Abstract

Involuntary attention shifting, i.e., detecting and orienting to unexpected stimulus changes, may be altered at low brain serotonin (5-hydroxytryptamine; 5-HT) levels. This was studied in 13 healthy subjects (21-30 years old; 6 females) by using a dietary challenge, acute tryptophan depletion (ATD), which decreases 5-HT synthesis in the brain. Five hours after ingestion of either ATD or control mixture (randomized, double-blinded, crossover design), brain responses indexing involuntary attention were measured with simultaneous 64-channel electroencephalography (EEG) and 122-channel magnetoencephalography (MEG). During the measurement, the subjects were instructed to discriminate equiprobable 200- and 400-ms tones by pressing one of two buttons rapidly. Occasionally, the frequency of the tones changed (10% increase/decrease), causing involuntary attention shifting. ATD significantly lowered plasma tryptophan concentrations (total tryptophan decreased by 75%, free tryptophan decreased by 35%). As compared to the control condition, ATD reduced the amplitude of the deviant-tone N2 wave, including the overlapping mismatch negativity (MMN) and N2b subcomponents, which are suggested to reflect change detection in the brain. The EEG results were accompanied by a significant increase in the peak latency of the magnetic counterpart of MMN. However, no ATD effects were observed in P3 to task-irrelevant frequency change. Reaction time (RT) to deviants per se was not significantly affected, but RT in trials succeeding the deviant-frequency tones was increased by ATD, which suggested impaired reorienting to the task-relevant activity. In conclusion, the results suggest that decreased level of central 5-HT function after ATD may decrease involuntary attention shifting to task-irrelevant sound changes and thus modulate resource allocation to the task-relevant activity.