Date of Award


Publication Type

Doctoral Thesis

Degree Name




First Advisor

Abeare, Christopher (Psychology)


sychology, Clinical Neuropsychology.




The purpose of this study was to examine the effects of tryptophan, the amino acid precursor to serotonin, on neuropsychological performance. A dietary method of tryptophan manipulation was employed to temporarily raise, lower, or maintain circulating levels of tryptophan within the body, allowing the resulting cognitive and affective sequelae to be measured. A total of 73 participants (50 females, 23 males) completed this mixed-design, double-blind study. Participants were quasi-randomly assigned to one of three tryptophan conditions (augmented, depleted or balanced) and were provided with breakfast, lunch and a snack. A comprehensive neuropsychological test battery was administered 1.5 hours after completion of lunch. Analyses were conducted on each gender separately. No significant results were found for females. For the males, however, significant between-group differences were found for the Rey-O delayed recall, with those in the Depletion group scoring significantly higher than those in the Balanced or Augmented conditions. Males in the Augmented condition also performed better than those in the Balanced or Depleted condition on the speed component of the Ruff 2 & 7 Selective Attention Test. With regards to affect, males in the Augmented group demonstrated a near-significant difference on the PANAS positive affect scale on the fourth PANAS administration compared to those in the Depleted group. These differences in positive affect levels seem to be primarily driven by the trend in excitement levels between males in the Augmented and Depleted conditions over time. Results of this study support the hypothesis that dietary manipulations aimed at altering tryptophan levels had an effect on some cognitive tests and positive affect, at least with regards to males.