Taste and Smell, Psychology of

There are five types of taste receptor cell, sweet, salt, bitter, sour, and umami (protein taste). There are 1000 olfactory receptor genes each specifying a different type of receptor each for a set of odors. Tastes are primary, unlearned, rewards and punishers, and are important in emotion. Pheromones and some other olfactory stimuli are primary reinforcers, but for many odors the reward value is learned by stimulus–reinforcer association learning. The primary taste cortex in the anterior insula provides separate and combined representations of the taste, temperature, and texture (including fat texture) of food in the mouth independently of hunger and thus of reward value and pleasantness. One synapse on, in the orbitofrontal cortex, these sensory inputs are for some neurons combined by learning with olfactory and visual inputs, and these neurons encode food reward value in that they only respond to food when hungry, and in that activations correlate with subjective pleasantness. Cognitive factors, including word-level descriptions, and attention, modulate the representation of the reward value of taste, odor, and flavor in the orbitofrontal cortex and a region to which it projects, the anterior cingulate cortex. Further, there are individual differences in the representation of the reward value of food in the orbitofrontal cortex. Overeating and obesity are related in many cases to an increased reward value of the sensory inputs produced by foods, and their modulation by cognition and attention that override existing satiety signals. Rapid advances have been made recently in understanding the receptors for taste and smell, the neural systems for taste and smell, the separation of sensory from hedonic processing of taste and smell, and how taste and smell and also the texture of food are important in the palatability of food and appetite control. Emphasis is placed on these advances. Taste and Olfactory Receptors Taste receptors. There are receptors on the tongue for sweet, salt, bitter, sour, and the fifth taste, umami as exemplified by monosodium glutamate (Chandrashekar et al., 2006; Chaudhari and Roper, 2010). Umami taste is found in a diversity of foods rich in glutamate like fish, meat, human mothers’ milk, tomatoes and some vegetables, and is enhanced by some ribonucleotides (including inosine and guanosine nucleotides), which are present in, for example, meat and some fish. The mixture of these umami components, which act synergistically at the receptor, underlies the rich taste characteristic of many cuisines (Rolls, 2009). Olfactory receptors. There are approximately 1000 different types of odor receptor each coded for by a different gene (Buck and Bargmann, 2013). This is a significant proportion of the total number of genes in humans, which is in the order of 30 000. This is a simple but genetically expensive way to build specificity into a sensory system. In humans, only approximately one-third of these receptor types are used, with stop codes in the majority of the olfactory receptor genes. This may reflect the greater reliance of primates including humans on stimuli in other sensory modalities, especially vision and hearing. In addition to this main olfactory system using the main olfactory epithelium and its connections to the main olfactory bulb, there is an accessory olfactory system with its peripheral receptors located in the vomeronasal organ (VNO), which projects to the accessory olfactory bulb. This system is involved in the detection of pheromones, present, for example, in urinary volatiles, which are used for mate attraction, territory marking, etc (Wyatt, 2014). The VNOs of humans and Old World monkeys are nonfunctional, although pheromones or pheromone-like substances acting through the main olfactory system may be involved in social and reproductive behavior in these species. Taste, Smell, Reward, and Emotion Emotions can be defined as states elicited by rewards and punishers (Rolls, 2014). A reward is any stimulus or event for which an animal (and that includes humans) will work for (i.e., learn and perform an action instrumental in obtaining the reward), and a punisher is any stimulus or event that an animal to work to escape from or avoid. All tastes are primary, unlearned, reinforcers. For example, animals show that a sweet taste (if hungry) and a salt taste (if salt deprived) are rewards the first time that taste is made available. Animals will similarly reject bitter tastes (which are generally produced by plants for exactly that function). Taste is thus very important in emotion, for it serves as an unlearned reward or punisher. Other primary reinforcers include soft pleasant touch, and pain, with many other examples provided in Emotion and Decision-Making Explained (Rolls, 2014). Emotions, as states elicited by instrumental reinforcers (e.g., a warm pleasant feeling produced by a soft caressing touch), can thus be seen to provide an evolutionary adaptive value account of why genes, for example, taste receptor genes, code for the reward or punisher value of some stimuli. Genes do this 26 International Encyclopedia of the Social & Behavioral Sciences, 2nd edition, Volume 24 http://dx.doi.org/10.1016/B978-0-08-097086-8.55050-8 International Encyclopedia of the Social & Behavioral Sciences, Second Edition, 2015, 26–31