Chemical Senses: Cognition and Chemosensory Processing

Introduction to Chemosensory Functioning

Smell and taste are important components of the sensory system, playing a critical role in food and nutrition selection, the hedonic experience of eating, and the support of healthy metabolism for quality of life. The olfactory and gustatory systems display considerable variety in their mechanisms of transduction, and over the past decade there has been substantial progress in understanding the core mechanisms of smell and taste. Understanding the functions of normal chemosensory organs has helped clarify the molecular actions underlying disorders of smell and taste. More than two million Americans are affected by chemosensory disorders, and this number continues to grow alongside the broader population.

Disorders of smell are more common than taste disorders, due in part to anatomical differences in the olfactory system and the fact that a decline in olfactory function is a recognized part of normal aging. Common complaints involving olfactory and gustatory function arise from medications, respiratory infections, sinus and paranasal disease, and damage to the peripheral nerves that supply smell and taste. Most chemosensory complaints have identifiable causes. Although the diagnosis of smell and taste disorders has improved considerably over the past twenty years, treatment options remain limited to reversible and discernible causes. Further research is needed to understand the mechanisms of chemosensory function, establish reliable diagnostic procedures, and raise public awareness (Spielman, 1998).

The study of flavor has been approached in a manner parallel to the ecological view of perception. Gibson (1966) argued that the primary purpose of perception is to identify objects in the environment, particularly those that are biologically significant. From this perspective, the psychological origin of sensory data is less important than its utility in object identification; successful perception is that in which sensory data is decoded as the qualities of an object. Accordingly, flavor can be understood as a functionally distinct sense, constructed from the psychologically integrated inputs of separate sensory systems — specifically gustation and olfaction (Prescott, 2012).

Chemical Interactions and Cross-Modal Integration

Cross-modal integration of the sensory systems is most often inferred from multimodality responses — typically an enhanced response to information received from one sensory system when another is also engaged (Calvert et al., 1999). For example, speech comprehension in a noisy environment improves when the listener can also observe the speaker's lip movements. Similarly, information from one modality can increase the neural response to stimuli in another, augmenting both behavior and performance. There is also evidence that when odors and tastes are combined into a single flavor, they interact in ways that alter the perception of each (Prescott, 2012).

The most widely recognized expression of this interaction is the attribution of taste-like qualities to odors. When asked to describe the smell of vanilla or caramel, people commonly report that it "smells sweet." Similarly, vinegar is described as smelling sour (Stevenson & Boakes, 2004). In a detailed analysis of odor descriptions, 65% of participants selected "sweetness" as an appropriate characteristic of vanilla, while 33% used "sour" to describe hexanoic acid. These attributions resemble synesthesia, in which stimulation of one sensory modality produces a consistent and automatic response in another (Stevenson et al., 1998; Martino & Marks, 2001). Unlike rare forms of synesthesia in other modalities, the attribution of taste qualities to smells appears to be largely universal (Prescott, 2012).

Conclusion

The studies discussed here reveal the diversity of approaches to studying chemosensory processes. Across these varied perspectives, several recurring themes emerge that unite these studies into a coherent picture. One such theme is the critical role of receptor cells: although chemosensory receptor cells are broadly tuned in many cases, some show a high degree of chemical specificity. This specificity represents a mechanism for extracting detailed information about the chemical features of tastes and smells. Receptor cells also serve as an important site of signal gating, controlling input to the brain and playing a central role in chemical perception (Katz et al., 2008). In this sense, many of the fundamental features of chemical processing are established early in the system.

A second major theme is that higher-order cognitive processes influence the early stages of chemosensory processing. Neural correlates of value, expectation, task difficulty, attention, and alertness have been observed at the initial stages of the system, reflecting the influence of these factors on perceptual judgments and sampling behavior. One of the key challenges for future research will be to understand how neural activity unfolds across different time scales as taste and smell perception develops dynamically, and how these early processes might be manipulated to alter chemosensory behavior and perception (Katz et al., 2008).

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