The current study would examine whether increases in endogenous dopaminergic activity via tyrosine and the (presumed) excitation of these by anodal tDCS of the dlPFC could causally be related to cognitive flexibility as measured by task switching and reversal learning. Additionally, the study will test whether the Val158Met-polymorphism in the catechol- O-methyltransferase (COMT) gene could also predict the effect of TYR supplementation, as this gene is involved in DA degradation in the prefrontal cortex.
Name: tdcs (sham/anodal) + drug (placebo/tyrosine)
Description: tDCS= A DC Stimulator Plus (neuroConn, Germany) with one 5 cm x 7 cm rubber electrode (anode) and a 10 cm x 10 cm (cathode; reference electrode), encased in saline soaked sponges will be used. The anode will be positioned over the left dlPFC, centered on F3 in the 10e20 electroencephalography (EEG) system, while the cathode on the contralateral supraorbital ridge (Fp2).Current will be delivered at 1.5mA for 20 min plus 30 s fade in/fade out periods.For sham stimulation, the current will be faded in over 30 s, at 1.5mA and then will be switched off. Drugs= 2.0 g of L-Tyrosine and 2.0 g of the placebo microcrystalline cellulose will be dissolved in 400ml of orange juice as per previously published protocols.Type: Combination ProducttDCS sham + placebo tDCS sham + tyrosine tDCS anodal + placebo tDCS anodal +tyrosine
Description: Measuring change in perseverative errors in the WCST
Measure: Wisconsin Card Sorting Test (WCST) performance Time: Measured twice in each session (4 arms): at time 0 and 80 minutes into testing.Description: Measuring change in reversal errors in the WCST
Measure: Probabilistic Reversal Learning (PRL) performance Time: Measured twice in each session (4 arms): at time 0 and 80 minutes into testing.Description: Measuring change in conflict cost (defined as the difference in reaction time between congruent and incongruent responses)
Measure: Flanker Task performance Time: Measured twice in each session (4 arms): at time 0 and 80 minutes into testing.Allocation: Randomized
Crossover Assignment
There is one SNP
Additionally, the study will test whether the Val158Met-polymorphism in the catechol- O-methyltransferase (COMT) gene could also predict the effect of TYR supplementation, as this gene is involved in DA degradation in the prefrontal cortex. --- Val158Met ---