Mere odor exposure induces olfactory perceptual expertise
Subjects were presented with four odorants that systematically differed in odor quality (mint vs. rose) and functional group (ketone vs. alcohol), both before and after a 3.5-minute period of odor exposure (habituation) to one of the four odors (Li et al., Neuron 2006). We found that continuous exposure to this "target" odorant resulted in enhanced perceptual differentiation for odorants related in odor quality or functional group, an effect that lasted up to 24 hours. Thus, with exposure to a mint-smelling alcohol (i.e., menthol), subjects effectively became floral "experts" and simultaneously became experts for the underlying molecular group. Such learning did not generalize to odorants outside of the experienced dimensions (that is, floral experts did not become mint experts, and alcohol experts did not become ketone experts), highlighting a psychological specificity that is a common characteristic of perceptual learning.
This behavioral improvement was paralleled by learning-induced fMRI response increases in posterior PCx and orbitofrontal cortex (OFC). Notably, the magnitude of OFC activation predicted subsequent improvement in behavioral differentiation. Our findings suggest that neural representations of odor quality are not fixed or pre-determined, but can be rapidly updated through mere perceptual experience. This process of odor feature differentiation, via sensory exposure, may underlie much of the way that humans naturally learn to identify odors in the environment, with progressive and ever more refined differentiation, to the point where we are able to recognize thousands, if not tens of thousands, of different smells.