Frontal cognitive impairment and changes in neuroimaging are seen in individuals depleted of tryptophan, a serotonin precursor. These cognitive changes do not improve following serotonin-specific reuptake inhibitor treatment and at least one study has found that executive dysfunction predicts non-response to fluoxetine. In many patients, remission of mood symptoms in depression requires medications to target non-serotonergic neurotransmitter systems. Brain areas mediating executive functions receive rich noradrenergic inputs, and norepinephrine is known to be intimately involved in many of the executive functions.