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Chapter 14: Olfaction: Perceiving Odors (Odors, Emotion, and Memory…
Chapter 14: Olfaction: Perceiving Odors
Odorants
What Is an Odor?
olfaction based on the responses from a very large variety of receptors- about 350 different receptor types, in contract to the 4 receptor types that enable vision and the dozen or so types that underlie body senses
thousands of tiny nerve fibers connecting neurons in her nose to structures in brain severed, leading total loss of sense of smell (anosmia)
almost entirely takes away ability to taste foods
taste and smell both based on the transduction of molecules in the air
transduction occurs when molecules activate receptors in the nose, resulting in neural activity that evokes the subjective experience of an odor
humans use smell to detect hazardous environmental conditions and to identify nutritious food
possibly also use smell to recognize others and/or select mates
discovery of gene family that governs the development/functioning of olfactory receptors provided a basis for the use of new techniques in molecular biology and cellular imaging to make unprecedented advances in uncovering the neural code for smell
many substances emit molecules that mix with the air at normal environmental temperatures
if we draw these molecules into our nose when we inhale, bringing them into contact with our olfactory receptors, perceive an odor, depending on whether two conditions are met
molecules must be odorants
odorants:
molecules that olfactory receptors “recognize” and respond to by producing neural signals that the brain represents as perceptions of different odors
molecules must be present at a great enough concentration to evoke a response
molecule:
two or more atoms bound together by electromagnetic forces
animals can smell the odorant molecules that their evolutionary ancestors needed to smell in order to survive/reproduce, such as those given off by food and potential mates
in olfaction, only some molecules are detectable, and capacities are largely determined by the conditions under which different organisms evolved
for humans, most odorants made up by combination of just five kinds of atoms- carbon, hydrogen, oxygen, nitrogen, sulfur, but can combine to produce different molecules
odorant molecules with similar molecular structures can smell very different, and odorants with very different structures can smell almost the same
any given environments typically contains mixture of many different odorant molecules; even single source can emit hundreds of different molecules
able to organize array of different odorants typically present in environment into groups of odorants as belonging to different sources
Detection and Identification of Odors
Detection Thresholds and Difference Thresholds
can’t ask which molecule smells stronger without identifying concentration
more of a spilled substance able to enter the air than a substance in a container, even if container open
detection threshold:
the concentration of an odorant necessary for a person to detect it
describes the strength of an odorant
determined by asking participants to smell two containers- one that contains odorless substance and one that contains odorant at different concentrations- and asking them to identify which one has odorant
generally, as concentration increases, proportion of correct responses also increases
concentration that leads to a predetermined performance level (e.g., 75% correct) is defined as participant’s detection threshold for that substance
different odorants can have very different detection thresholds
detection threshold doesn’t tell us by how much the concentration must change in order to make a just noticeable difference
studies found difference thresholds as low as 5% for some odorants
fact that difference threshold is typically a fixed proportion of the odorant concentration is another example of Weber’s law
at higher concentrations, larger change is required to make a detectable difference
Identifying and Discriminating Odors
not uncommon to be able to identify what the source of the smell is
when people are presented with an odor out of context, without any information that would help determine even what kind of odor it is, their ability to identify even familiar odors is quite poor
however, if give them options to choose from, performance nearly perfect
giving choices provides specific contextual information, but even when clues aren’t as specific as naming the source as one of the options, context tends to generate expectations that usually help identify odors
when in the kitchen, likely to be much better at identifying food-related smells than park-related smells
ability to identify odors and discriminate between similar ones depends not just on context but also on training and experience
professional wine tasters also better than nonprofessionals at identifying and discriminating the odors of substances other than wine, suggesting that their olfactory expertise is not domain specific
not able to better detect whether smell is present or not
The Role of Odors in Sensing Flavor
the accuracy with which people can identify tastes is significantly reduced in people who have impaired olfactory sensitivity
people who acquire anosmia often experience their loss of smell primarily as a loss of the ability to experience the flavor of food
96% of patients who report “taste” deficiencies actually have only an olfactory deficit
Olfactory Impairments: Age and Other Factors
olfactory (and all other senses) performance declines with age
small decrements in performance (on average) on UPSIT begin appearing around 60, followed by a dramatic decrease after age 70
normal score defined at 35
overall, women preform better than men
anosmia:
loss of the ability to perceive odors
defined by 18 or below on UPSIT
impairment doesn’t reach this level until 90s for men
olfactory performances decline with number of years people have smoked, but begins to recover once quit
congenital anosmia often associated with deformed/absent olfactory bulbs, the first brain areas to receive olfactory signals from the nose
many diseases/clinical syndromes are assisted with impairments in the ability to identify smells
could also be caused by injury to the anatomical structures that support olfaction
olfactory system remarkably self-healing
study on mice/rats shows that when receptor neurons in the nose are damages, they degenerate over a period of several days, but stem cells in the nose soon become new receptor neurons that reconnect the nose to the brain
rare genetic mutation that leads to inability to experience pain also leads to anosmia
a crucial factor in detecting an odor is the concentration of odorant molecules, typically measured as the number of odorant molecules present in 1 million molecules of air, or parts per million (ppm)
concentrations depend on factors such as source, distance of source, whether confined in small space
in general, perceived intensity of odor increases as concentration of the odorant molecules increases
in some cases, change in concentration can also affect quality of an odor
Adaptation to Odors
olfaction especially sensitive to change, for same reason as other senses- must rapidly detect the presence of something new, orient to that new thing, and decide what to do about it
once smell has been continuously present for a while, or repeatedly present over a short period of time, has most likely already been identifies, evaluated, and responded to if necessary
organism no longer needs to perceive the smell in order to assess current situation
by adapting to smell so that it’s no longer perceived, the organism is better prepared to detect new smells, because the background of odors is relatively clear
can also result in reduced sensitivity to similar odors
cross-adaption:
in olfaction, reduced sensitivity to odorants that are chemically or perceptually similar to odorants to which the person has been continuously or repeatedly exposed
for rats, investigation time (longer indicating new odor) is greatest for test odorants that are most dissimilar to the adapted odorant
investigation time increases with the differences in carbon chain length suggests physical property of molecules at least partly responsible for perceptual similarity
study with humans demonstrated cross-adaption between some perpetually similar odorants but not between others, despite similarity in molecular structure
many situations in which two odorants that are perceptually very dissimilar can nevertheless produce cross-adaptation
no simple relationship between the molecular structure of an odorant and the perceived odor and perceptually dissimilar odors can evoke responses suggesting that the odors are somehow similar
Anatomical and Neural Basis of Odor Perception
The Olfactory System: From Nose to Brain
Olfactory Transduction and the Large Variety of Olfactory Receptors
each ORN has numerous hairlike cilia, which project into the mucus layer
surface of each cilium studded with olfactory receptors
olfactory receptors:
G-protein coupled receptors in the cilia of ORNs
odorant molecules contact these receptors when they dissolve into and flow through the mucus
olfactory receptors are members of the large family of G-protein coupled receptors
G-protein coupled receptors (GPCRs):
a large family of proteins that function as receptors; they provide a mechanism for molecules outside a cell to influence the inner workings of the cell
when odorant molecule binds to an olfactory GPCR, ion channels in the ORN’s cell membrane open, positively charged calcium and sodium ions enter the cell, and the cell membrane slightly depolarizes
if odorant molecules bind to many receptors on an ORN’s cilia at the same time, cell membrane depolarizes enough to generate an action potential in the ORN’s axon
axons of ORNs form the olfactory nerve
olfactory nerve:
the axons of ORNs, carrying neural signals from ORNs to the olfactory bulb via tiny holes in the cribriform plate
travel to olfactory bulb through a grid of tiny holes in the cribriform plate
cribriform plate:
the part of the skull immediately above the nasal cavity; the axons of ORNs pass through a grid of tiny holes in the plate
within olfactory bulb, ORN axons enter small, more or less spherical structures called glomeruli where they make synapses with the dendrites of two types of relay neurons (mortal cells and tufted cells), the axons of which form the olfactory tract, carrying signals with olfactory information to higher areas of the brain
glomeruli:
small, more or less spherical structures in the olfactory bulb; within the glomeruli, the axons of ORNs make synapses with the dendrites of mitral cells and tufted cells
mitral cells:
relay neurons within the glomeruli in the olfactory bulb; the axons of mortal cells and tufted cells form the olfactory tract
tufted cells:
relay neurons within the glomeruli in the olfactory bulb; the axons of tufted cells and mitral cells form the olfactory tract
olfactory tract:
the axons of mortal cells and tufted cells, carrying neural signals from the olfactory bulb to higher areas of the brain
each of the 350 different types of olfactory receptors can be activated by only a restricted set of odorant molecules
each ORN has only one type of receptor on its cilia
Adaptation by Olfactory Receptor Neurons
an ORN’s response to brief pulses of odorant molecules is measured by using voltage clamping technique in which the electrical current is applied to the ORN to hold the membrane potential at fixed value
when an ORN is stimulated by two pulses of the same odorant 1 second apart, the response to the second pulse is much less than the response to the first pulse
the ORN has adapted to the odorant and barely registers the stimulation, meaning that the rate of transduction declines dramatically
ORNs recover relatively quickly from this adaption- when two pulses separated by 5 seconds, the response to the second pulse is almost as great as the response to the first
left and right nostrils are entrances to the left and right nasal cavities, which are separated by a wall of cartilage called the nasal septum
turbinates:
bony convolutions of tissue protruding into the nasal cavities, functioning to disperse air evenly throughout the nasal cavities
at the back, both nasal cavities job with the pharynx (upper part of the throat)
odorant molecules in the outside air enter the nasal cavities via the nostrils (called the orthonasal pathway)
odorant molecules released from food or other substances in the oral cavity are carried into the nasal cavities via the pharynx (called the retronasal pathway)
transaction into neural signals carried out by ORNs embedded in the olfactory epithelium
olfactory receptor neurons (ORNs):
neurons that transduce odorant molecules into neural signals
olfactory epithelium:
a patch of tissue in the upper reaches of each nasal cavity; the epithelium contains ORNs and is covered by a layer of olfactory mucus
also contains supporting cells, basal cells, Bowman’s glands in addition to ORNs
supporting cells provide structural matrix for ORNs
since each ORN lasts only a few weeks before dying, replaced by basal cells, which become ORNs
Bowman’s glands continually secrete mucus which covers the olfactory epithelium and flows toward the back of the nasal cavity and into the pharynx and then is swallowed
effectively washes out odorant molecules from the epithelium, which otherwise could create a problem by continuing to stimulate odor perceptions after the sources of the molecules are no longer present in the environment
mucus layer also provides a barrier against irritants and against harmful microorganisms that might otherwise penetrate into the central nervous system
Neural Code for Odor
about 1,000 genes are devoted to the genetic code for approximately 350 different receptor types
estimated 4% of the entire human genome
frequently estimate humans can discriminate between 10,000-100,000 or more different odorants, but don’t really know
raises question of how large array of odorants encoded by the 350 different types of ORNs
olfaction uses a population code, one based on the responses of the 350 different types of ORNs
increased number of receptor types means that each type can be relatively narrowly tuned to respond to only a few different odorant molecules
find two complementary findings from study with exposing two different odorant molecules to each of four different chemical families to olfactory receptor neurons
any given odorant molecule evoked a response from some ORNs but not from others
any given ORN might respond strongly to some odorant molecules, weakly to others, and not at all to still others
results indicate that odorant molecules of a specific type activate a specific subset of 350 different types of ORNs
odorant molecules of a different type activate a different subset
the pattern of ORN responses (the particular subset of ORN types activated and the relative strength of their responses) determines how that odorant smells
patterns of ORN responses can be quite different for odorant molecules that are structurally similar
helps explain why odorants with similar molecular structures can smell very different from one another
each glomerulus within the mouse olfactory bulb receives the axons of just one type of ORN
the population code for each particular odorant, based on a unique pattern of responses across all the different ORN types, is maintained within the bulb
each odorant produces a characteristic pattern of responses within a subset of glomeruli in the olfactory bulb, this pattern is sent to higher areas of the brain via the tufted and mitral cell axons, which together form the olfactory tract
some odorants smell very different at low concentrations than at high concentrations
change in odor quality can be seen in the pattern of ORN responses in the olfactory bill
the distribution of glomeruli in the olfactory bulb isn’t random, distribution exhibits a coarse chemotopy- the axons of the ORNs activated by odorants with similar molecular structures tend to travel to glomeruli in the same part of the olfactory bulb and glomeruli in different parts of the bulb tend to receive the axons of ORNs activated by odorants with different molecular structure
Representing Odors in the Brian
Separate Cortical Representations of Odor Identity and Pleasantness
the piriform cortex consists of two subdivisions that have different functions in olfaction
anterior piriform cortex (APC):
the anterior (front) portion of the piriform cortex; it produces representation of features of the chemical structure of odorant molecules
represents things such as length of carbon chain, which affects things such as detection thresholds, cross-adaptation, perceptual similarity of odorants
tend to be narrowly tunes, which means that odorant features are represented in fine enough detail to support odorant identification
posterior piriform cortex (PPC):
the posterior (rear) portion of the piriform cortex; it produces representations of the quality of an odor as a whole, regardless of whether the odor is simple or complex
if odor is simple, results from presence of just one type of odorant molecule; if complex, reflects presence of many different types of odorant molecules
represents odors as ‘olfactory objects’ that can be named and that have associated representations in long-term memory
evidence for distinction between functions of APC and PPC in humans provided by fMRI study in which patterns of activity in four different olfactory brain regions were recorded while sniffed three odorants from each of three odor categories (minty, woody, citrus)
odorants within each category had very different molecular structures despite their perceptual similarity
results showed that patterns of PPC activity were very similar for odorants within a category and different for odorants in different categories, but that activity in the there areas exhibited no such difference corresponding to differences in odor category
provides evidence that PPC represents odor quality and not merely the chemical structures of odorants
in study in which odorants varied from mildly to very unpleasant, in both the amygdala and the OFC, the magnitude of activation correlated with participants’ subjective ratings of the aversiveness of the odor
results from two studies together suggest that amygdala and the OFC are involve in representing the emotional dimension of odor perception and not the identity of odors, whereas the PPC plays a role in representing the perceptual identity of odors
there is coarse chemotopic organization of the olfactory bulb- odorants with similar molecular structures activate adjacent glomeruli
experiments revealed that each odorant evoked a highly consistent pattern of activity in the piriform cortex neurons but that there was no consistent clustering in the location of this activity; no apparent chemotopic organization
Cortical Adaptation to Odors
adaptation that can last for several minutes appears to be mediated by the responses of neurons in the piriform cortex
essential for detecting the presence of a new odor in an otherwise unchanging olfactory background
found that there is strong activation in the piriform cortex in response to the introduction of a new odor, but the activation declines over 40-50 seconds, presumably as a result of adaptation by neurons in that cortical region
axons of mitral cells and tufted cells carry signals via the olfactory tract from the olfactory bulb to number of brain regions, including the piriform cortex, the amygdala, and the entorhinal cortex
piriform cortex:
the brain region considered to be the primary olfactory cortex, because it’s the only region that both receives signals directly from the olfactory bulb and is known to be dedicated solely to olfaction
amygdala and entorhinal cortex are both involved in other functions in addition to olfaction
unpleasant and pleasant smells are more effective at activating this emotional center of the brain than are sights and sounds
amygdala sends signals to the hypothalamus, which is involves in a wide range of functions through the release of hormones and through neural activity, including the regulation of thirst, hunger, and sexual behavior
entorhinal cortex is the gateway to the hippocampus, where long-term memories are stored and retrieved
the piriform cortex, amygdala, and entorhinal cortex send signals to the orbitofrontal cortex, which plays a role in evaluating the incoming stimuli as positive or negative
olfactory system is the only sensory system in which the pathways from the sensory receptors to the cortex don’t go through a nucleus within the thalamus
thought to reflect fact that the olfactory system evolved first among the senses and the thalamus didn’t emerge as part of the sensory pathways until after the olfactory pathway was established
begins in the nose, which funnels stimuli to the neurons responsible for transduction
stimuli are odorant molecules and neurons that transduce them are located near the most environmentally exposed part of the brain
Odors, Emotion, and Memory
judgements about odor quality (about identity or familiarity of odors) can be easily influenced by such factors as visual or verbal cues
people very good at consistently assessing an odor as pleasant or unpleasant
makes sense from evolutionary perspective since important to be able to distinguish between substances that are positive and negative
suggests that most important perceptual dimension of olfaction is the emotional one of pleasantness versus unpleasantness
reinforced by the fact that two important centers of emotional processing in the brain (amygdala and OFC) are on the olfactory pathways
some odors experienced as pleasant/unpleasant depending on the state of smeller
other odors seem always to be experienced in the same way
odors given off of substances such as bodily waste, decomposing animals, and spoiled food are interesting case
some evidence that newborn infants have innate responses to some odorants that elicit reactions of disgust in adults
other research indicates that disgust reactions to many substances are actually learned, not innate
facial expressions of adults and children 2-15 recorded while sniffed fecal and urinous odorants, adult participants provided disgust rating for each odorant
more than 65% of the adult participants spontaneously produced facial expressions of disgust, but only a small percentage of children under the age of 5 did so
found that the children with the most obvious disgust reactions were those of parents with the most obvious disgust reactions
other studies have shown that disgust reactions aren’t inherited, so suggests that similar disgust reactions of children and their parents were learned
for many species, ability to rapidly learn the importance of certain odors is essential for survival
have to learn to recognize scents of mother and of specific predators or avoid ingesting harmful substances
humans have ability to rapidly acquire long-lasting memories of odor
study demonstrated that when asked to smell/name 20 quite different substances and then after 10 minutes, 1 day, or 7 days, performance in identifying 20 scents among 40 was only slightly worse after 7 days than after 10 minutes
indicates that memories of specific odors are formed quickly and decline slowly
results often showed that substances more accurately named when first smelled were more accurately identified as old when smelled again
suggests that odors might have been, to some degree, remembered as names rather than as smells
another study compared memory for odors that varied in familiarity to check possibility of remembering names rather than smells
after 6months, familiar, easily named odors were remembered a little better than unfamiliar odors, but even unfamiliar odors were remembered well above chance
suggests that naming probably does play some role in memory for odors but that ‘pure’ odor memory is still quite good
our memory for odors, as well as our assessment of the quality of odors, also affected by olfactory context (whether odors are smelled alone or in combination with other odors)
when smelled odors alone, in combination, and then alone again, the original odors tended to acquire the characteristics of the odors they had been combined with, and that the pairs of combined odors could even be mistaken for one another
not only does smelling odors in combination influence later perception of individual odors, but also combination of odors undergoes a kind of perceptual grouping, where the distinct odors are bound together into a single olfactory
when new odor arrives at the nose against an unchanging odor background, it evokes a distinct pattern of responses in the piriform cortex that is thought to reflect the representation of an odor ‘object’ for potential identification and evaluation
property of odor memory examined ins study in which people 65-80 smelled 20 different substances and provided brief written description for each smell that evoked a specific autobiographical memory and recorded when memory was from
other groups saw words/pictures rather than smelling odors
for all three groups, the percentage of memories is higher for the most recent decade than for earlier decades (recency effect)
for the odors group, nearly half of all memories came from the first decade of life, whereas for the words and pictures groups, the highest percentage of memories came from the second decade
participants in odors group also more strongly indicated feeling as if they had been ‘brought back in time’ than did participants in pictures/words group
memories evoked by smells also more emotionally charged
possible that odor-evoked memories are acquired so quickly, are so long lasting, and so emotionally vivid because of brain anatomy
centers of emotion (amygdala and OFC) on olfactory pathways, and so is hippocampus
olfactory information travels to these brain structures more directly than does information from any other sensory modality
anatomical organization is thought to be consequence of fact that olfactory systems evolved earlier than did most other sensory systems, which tend to be centered in the more elaborately developed areas of the cerebral cortex
enable emotionally charged experiences need to be quickly engraved in memory so that when situations that gave rise to the memories recur, the organism can take appropriate action without delay, such as fleeing a predator or courting a potential mate
Effects of Odor on Social and Reproductive Behavior
Pheromones, Sweat, and Tears
pheromone:
a chemical substance emitted by individual organisms that evokes behavioral or hormonal responses in other individuals of the same species
unlike hormones, pheromones operate not on the individual secreting them, but on others; function as a form of chemical communication among individuals of the same species
debates over whether pheromones play an important role in the social and reproductive functions of mammals as well as insects
some argue that complex social behaviors of mammals make it impossible to define what would/wouldn’t qualify as pheromone
others cite studies, such as one where male pigs release chemical that initiates receptive mating behavior in female pig, which happens even when chemical is released when male pig is not around
in many species, chemicals that would seem to qualify as pheromones are sensed via a vomeronasal olfactory system
vomeronasal olfactory system:
in many species, an olfactory system that senses pheromones; it is distinct from the main olfactory system used to smell most substances
contains receptor neurons, support cells, and basal cells
axons of vomeronasal receptor neurons synapse with relay cells in structure called accessory olfactory bulb
from there, signals travel to the amygdala and the hypothalamus, but not to brain areas thought to be involved in identifying odors
such system may not be necessary for sensing pheromones since rabbits don’t have vomeronasal system, but still release/sense pheromones
when placed underarm secretions from women on upper lips of other women, secretions from donors in the late follicular phase of their menstrual cycle had the effect of shortening the menstrual cycle of the recipients, whereas secretions from donors in the ovulation phase had the effect of prolonging recipients’ cycle
question of whether humans emit substances that can have positive effects on the sexual thoughts, moods, or behaviors of other individuals
in one study, women who sniffed samples from androstadienone, found in male sweat, reported bot elevated mood and increased sexual arousal
obtained sweat from men watching erotic videos and men watching neutral videos
change in activity in the right OFC was greater while women were smelling the sexual sweat
after sniffing either tears or saline, men asked to rate sexual attractiveness of photos of women
most men rated women as less sexually appealing after sniffing tears than after sniffing saline
activity in region of hypothalamus known to be related to sexual arousal much lower after sniffing tears than after sniffing saline
smelled teeshirts that had been worn under different conditions by four women as they slept
measured testosterone levels before/after smelling and asked to rate pleasantness of odor
teeshirts that had been worn during the nights closest to ovulation produced higher levels of testosterone after smelling them and rated odor as more pleasant
studies provide evidence that substances emitted by individuals of one sex and sensed via the olfactory system by individuals of the other sex can indeed evoke changes in sex-related mood, levels of arousal, approach behaviors, and hormone levels
Human Leukocyte Antigen Detection
question of whether olfactory detection of certain genes can influence human mating behavior less controversial than existence of human pheromones
offspring will have better chance of survival if have effective immune system
human leukocyte antigens (HLAs):
the genes responsible for regulating the immune system; women can detect HLA differences in odors from men, possibly influencing their selection of a mate
children of parents with dissimilar HLAs tend to have more protective immune systems
women preferred smell of teeshirts worn by men whose HLAs were dissimilar to their own
many animals, including humans, emit odors whose function is to be sensed by individuals in their own species and that have effects on the behavior of those individuals