Environmental stimuli generate complex emotional responses in the human brain. These stimuli include visual, auditory and olfactory information, which is primarily characterized by two dimensions: intensity and valence. It has been unclear if the brain has separate circuits for processing intensity and valence. In the January 20 online Nature Neuroscience, A.K. Anderson and colleagues at the University of California at Berkeley, USA, show that for the olfactory stimuli there are distinct brain regions that analyze the degree and quality of smell (Nature Neuroscience, DOI:10.1038/nn1001, January 20, 2003).

Anderson et al. used event-related functional magnetic resonance imaging (fMRI) and observed that in normal volunteers activation in the amygdala area was associated with intensity, and not valence, of odors. In contrast, activities in regions of orbitofrontal cortex were associated with valence independent of intensity.

"Thus, unlike the evolutionarily conserved functions of the amygdala, it seems that the malleability of human hedonic experience is characteristic of the more flexible, integrative and evolved functions of the prefrontal cortices," conclude the authors.