Effects of elevated plasma tryptophan on brain activation associated with 
the Stroop task.

Psychopharmacology (Berl). 2006 Dec 19; [Epub ahead of print]

Morgan RM, Parry AM, Arida RM, Matthews PM, Davies B, Castell LM.

Cellular Nutrition Research Group, Nuffield Department of 
Anaesthetics, University of Oxford, Radcliffe Infirmary, Oxford, OX2 6HE, UK,
lindy.castell@nda.ox.ac.uk.

NLM Citation: PMID: 17180619


RATIONALE: Central fatigue, such as that found in chronic fatigue syndrome, 
is a state in which cognition and action require increasing effort and 
performance is impaired without evidence for reduced peripheral motor 
responsiveness. Previous studies identified functional changes in 
subcortical regions in patients who experience central fatigue but did not 
address neural correlates of the subjective experience of fatigue.

OBJECTIVES: This study investigated responses to acute tryptophan feeding 
(after administration of 30 mg/kg body mass) using functional magnetic 
resonance imaging to investigate neural correlates of central fatigue 
during a cognitively demanding exercise, the counting Stroop task.

MATERIALS AND METHODS: In a double-blind, cross-over study, eight subjects 
ingested L: -tryptophan (Trp) or placebo (Plac) on two separate test days. 
Neutral (N) and interference (I) Stroop tasks were carried out.

RESULTS: Plasma-free tryptophan (p[FT]) increased tenfold after L: -Trp 
administration (P < 0.01). Although reaction times were longer after Trp 
(mean+/-SD, Plac-Neut 669 +/- 163 ms, I 715 +/- 174 ms, P < 0.01; Trp-Neut 
712 +/- 193 ms, I 761 +/- 198 ms, P < 0.05), the Stroop effect was not 
significantly different between Plac and Trp. L: -Trp administration was 
associated with relatively decreased activation in regions, including the 
left postcentral, angular, inferior frontal, and the lateral orbital gyri 
and the inferior frontal sulcus relative to Plac. Relatively increased 
activation was found after Trp in the left precuneus and in the posterior 
cingulate gyrus.

CONCLUSIONS: Thus, Trp administration before the Stroop task caused 
distributed functional changes in primary sensory and in multimodal 
neocortex, including changes in a brain region, the activity of which has 
been shown previously to vary with conscious awareness (precuneus). 
Previous reports suggest that primary mechanisms of central fatigue may be 
predominantly subcortical. The present results demonstrate that neocortical 
activity changes are also found. Whether this activity contributes to the 
primary mechanisms underlying central fatigue or not, the neocortical 
activity changes may provide an index of the conscious experience.


[Note: The Stroop task is a psychological test of our mental vitality and 
flexibility. The task takes advantage of our ability to read words more 
quickly and automatically than we can name colors. If a word is printed or 
displayed in a color different from the color it actually names; for 
example, if the word "green" is written in blue ink we will say the word 
"green" more readily than we can name the color in which it is displayed, 
which in this case is "blue."  The cognitive mechanism involved in this 
task is called inhibition, you have to inhibit or stop one response and say 
or do something else.]