LHB and major depressive disorder: Morris et al. (1999) observed that in human patients where depressive relapses were triggered by rapid depletion of blood tryptophan (the precursor to serotonin), positron emission tomography signals displayed correlated increases in the habenula and dorsal raphe as patients' rating of their depressed mood worsened. Recently, Yang et al. (2008) have reported that in two rat models of depression (where serotonin levels in the dorsal raphe are depleted), LHb lesions improved the behavioural responses of the rats and this was accompanied by an increase in the dorsal raphe serotonin levels.