Effects of acute tryptophan depletion on neural processing of facial expressions of emotion in humans

Acute tryptophan depletion (ATD) temporarily lowers brain serotonin (5-HT) synthesis, and behavioral studies have shown that this alters the processing of facial expressions of emotion. The neural basis for these alterations is not known. Therefore, we employed ATD and event-related functional magnetic resonance imaging (fMRI) to examine neural responses during incidental processing of fearful, happy, sad, and disgusted facial expressions. Fourteen healthy male controls (age, 28 +/- 10) were scanned under both placebo (SHAM) and depletion (ATD) conditions. We predicted that ATD would be associated with changes in neural activity within facial emotion-processing networks. We found that serotonergic modulation did not affect performance on the fMRI tasks, but was associated with widespread effects on neural response to components of face processing networks for fearful, disgusted, and happy but not sad expressions across differing intensities. Hence, the 5-HT system affects brain function (in 'limbic' and 'face processing' regions) during incidental processing of emotional facial expressions; but this varies with emotion type and intensities.