Linking peripheral taste processes to behavior
Introduction
Our understanding of peripheral gustatory mechanisms continues to advance at a rapid pace. Ultimately, these neurobiological processes must be linked to behavioral outcomes. At times, such efforts have produced seemingly paradoxical results; for example, knocking out a taste receptor caused severe impairments in one behavioral task but not in another. To explain these apparent disparities, it is important to realize that there are at least three categories of taste processing [1]. Stimulus identification is the detection or discrimination of sensory signals arising from taste cell activation. Ingestive motivation involves processes that promote or discourage ingestion. Digestive preparation refers to feed-forward physiological reflexes that protect oral tissues, aid digestion, and facilitate homeostasis. It must also be recognized that behavioral responses to taste stimuli can be influenced by nongustatory factors, including olfactory, somatosensory, and visceral signals. We propose that integrating these perspectives into studies of taste function will help establish more logical links between neural processes and taste-related behavior.
Section snippets
Stimulus identification
Stimulus identification refers to the ability of animals to discriminate between the gustatory signals generated by different taste stimuli. Such processes allow animals to learn about foods by associating particular tastes with other stimuli and/or outcomes, ultimately facilitating survival. In humans, stimulus identification can be assessed through verbal qualitative descriptors such as ‘sweet,’ ‘sour,’ ‘salty,’ ‘bitter’ and ‘umami.’ In nonverbal animals, more objective approaches, such as
Ingestive motivation
The motivational function of taste has been referred to as affect, hedonics, palatability, and reward. All of these processes share the same fundamental property of facilitating or inhibiting ingestion. It is important to recognize that two taste compounds can be equally preferred or avoided but have distinct taste qualities. For instance, even though rats avoid high concentrations of quinine and NaCl, they can nevertheless discriminate the tastes of these stimuli.
Digestive preparation
A third function of gustatory input is the activation of physiological reflexes that produce effects like delaying gastric emptying, protecting the oral cavity, facilitating digestion, and maintaining homeostasis. These are commonly referred to as cephalic-phase reflexes because they are triggered by the stimulation of head receptors. For instance, a recent study documented that bitter taste alone can delay gastric emptying in human subjects [54•]. This could have adaptive value in that it
Conclusion
The three categories of taste function discussed here must have dissociable neural substrates at some level in the gustatory neuraxis (Figure 1) [61]. Although these substrates have yet to be clearly delineated, there are hints in the literature. For example, in rats, stimulus identification relies on input from the gustatory branches of the facial nerve, whereas taste signals carried by the glossopharyngeal nerve appear unnecessary to support this function [see [62, 63]]. Neurons in the
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
We would like to thank Clare Mathes and Yada Treesukosol for providing feedback on the article. ACS would also like to acknowledge research support from the National Institute on Deafness and Other Communication Disorders (R01-DC-004574).
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