We are using a technique known as Acute Tryptophan Depletion (ATD) to establish whether serotonin (5-HT) differentially modulates perceptual, cognitive and behavioural responses to affective touch. ATD is a dietary method of reversibly reducing brain 5-HT levels. Tryptophan is an essential amino acid and the precursor of 5-HT. ATD involves the administration of an amino acid drink containing all amino acids excluding tryptophan. This induces protein synthesis which reduces endogenous tryptophan levels. The ratio of plasma tryptophan to other large neutral amino acids is reduced by ATD, thus reducing transportation of tryptophan across the blood brain barrier via the large neutral amino acid transporter. Plasma tryptophan levels are maximally reduced after 5 hours. The effects of ATD are simply reversed by the consumption of a tryptophan containing meal.