Tryptophan depletion effects on EEG and MEG responses suggest serotonergic modulation of auditory involuntary attention in humans |
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Authors: | Ahveninen Jyrki Kähkönen Seppo Pennanen Sirpa Liesivuori Jyrki Ilmoniemi Risto J Jääskeläinen Iiro P |
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Affiliation: | a Cognitive Brain Research Unit, Department of Psychology, P.O. Box 13, University of Helsinki, Helsinki, FIN-00014, Finland;h Apperception & Cortical Dynamics, Department of Psychology, P.O. Box 13, University of Helsinki, Helsinki, FIN-00014, Finland;c Department of Neurology, University of Helsinki, Helsinki, Finland;b BioMag Laboratory, Engineering Centre, Helsinki University Central Hospital, Helsinki, Finland;e Occupational Hygiene and Toxicology Section, Kuopio Regional Institute of Occupational Health, Kuopio, Finland;f Department of Pharmacology and Toxicology, University of Kuopio, Kuopio, Finland;g Massachusetts General Hospital-NMR Center, Harvard Medical School, Charlestown, Massachusetts, 02129;d Helsinki Brain Research Center, Helsinki, Finland |
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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. |
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Keywords: | attention auditory cortex, event-related potentials electroencephalography magnetoencephalography mismatch negativity P3 P3a RON serotonin tryptophan depletion |
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