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Chapter 15: Gustation: Perceiving Tastes and Flavors (What Is Taste? What…
Chapter 15: Gustation: Perceiving Tastes and Flavors
Individual Differences in Taste and Flavor Perception
people with genetically determined differences in taste receptors experience taste differently
most common genetic difference affects ability to taste certain bitter substances
gene that codes for specific bitter-taste receptor (T2R38) occurs in two versions, known as PAV and AVI
about 75% people have PAV, which lets them detect the presence of bitter compounds PTC and PROP at low concentrations
tasters:
people with a form of a gene that allows them to detect certain bitter tastants; approximately 75% of people are tasters
remaining have AVI version and require much higher concentration to detect these substances
nontasters:
people with a form of a gene that allows them to detect certain bitter tastants only at very high concentrations; approximately 25% of people are nontasters
some tasters experience these and other bitter substances much more intensely than other tasters
supertasters:
tasters who have about twice as many fungiform papillae as others and are especially sensitive to certain bitter tastants; approximately one-third of tasters are supertasters
find certain foods/beverages so intensely bitter that they are practically inedible/undrinkable
estimated 25% of people worldwide are supertasters, women more likely than men along with people from Asia, Africa, South America
factors such as the availability of certain foods, experiences during/after eating, cultural practices also lead to individual differences in taste preferences
study brain activity of chocolate cravers and nonchocolate cravers while held chocolate solutions in mouths
activity in primary taste cortex same in both groups but chocolate cravers showed more activation in OFC when even looking at picture
even greater differences in OFC activity when saw picture and had chocolate in mouth
suggests that reward value of chocolate is greater for cravers than for non cravers
another dramatic difference is preference for capsaicinoids (compounds that make chili peppers spicy hot
may be that prefer items to which they are repeatedly exposed
social factors might influence taste preferences
good feelings experienced during meals has led to preference for those foods
as concentration of capsaicin in chili peppers increase, so does perceived spiciness
detect lowest concentration at which taster can detect spiciness
individual differences in sensitivity to capsaicin and the inherent subjectivity of psychophysical measurement have led to widespread adoption of an alternative method for quantifying pungency of chili peppers
What Is Taste? What Is Flavor?
Tastants and the Basic Tastes
tastants:
molecules that taste receptors “recognize” and respond to by producing neural signals that the brain represents as perceptions of different tastes
dissolve in saliva and activate taste receptor cells within taste buds, which then produce neural signals that the brain represents as perceptions of different tastes
different tastants have different thresholds
basic tastes:
the five well-established taste categories—sweet, salty, umami, sour, and bitter
qualities sweet, salty, umami, and mild sour typically associated with tastants that fill particular nutrition needs
strong sour and bitter typically serve to warn us of toxic/inedible substances
sweet taste evoked by tastant molecules from sugar family and some amino acids/proteins
glucose is essential nutrient
salts are molecules containing a positively charged ion and a negatively charged ion that together have no net charge
typically evoked by salts containing the sodium ion
table salt (Na+ and Cl-) contains essential biological nutrients
umami:
a basic taste evoked by tastants such as MSG and certain amino acids found it meats; it generally signals the presence of protein in food
sour taste evoked by acids, can be pleasant/desirable when relatively mild
very acidic substances have strong, aversively sour taste and are typically avoided
bitter taste evoked by many different types of molecules
most often associated with toxic substances
The Perception of Flavor
five basic taste qualities aren’t only attributes of tastants that determine overall perception of taste
quality of tastant helps identify substance we’re tasking, but tastants also differ in hedonics (the degree to which the taste is pleasant/unpleasant), intensity, onset and aftertaste, and localization (where in the mouth it is sensed)
all combine for perception of flavor
flavor:
the total sensory experience evoked by ingesting something; it includes the perception of the basic tastes, the perception of other attributes of tastants such as pleasantness and intensity, and other sensory properties, the most important of which is smell
strongly depends on taste, but also strongly depends on food's aroma, which comes both from odorants released by the food before it is placed in the mouth, which enter the nasal cavity through the nostrils, and from the odorants released while the food is in the mouth and being chewed/swallowed, which enter the nasal cavity via the retronasal pathway
several senses other than olfaction contribute to perception of flavor
trigeminal sense:
the sense that signals the presence of irritants in the mouth, such as menthol and chili pepper; it contributes to much of the “mouth feel” of foods
based on inputs via the trigeminal nerve, which transmits signals from receptors in nose and mouth
vision and audition- food that looks appetizing often seems especially flavorful and the audible crunch produced by biting down on some foods make it taste crisper and fresher
study showed good at identifying flavor of drink when color was compatible, but very poor at identifying the flavor when color didn’t match
could reflect influence of flavor expectations that participants formed when initially saw drinks, but similar results when informed that colors unrelated to flavors
second major chemical sense
gustation plays important role in helping us assess quality of material we allow to enter our body
main way many animal select health foods is by perceiving their flavors
things that taste good are usually edible and healthful, whereas things that taste bad are often inedible and potentially toxic
misleading because implies that taste and flavor are the same thing
flavor perception is more complex and multidimensional than taste perception
flavor = taste + olfaction (and more)
Regulating Food Intake
Sensory-Specific Satiety
satiety:
a reduction in appetite for food
sensory-specific satiety:
a reduction in appetite specifically for foods that have been recently consumed
also experience sensory-specific satiety without having to be completely full
could have beneficial effect of ensuring that diet contains variety of foods
one factor accounting for this type of satiety might be memory
memory of having recently eaten/being in the process of eating particular food might make people less desirous of that food
might explain why people eat more if offered variety of food than if offered unlimited amount of single food
may remember having eaten one food and actively seek out a different food
however, study with patients with memory deficits suggests explicit memory may have little to do with choice of food
rated four foods then given choice of which one to have a meal of and re-rated foods 15 minutes later, when had completely forgotten about original ratings
ratings decreased only for food that they had eaten eat of
Regulating Food Intake in the Absence of Taste
studies with mice suggest we would still desire specific foods of we were without any taste
mice who could taste sweet strongly preferred sugar water, whereas mice without ability to taste sweet had no preference
question of whether mice who couldn’t taste sweet could learn to recognize and prefer nutritious food that couldn’t identify by taste
conditioned over 6 days, where only allowed to drink from one (sugar water or regular water) each alternating day
after 6 days, both spouts made available, but with both delivering plain water
mice who couldn’t taste sweet now had strong preference for spout that had previously delivered sugar water
even without taste information, had learned to recognize/prefer nutritious sugar water based on metabolic or gastrointestinal signals received after ingesting it
when replaces with artificial sweetener, normal mice initially preferred sweet water, but didn’t after 6 days because had no nutritional value
knockout mice showed no preference before or after
gustation and state of hunger play important roles in regulating choices of what, when, how much to eat
Anatomical and Neural Basis of Taste and Flavor Perception
Taste Buds and Taste Receptor Cells
taste buds:
structures that contain taste receptor cells, within papillae in the mouth
humans have on average 3,000-12,000
papillae:
tiny structures on surfaces in the mouth, mainly on the tongue; three different types of papillae contain taste buds
fungiform papillae:
tiny mushroom-shaped structures located along the edges and top of the front two-thirds of the tongue; each fungiform papilla contains 3-5 taste buds on its upper surface
foliate papillae:
ridgelike folds of tissue located on the sides of the tongue near the back; a few hundred taste buds are tucked into each fold
circumvallate papillae:
mushroom-shaped structures (much larger than fungiform papillae) situated in a row at the back of the tongue; each circumvallate papillae contains 200-700 taste bunds around its sides
fourth type—filiform papillae—doesn't contain taste buds, covers much of the tongue’s surface, thought to be helpful in manipulating food on the tongue
taste receptor cells (TRCs):
elongated neurons, peached within taste buds, that transduce tastants into neural signals
have cilia on outer ends and don’t have axons
each taste bud contains 40-100 TRCs
each TRC lives for about a week then replaced by developed basal cells
cilia of TRCs project into taste pore (opening onto surface of the tongue) at the top of the taste bud, where cilia come into contact with tastant molecules dissolved in saliva
two types of TRCs
receptor cells:
a type of taste receptor cell containing receptors that initiate transduction of sweet, umami, and bitter tastants
typically, each receptor cell contains only single category of receptors on its cilia
do not have synapses with cranial nerve fibers
presynaptic cells:
a type of taste receptor cells in which the receptors take the form of ion channels where transduction of salty and sour tastants is inhibited
each presynaptic contains channels for both types of tastants
inner ends of presynaptic cells release neurotransmitters into synapses with cranial nerve fibers that send taste signals to the brain
receptors in the membranes of receptor cell cilia are GPCRs that have been characterized biochemically and genetically and linked via behavioral and genetic studies in mice to transduction of sweet, umami, and bitter tastants
receptor cells that transduce sweet/umami tastants contain different pairs of GPCRs known as T1R1, T1R2, and T1R3
sweet tastant molecules transduced by receptor cells that contain T1R2-T1R3
umami tastant molecules transduced by receptor cells that contain T1R1-T1R3
bitter tastants are transduced by receptor cells containing a category of GPCRs knows as T2Rs, thought to include 25-30 different types
diversity probably reflects diversity of toxic substances that must be avoided
presynaptic cells are connected via a synapse to a cranial nerve fiber
when salty/sour tastant ions are transported through corresponding ion in membrane of presynaptic cell’s cilia, chain of reactions is initiated within the cell that results in the release of neurotransmitters into the synapse
neurotransmitters are taken up by the nerve fiber, which responds by sending action potentials to the brain
sour tastants are acids, which contain hydrogen ions, and ion channel that transports hydrogen ions into cell body of presynaptic cells and thought to be responsible for initiating the transduction for sour tastants has been identified
salty tastants can be divided into two categories- everyday table salty and other salts such as potassium chloride and magnesium chloride
thought to be two distinct transduction mechanisms for salty tastants- one for transduction of sodium ions in relatively low concentrations (epithelial sodium channel ENaC) and another for transduction of a wide variety of salty ions (sodium and others) in aversively high concentrations
question of how taste receptor cells communicate to the brain the presence of sweet, umami, and better tastants, given that they don’t have synapses with the cranial nerve fibers
cell-to-cell signaling:
in taste perception, signals from receptor cells to presynaptic cells, causing the presynaptic cells to release neurotransmitters in a way that carries information about sweet, umami, and bitter tastants
receptor cells release adenosine triphosphate (ATP) into extracellular flood within the taste buds, and ATP molecules are taken up by presynaptic cells at their synapse with the cranial nerve fibers
differences in release of ATP and the consequent release of serotonin release provide information about type of tastant transduced by receptor cell
ATP also thought to stimulate free endings of cranial nerve fibers within taste buds, providing another means by which taste information from receptor cells is sent to the brain
From Taste Buds to the Brain
most nerve fibers from the tongue to the brain respond to more than just single taste category, although any given fiber tends to respond more strongly to one taste category than the others
two models of taste representation; difficulty to find evidence that definitely rules out either one
labeled-line model:
a model of taste perception proposing that each cranial nerve fiber carries signals with information about just one of the five taste qualities, and that the cortical neurons on the receiving end of these signals also respond only to information about a single type of tastant
there is single labeled line from specialized TRCs in taste buds to correspondingly specialized cranial nerve fibers to correspondingly specialized cortical neurons
across-pattern fiber model:
a model of taste perception proposing that cranial nerve fibers can carry signals from multiple taste receptor types, and that the cortical neurons receiving these signals are broadly tuned to respond to signals carrying information about multiple types of tastants
two models agree on how tastants are transduced
experiments don’t rule out either model, both models would predict what experiments have shown- that mice lacking a particular type of receptor are unable to perceive tastants transduced by that receptor
studies of single cranial nerve fibers found that single fibers respond most strongly to just one of the five taste categories, but other studies have found that while some fibers respond to only one type of tastant, others respond to two or more
would seem to support across-fiber pattern model, since this model doesn’t exclude possibility that some fibers respond only to one type of tastant, whereas labeled-line model has no place for multiple-tastant responses in single fibers
studies with rats and monkeys have indicated that neurons in the primary taste cortex tend to have fairly broad tuning profile, which against supports across-fiber pattern model
conclude that cranial nerve fibers are seen as carrying information about differences in taste quality based on the relative activity of a few different types of receptors
Representing Taste and Flavor in the Brain
neural signals originating in taste receptor cells in taste buds are transmitted to the brain via three cranial nerves
the facial nerve (cranial nerve VII) innervates the front 2/3 of the tongue
the glossopharyngeal nerve (cranial nerve IX) innervates the back 1/3 of the tongue
the vagus nerve (cranial nerve X) innervates the epiglottis and the upper esophagus
travel first to the nucleus of the solitary tract (aka gustatory nucleus) in the medulla, then pass through the ventral posterior medial nucleus of the thalamus on their way to the cortex
first cortical areas to receive taste signals via thalamus are anterior insular cortex and frontal operculum
primary taste cortex:
the first cortical areas to receive taste signals, consisting of the anterior insular cortex and the frontal operculum
then go to orbitofrontal cortex (where reward value of food thought to be represented), the amygdala (emotions) and hypothalamus (hunger)
neurons in primary taste cortex respond to signals containing information about basic tastes, temperature, viscosity, fattiness, grittiness of foods in mouth and represent taste qualities regardless of particular interest in the food
study recorded individual neurons in insular cortex in monkeys in response to glucose solution (sugar water) over one-hour period
be end of session, satiated and resisting feeding
as session progressed, monkey became less hungry, but firing rate of the neuron remained high throughout the session (well above baseline rate), showing that primary taste cortex represents the taste quality of foods regardless of the state of hunger or satiation at time they’re being consumed
when also recorded neurons from orbitofrontal cortex (OFC), found that OFC neurons responded strongly only when monkey was hungry, when the firing rate declining to baseline over course of session as monkey became satiated
implies that OFC represents reward value of food, which changes with animal’s state of huger, not taste quality of food
compared neural responses of tasting sugar or smelling strawberry to those of tasteless/odorless substances
both sweet taste and smell activated same region of anterior insular cortex, part of primary taste cortex that also receives olfactory signals
region of OFC activated by combination of taste and smell, but not either individually and strength of activation strongly correlated with judgements about pleasantness of smell/taste
suggests that information from several sensory modalities converges in the OFC to produce perception of flavor
Adaptation and Cross-Adaptation
taste sensitivity reduced after continuous exposure to particular stimulant
however, usually tastants don’t remain in the mouth long enough to produce adaptation while you’re eating
exception might be when suck on hard candy
in studies, find that tastants from one basic taste don’t reduce sensitivity to other basic tastes, allowing person to remain sensitive to tastes of new, potentially toxic substances
cross-adaptation between similar tastants can occur
adaptation to one type of salt reduces the ability to taste other salts
different type of cross-adaptation occurs when adaptation to one basic taste enhances the salience of other basic tastes
Cognitive Influences in the OFC, and the Flavor of Expensive Wine
cognitive factors such as expectations can influence the response to foods
knowledgeable wine students asked to describe flavor of same wine, but one from expensive bottle and one from basic bottle
52 of 57 participants reported that they preferred high-end wine
question of whether perceive wines as having different flavors or whether shape descriptions of wines to conform to expectations
study where recorded brain activity while drinking same wine- one said to be $10 and one said to be $90
area in OFC more active in response to $90 wine; increased more quickly and remained elevated for longer
suggests expectations about food and drink can influence activity in parts of the brain involved in signaling flavor and expectations can truly change flavor that you perceive
also slowed brain activity in primary taste cortex did not differ between two wines
suggests primary taste cortex provides information about the basic sensory attributes of the tastants in your mouth, regardless of interest in them or value to you
taste perception starts with transduction of tastants into neural signals by specialized cells within taste buds on the tongue and other surfaces of the mouth
depending on location of taste bud, signals transmitted by one of three cranial nerves to a nucleus in the brain stem and eventually various parts of the cortex, where combined with information in signals from olfactory and somatosensory systems
mouth also has free nerve endings associated with trigeminal sense and somatosensory receptors that let us detect texture/temperature of food