Previous research has suggested that room air may contain odors that affect central nervous system activity. This research presumes that odors can produce significant changes in brain activity even though they are undetected. In the present study, we examined this hypothesis. Sixteen subjects were exposed to four concentrations of two odorants while EEG data were recorded. The concentrations used in the study were no odor, low (undetected in half the subjects), medium, and high. The results of the EEG period analyses indicated that the distribution of EEG theta activity differed as a function of odor concentration. In addition, more detailed analysis of the two lowest odor-presentation conditions revealed that EEG beta activity differed as a function of odor and concentration for the subjects who were unable to detect the presence of the odors. These subjects also reported being significantly less happy during the administration of the undetected odors. These results support the hypothesis that undetected odors, which may exist in room air, have the capacity to alter brain activity and possibly mood.
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