title: Tryptophan Depletion Promotes Habitual over Goal-Directed Control of Appetitive Responding in Humans.
abstract: BACKGROUND
  Optimal behavioral performance results from a balance between goal-directed and habitual systems of behavioral control, which are modulated by ascending monoaminergic projections. While the role of the dopaminergic system in behavioral control has been recently addressed, the extent to which changes in global serotonin neurotransmission could influence these 2 systems is still poorly understood.
  
  
  METHODS
  We employed the dietary acute tryptophan depletion procedure to reduce serotonin neurotransmission in 18 healthy volunteers and 18 matched controls. We used a 3-stage instrumental learning paradigm that includes an initial instrumental learning stage, a subsequent outcome-devaluation test, and a slip-of-action stage, which directly tests the balance between hypothetical goal-directed and habitual systems. We also employed a separate response inhibition control test to assess the behavioral specificity of the results.
  
  
  RESULTS
  Acute tryptophan depletion produced a shift of behavioral performance towards habitual responding as indexed by performance on the slip-of-action test. Moreover, greater habitual responding in the acute tryptophan depletion group was predicted by a steeper decline in plasma tryptophan levels. In contrast, acute tryptophan depletion left intact the ability to use discriminative stimuli to guide instrumental choice as indexed by the instrumental learning stage and did not impair inhibitory response control.
  
  
  CONCLUSIONS
  The major implication of this study is that serotonin modulates the balance between goal-directed and stimulus-response habitual systems of behavioral control. Our findings thus imply that diminished serotonin neurotransmission shifts behavioral control towards habitual responding.
authors: Behavioral and Clinical Neuroscience Institute (Drs Worbe, Fernandez-Egea, and Robbins), Department of Psychiatry (Drs Savulich and Fernandez-Egea), and Department of Psychology (Dr Robbins), University of Cambridge, Cambridge, United Kingdom; Department of Clinical Psychology, University of Amsterdam, Amsterdam, The Netherlands (Dr de Wit). yworbe@gmail.com.
authors: Yulia
authors: Y
authors: Worbe
authors: Behavioral and Clinical Neuroscience Institute (Drs Worbe, Fernandez-Egea, and Robbins), Department of Psychiatry (Drs Savulich and Fernandez-Egea), and Department of Psychology (Dr Robbins), University of Cambridge, Cambridge, United Kingdom; Department of Clinical Psychology, University of Amsterdam, Amsterdam, The Netherlands (Dr de Wit).
authors: George
authors: G
authors: Savulich
authors: Behavioral and Clinical Neuroscience Institute (Drs Worbe, Fernandez-Egea, and Robbins), Department of Psychiatry (Drs Savulich and Fernandez-Egea), and Department of Psychology (Dr Robbins), University of Cambridge, Cambridge, United Kingdom; Department of Clinical Psychology, University of Amsterdam, Amsterdam, The Netherlands (Dr de Wit).
authors: Sanne
authors: S
authors: de Wit
authors: Behavioral and Clinical Neuroscience Institute (Drs Worbe, Fernandez-Egea, and Robbins), Department of Psychiatry (Drs Savulich and Fernandez-Egea), and Department of Psychology (Dr Robbins), University of Cambridge, Cambridge, United Kingdom; Department of Clinical Psychology, University of Amsterdam, Amsterdam, The Netherlands (Dr de Wit).
authors: Emilio
authors: E
authors: Fernandez-Egea
authors: Behavioral and Clinical Neuroscience Institute (Drs Worbe, Fernandez-Egea, and Robbins), Department of Psychiatry (Drs Savulich and Fernandez-Egea), and Department of Psychology (Dr Robbins), University of Cambridge, Cambridge, United Kingdom; Department of Clinical Psychology, University of Amsterdam, Amsterdam, The Netherlands (Dr de Wit).
authors: Trevor W
authors: TW
authors: Robbins
mesh_terms: D000328:Adult; D001070:Appetitive Behavior / Q000502:physiology*; D003216:Conditioning, Operant / Q000502:physiology*; D056344:Executive Function / Q000502:physiology*; D005260:Female; D006040:Goals; D006184:Habits; D006801:Humans; D007266:Inhibition, Psychological; D008297:Male; D011581:Psychological Tests; D011597:Psychomotor Performance / Q000502:physiology; D012701:Serotonin / Q000378:metabolism; D009435:Synaptic Transmission / Q000502:physiology; D014364:Tryptophan / Q000097:blood / Q000172:deficiency*
chemical_list: D012701:Serotonin; D014364:Tryptophan
keywords: 5-HT; goal-directed behavior; inhibitory control; serotonin; stimulus-response habit learning; tryptophan depletion