"The representation of unpleasantness in the brain emerged earlier than pleasantness and perceived quality," said Project Associate Professor Masako Okamoto, also from the Graduate School of Agricultural and Life Sciences. When unpleasant odors (such as rotten and rancid smells) were administered, participants' brains could differentiate them from neutral or pleasant odors as early as 300 milliseconds after onset. However, representation of pleasant odors (such as floral and fruity smells) in the brain didn't occur until 500 milliseconds onwards, around the same time as when the quality of the odor was also represented.
— Researchers at the University of Tokyo used EEG to look at when and where odors are processed in the brain. Read more in Seeing how odor is processed in the brain at ScienceDaily.