duration. This presents a considerable clinical challenge across the lifespan. It is well documented that depression across the life span is associated with poor maintenance of homeostasis in executive and affective control networks [The N-methyl-D-aspartate receptor (NMDAR) antagonist ketamine has shown considerable clinical efficacy for TRD over the last decade [Little is known about mechanisms of action for ketamine for adults experiencing TRD late in life (LL-TRD). In a recent study [One promising biomarker is signal complexity, which has previously been used to estimate regional and global effects of anesthetic-dose ketamine on consciousness [