GSTM1 and depressive symptom measurement: Based on the observations that ketamine enhances the connectivity between frontal and limbic structures and that clinical improvement in depression is associated with better cognitive control of emotions via prefrontal cortical regions (Johnstone et al., 2007; Murrough et al., 2011), we hypothesized that improvement of depression severity would be associated with greater connectivity between the Hb and prefrontal areas.