Integrated Neural Representations of Odor Intensity and Affective Valence in Human Amygdala

Rationale and experimental hypotheses. a, Previous experiments using pleasant and unpleasant stimuli (Anderson et al., 2003; Small et al., 2003) have shown a main effect of intensity in chemosensory processing, suggesting that the amygdala is preferentially tuned to intensity. However, because these studies did not use valence-neutral stimuli, it remains unclear how the amygdala would respond to the neutral mid-range of valence space (dashed box), which has important implications for interpretation of amygdala coding. In the current study, odors of pleasant, neutral, and unpleasant valence were each presented at high and low intensity, permitting a test of two competing hypotheses. b, Hypothesis I: main effect of intensity. Here, the amygdala responses are enhanced to high (vs low)-intensity odors, independent of valence, which would fit with previous findings suggesting the amygdala operates as an intensity detector across the entire valence spectrum. c, Hypothesis II: interaction of intensity and valence. Here, amygdala responses are enhanced only to high-intensity versions of pleasant and unpleasant, but not neutral, odor, implying a greater complexity to amygdala coding involving combined sensitivity to both intensity and valence. Thus, although the diagrams in b and c show equivalent response profiles at the extremes of valence, the inclusion of neutral odor allows the two hypotheses to be distinguished.