Neural and personality correlates of individual differences related to the effects of acute tryptophan depletion on future reward evaluation.
OBJECTIVE
In general, humans tend to discount the value of delayed reward. An increase in the rate of discounting leads to an inability to select a delayed reward over a smaller immediate reward (reward-delay impulsivity). Although deficits in the serotonergic system are implicated in this reward-delay impulsivity, there is individual variation in response to serotonin depletion. The aim of the present study was to investigate whether the effects of serotonin depletion on the ability to evaluate future reward are affected by individual personality traits or brain activation.


METHODS
Personality traits were assessed using the NEO-Five Factor Inventory and Temperament and Character Inventory. The central serotonergic levels of 16 healthy volunteers were manipulated by dietary tryptophan depletion. Subjects performed a delayed reward choice task that required the continuous estimation of reward value during functional magnetic resonance imaging scanning.


RESULTS
Discounting rates were increased in 9 participants, but were unchanged or decreased in 7 participants in response to tryptophan depletion. Participants whose discounting rate was increased by tryptophan depletion had significantly higher neuroticism and lower self-directedness. Furthermore, tryptophan depletion differentially affected the groups in terms of hemodynamic responses to the value of predicted future reward in the right insula.


CONCLUSIONS
These results suggest that individuals who have high neuroticism and low self-directedness as personality traits are particularly vulnerable to the effect of low serotonin on future reward evaluation accompanied by altered brain activation patterns.
Division of Frontier Medical Science, Department of Psychiatry and Neurosciences, Graduate School of Biomedical Sciences, Hiroshima University, Higashi-Hiroshima, Japan.
Yoshihiko
Y
Demoto
Go
G
Okada
Yasumasa
Y
Okamoto
Yoshihiko
Y
Kunisato
Shiori
S
Aoyama
Keiichi
K
Onoda
Ayumi
A
Munakata
Michio
M
Nomura
Saori C
SC
Tanaka
Nicolas
N
Schweighofer
Kenji
K
Doya
Shigeto
S
Yamawaki
D000328:Adult; D000704:Analysis of Variance; D001921:Brain / Q000098:blood supply / Q000502:physiology*; D001931:Brain Mapping; D002755:Choice Behavior / Q000502:physiology; D004644:Emotions / Q000502:physiology; D007839:Functional Laterality; D006801:Humans; D007206:Individuality; D008279:Magnetic Resonance Imaging; D008297:Male; D009483:Neuropsychological Tests; D010100:Oxygen / Q000097:blood; D010551:Personality / Q000502:physiology*; D010555:Personality Inventory; D011930:Reaction Time; D012201:Reward; D018709:Statistics, Nonparametric; D014364:Tryptophan / Q000097:blood / Q000172:deficiency*; D055815:Young Adult
D014364:Tryptophan; D010100:Oxygen