4 Olfactory/Gustatory Processing

The chemical senses, taste and smell, are ancient sensory modalities that allow animals to evaluate and interact with their environment and make adaptive decisions to enhance survival. The most primitive embodiment of chemical sensing can be seen in the orientation toward nutrients shown by single-celled prokaryotes (Berg 1975). Even some plants appear to have the capacity to detect air-borne chemical signals that allow them to orient toward food sources (Runyon et al. 2006). Smell and taste are used by higher animals to guide food and mate selection, and to avoid environmental dangers such as fires and noxious chemicals. Perhaps more than any other senses, smell and taste are perceptually intertwined. In humans, and presumably other animals, the “flavor” of food is a perfect fusion of our perception of the taste of the food and its smell. Olfaction allows animals to detect volatile chemicals at a distance, whereas gustation requires direct contact with the relevant chemical. Thus, the sense of smell can be used to orient toward attractive stimuli, such as palatable food or a suitable mate, and to orient away from repulsive stimuli, such as spoiled food or fire. Since taste requires contact with the stimulus, this sense is crucial for sensory evaluation immediately before ingestion. Foods that smell attractive but contain nonvolatile substances that make them inedible can be vetted by the gustatory system before ingestion. Most animals show appetitive responses to sugars and rejection behaviors to bitter or sour compounds, a likely outcome of many generations of selective...