Audition and the chemosenses
Biological Psychology 1st ed.
John Wiley and Sons
The Physics of Audition:
Table 8. 1 Common sounds and associate decibel levels
Figure 8.1. Sine waves differing in frequency but not amplitude. Public domain.
The Human Auditory System
Figure 8.2a,b The Human Auditory System
Figure 8.3 Stapes
The Cochlea and the Organ of Corti
Figure 8.4 the cochlea. Image public domain
Figure 8.5 Cross section of the Organ of Corti.
Figure 8. 6 Outer hair cells..
Activity: video of hair cell responding to sound
Figure 8.7; Organ of Corti cross section showing inner and outer hair cells.
Amplitude and Frequency coding in the cochlea:
Media: Play wave files at 25 and 8000 Hz.
Figure 8.8 Deformation of the Basilar membrane in response to high and low frequency sound.
Media: dynamic visualizations of high and low pitch deformation of basilar membrane
Figure 8.8a,b,c Frequency response as a function of location on basilar membrane
. Activity: test the upper range of your hearing
Activity: test your own hearing
What is it like to have a hearing disability?
Activity: hear simulations of different auditory deficits
Learn about cochlear implants.
Figure 8.10 Pre 1900 ear trumpet.
Figure 8.11. Early transistorized hearing aid.
Figure 8.12 Example of an in the ear hearing aid.
Activity: a cochlear implant patient talks about their experience
Activity: hear simulations of speech and music as experienced by an individual with a cochlear implant
Auditory pathway and Primary Auditory Cortex
Figure 8.13 The Auditory Pathway.
Activity: Demonstration of auditory localization
Figure 8.14 Sound localization:
Chemosenses: Olfaction and Gustation
Evolution of Chemosenses
Olfaction the biological basis of smell
Perceptual characteristics of Olfaction:
Anatomy and physiology of olfaction:
Figure 8. 15 the human olfactory system.
How do olfactory receptors work?
Genetics and olfaction:
Secondary Olfactory system:
Activity: explore the web site of the Monell Center.
Gustation: the biological basis of taste.
Anatomy and Physiology of the tongue:
Figure 8.16 the tongue. .
Figure 8.17 A typical taste bud
Basic dimensions of taste
Blocking taste receptors:
Genetic influences on taste:
Genetics, taste and alcoholism:.
Activity: are you a supertaster?
Bob graduated from College with a degree in psychology and found a job in the marketing department of a large firm. While he enjoyed his career, his true passion was music. He played guitar and bass in a number of bands and enjoyed both the pleasure of making music and also the notoriety his stage presence brought him in his circle of friends.
Bob often set up directly in front of his amplifier and he prided himself on having a rig that was one of the most powerful in his circle of friends. He could always be heard, no matter how loud his band mates or the drummer played. Bob often experienced a ringing in the ears after a show, but gave it no thought. In his late 20’s however, his band mates began to complain that Bob was playing so loud that he was drowning out the other instruments and the vocalist. At first, Bob thought they were just jealous but after a while, he began to realize that something had changed in his hearing. He was always telling his friends to speak up in conversations, and when guests would drop over to watch television or a movie, they complained that he had turned the volume up too loud. It all came to a head the day he was walking downtown with a friend and while crossing a street and deep in a conversation, he walked out in front of a car and was almost run down. His friend pulled him back just in time and said ‘Didn’t you hear the car coming? Bob had not heard a thing. That day Bob made an appointment with an audiologist.
The audiologist had him listen through headphones as very faint tones were presented across a wide range of frequencies from very low to very high. Bob indicated when he was able to detect the tone. At the end, the audiologist told Bob that he was suffering from moderate hear loss that was likely due to continued exposure to very loud sounds. Bob was fitted with a hearing aid that nestled discreetly in his ear. While that helped somewhat, Bob was distressed to find that the hearing aid did not solve his problem to his satisfaction
Overview of Audition
- Audition provides important information about the world; it supports oral speech and also information about objects that are in our vicinity.
- Damage to the auditory system can occur through disease or environmental hazards.
The 30,000 foot view: While vision is in many ways paramount, our sense of hearing provides important information about the world around us. We often hear an approaching object before we see it, and our sense of hearing facilitates our ability to converse and appreciate music. In this section, we will provide an overview of how your auditory system works and then discuss some of the more common auditory difficulties.
Activities in this chapter:
- Test your hearing with an on line audiometry test
- Experience a simulation of auditory damage
- Experience a simulation of hearing with a cochlear implant
- Experience the use of high frequency sound as a crowd deterrent
While vision is our most important sense, audition runs a close second. Taste, smell and touch require that we come into physical contact with some part of the distal stimulus, but hearing and vision inform us about objects that are distant. We use hearing to detect the stealthy approach of a predator creeping towards us through the forest; we use it to locate a brook filled with refreshing water, and we can use it to attract the attention of others. Hearing also supports language and music. Without a doubt, the ear is a mechanism of beauty and considerable complexity. How does our auditory system take physical vibrations in the air and transform them into meaningful sounds?
The Physics of Audition: Sound waves are really pressure waves and so our sense of hearing is really a sophisticated way of detecting and analyzing small changes in atmospheric pressure. What our brains perceive as the sound of a symphony, of waves crashing on a sandy beach or the sounds of someone saying ‘It’s time for dinner” begin their journey into your mind as pressure waves traveling through the air.
It may seem harsh to think of sound in purely physical terms without the emotion, mood or romance that sounds can evoke, yet it is true that all the sounds you have ever heard in your life, complex and varied though they might be, can be described at the most basic level as having only two characteristics: amplitude and frequency. Pressure waves that are larger in amplitude are perceived as louder while waves of lesser amplitude are perceived as being fainter. Perceived loudness is measured on the decibel scale, and Table 8.1 provides some examples to help you understand and evaluate decibel levels.
Whisper in a library
Normal conversation at 3 feet
Jackhammer at 50 feet
Power lawn mower at 3 feet
Jet engine at 100 feet
Permanent and likely total damage to hearing
Table 8. 1 Common sounds and associate decibel levels
Be aware that sustained exposure to sounds over 90 db leads to haring loss, and even short term exposure to sounds over 140db will cause permanent hearing loss.
While the amplitude of the pressure wave is interpreted as loudness, the frequency of the wave is coded as tone or pitch. Waves of greater frequency are perceived as being higher pitched while those of lower frequency are perceived as bass sounds. Figure 8.1 below illustrates five sounds of equal amplitude or loudness that differ in frequency. The lowest pitched sound would be at the top and the pitch or frequency of the sounds increases as you move down the graph.
With an understanding that our sensory world is built upon the foundations of the physical world, we can turn to an overview of how our auditory mechanism transforms those physical vibrations into meaningful sounds.
The basic structures of the human ear are presented in Fig. 8.2 below. There are three basic regions you need to know about to understand audition and the first is the external ear which consists of everything that air touches. The outer ear is made up of the cartilaginous appendage we call the ear (also known as the pinna or the auricle), the external auditory canal and the tympanic membrane. The middle ear consists of 3 bones known collectively as the ossicles and the inner ear is comprised of the cochlea, semicircular canals and the nerve tracts that leave the inner ear. Each has a specific role to play in audition and so we will briefly discuss them in turn.
Figure 8.2: The architecture of the ear.
External Ear: The external or outer ear starts at the Pinna and ends at the tympanic membrane. Sound waves traveling through the air first touch the Pinna. The Pinna is cupped and so reflects sound waves into the middle ear in the same way that scoop collects water. These sound waves travel down the auditory canal and strike the tympanic membrane, causing the membrane to move. The tympanic membrane (also known as the eardrum) vibrates in response to these incoming sound waves.
Middle Ear: The middle ear consists of the ossicles or three bones which are named after their shape. In English they are called the hammer, the anvil and the stirrup but their Latin or medical names are the incus, malleus and stapes. These three bones are quite small, as illustrated by the following comparison in relation to a tenth of a Euro coin. The stapes is about 3.5 mm in length. This scale is shown in Fig. 8.3.
What function do the ossicles provide? Why not have the eardrum connect directly to the auditory organ of sensation? The answer is that these 3 bones mechanically amplify the vibrations from the tympani by taking the energy from the tympani and transmitting that energy down into a smaller surface area. You understand the principle of mechanical amplification if you know why you would prefer to have someone step on your foot while wearing men’s wingtips over the same person stepping on your foot with a woman’s spike heel.
The stapes is stabilized by the smallest muscle in your body, the stapedius muscle, which is about 1mm in length and this muscle prevents the stapes from damaging the cochlea in situations where a sudden loud sound might tear the cochlea. If you have ever experienced brief deafness after exposure to a very loud sound such as thunder or an explosion, you can thank the reflexive response of your stapedius for protecting your hearing. If this muscle is paralyzed as is sometimes the case for patients with Bell’s palsy, the dampening of the stapes is reduced and the individual may find that many sounds are now uncomfortably loud.
Inner Ear: In the inner ear are the cochlea and the semicircular canals which are perched atop the cochlea. As the semicircular canals are not involved in hearing but are responsible for our sense of orientation in space (i.e. are we standing upright, tilted to the side or upside down) we will not discuss them in this context but focus instead upon the cochlea.
The Cochlea and the Organ of Corti:We saw with vision that the point of transduction from an external physical event into a format the nervous system can process was the photoreceptors of the retina. For audition, that point where physical energy out in the world enters our nervous system is the Organ of Corti in the cochlea. The basic structure of the cochlea is presented in Figure 8.4.
The Cochlea is a spiral organ filled with fluid; it is divided into two section by a membrane that goes down the middle and extends almost to the end. There are two windows into the cochlea; the oval and round windows. Movement of the tympanic membrane is transferred to the ossicles; the last of the ossicles is the stapes and it is in physical contact with the oval window. When the stapes oscillates in response to sound, that oscillation creates movement in the form of a wave in the cochlear fluid that begins at the oval window, travels down the cochlea until it reaches the tip where it reverses and continues back until its energy is dissipated against the round window. This is demonstrated in figure 8. 5 below.
This motion of fluid in the cochlea stimulates the Organ of Corti and from this stimulation, we perceive sounds. In order to gain a fuller grasp of the complexity of this process, we need to look at a cross section of the cochlea which depicts the anatomy of the Organ of Corti.
You will not be surprised to find that the Organ of Corti is a very complex structure; for our purposes, we can focus just on the basics of acoustic transduction. If you examine Figure 8.6 carefully, you will notice that the basilar membrane at the base of the Organ of Corti sports hair cells which extend outwards towards the tectoral membrane. The tectoral membrane is less of a membrane and more of a gelatinous construction that is anchored on only one end to the basilar membrane. The energy of sound waves entering the ear is transferred by the ossicles into the cochlea via wave forms moving down the cochlea. The movement of this fluid moves the basilar membrane where the hair cells reside. Imagine a shallow underwater cave with kelp that stretches from the floor to the ceiling; as waves reach into the cave, the kelp gently moves to and fro. That image gives you a good idea of what is occurring in your cochlea with every sound you hear.
Hair cells are delicate. Given that fact, you should be able to explain why repeated exposure to very loud sounds (those that create very powerful waves in the cochlea) leads to permanent hearing loss.
Hair cells themselves are rather complicated entities, but the general picture is fairly straightforward. There are two varieties of hair cells, outer and inner. Outer hair cells are arranged in sets of three in a V or W; inner hair cells are arranged in straight rows. Hair cells are topped with structures called stereocilia. The tallest of the outer hair cells have the tips of their stereocilia embedded in the overhead tectoral membrane . When stimulated by a passing pressure wave, the outer hair cells respond by shortening and elongating, thereby physically amplifying the original motion of the basilar membrane to the tectoral membrane which brushes against the inner hair cells.
You can see a simulation of how your stereocilia move in response to stimulation by going here.
Outer hair cells transmit and enhance the physical motion of the basilar membrane to the tectoral membrane (and selectively amplify low amplitude and high frequency signals); inner hair cells are stimulated by the movement of the tectoral and send that signal to the cochlear nerve. Without outer hair cells we would struggle to hear faint sounds and without inner hair cells, we could not hear at all.
While outer hair cells amplify the signal, it is the inner hair cells that actually send a signal further down the auditory processing stream.
While we cannot observe human hair cells without damage to the individual, we can observe hair cells in other organisms in an effort to understand how they work and the causes and treatments for auditory disorders. To see a short video of a guinea pig hair cell responding to sound, see http://auditoryneuroscience.com/?q=ear/dancing_hair_cell
The physical arrangement of the inner and outer hair cells is shown in Figure 8. 7.
Amplitude and Frequency coding in the cochlea: How does the Organ of Corti convert the physical movement of hair cells into the sounds we perceive? More than a century ago, scientists learned that damage to the cochlea leads to impaired hearing or even deafness and so it seemed logical in their search for the seat of hearing to turn to the contents of the cochlea for an answer. As we begin to consider this question, let’s remind ourselves that the amplitude and frequency of waves in air are transformed into a psychological reality that you and I perceive as loudness and pitch. How does our sense of audition create this representation of the physical world?
Intensity or loudness is coded by the hair cells in response to the amount of movement in the cochlear fluid. One way to understand how this works is to imagine that you were creating a simple wind velocity gauge by planting rows of thin reeds in the ground. It is easy to see that there would be a direct relationship between the bending of the reed and the velocity of the wind. In a similar fashion, hair cell deformation is coded as intensity and you and I then perceive loudness. How then is frequency or pitch coded by these same hair cells? The answer reflects the elegance of our sensory systems.
There are two mechanisms by which frequency is coded. The most obvious mechanism is to directly translate frequency of oscillation into perceptual frequency or tone. In this model, movement of the hair cells 25 times per second would be perceived as a 25 Hz or low base tone. Similarly, 800 waves per second would be perceived as a tone of 880 Hz. The idea that rate of fire of hair cells directly creates the perception of tone is known as Volley theory, or rate theory.
While at first it might appear that we have solved the puzzle of how our cochlea creates the perception of tone, there is a fly in the ointment. You may remember from your reading of Chapter X (neural function) that there is a maximum rate of fire for neurons and while estimates of that maximal rate of fire vary depending on how rate is calculated, our best guesses range from 200 times per second to no more than 1000 times per second. If rate of fire is coded as perceived frequency, then neither you nor I should ever perceive a tone of more than 1000 Hz. Yet, we know that under optimal conditions, young humans perceive sounds from 20-20,000 Hz and even older individuals can perceive tones between 50 Hz and 10-12K Hz. How is it that we can perceive tones whose frequency is clearly above the maximal firing rate of the hair cells in the cochlea?
Scientists of the 19th and early 20th centuries asked themselves the same question and their initial interest was directed into the Organ of Corti. One possibility is that the hair cells act like the strings of a harp and so low sounds might stimulate cells at the base of the membrane while high sounds are reflected in the movement of hair cells at the tip. As microscopes improved it became clear that hair cells could not be thought of as harp strings. A later revision of this approach suggested that our perception of pitch or frequency could be generated by deformations in specific regions of the basilar membrane itself. That is, a deformation in one place is perceived as a low organ note while a dip elsewhere generates the perception of a high reedy piccolo. This theory is known as place theory. Examples of how a standing wave in the cochlea could create a corresponding deformation at specific locations on the basilar membrane are provided below in Figure 8. 8; these examples are graciously provided by Dr. Fabio Mammano of the University of Padua. The first illustration shows the standing wave that a high pitched sound would create at the base of the basilar membrane where the structure is thin but also stiffer. This thin end is closest to the oval window.
Figure 8.8 a: Simulation of the standing wave created by a high pitch. Image courtesy Fabio Mammano. Figure 8.8 a: Simulation of the standing wave created by a high pitch. Image courtesy Fabio Mammano.
Below you see a second simulation but this time, a low pitched sound is creating the deformation at the flexible apex or tip of the basilar membrane.
We now know that there is a general relationship between point of stimulation on the membrane and sensory experience. In general, stimulation at the wide end of the membrane creates an experience we would describe as a low pitched tone, while stimulation further up the membrane leads to a higher pitch. A more detailed view of the relationship between where the standing wave resides and perceived tone is depicted below in portion A of Figure 8.9.
Thought Question: If you understand the concept of place theory, you should be able to explain why a patient might have a serious hearing loss only for frequencies between 100-200 Hz, or a loss between 700 and 1100 Hz.
We now have discussed two possibilities for the coding of frequency. The idea of a whole membrane being deformed in response to a wave traveling down its surface and the notion of cells all firing in concert with a specific frequency both have a certain intuitive grace to them. Which is correct? You don’t have to chose, as both are correct. Researchers have discovered that volley or rate coding accounts for our hearing of low frequency sounds while for higher frequency sounds, place coding takes over. At intermediate points, both rate and place coding is occurring, and so in a sense, both theories are correct.
Age and frequency sensitivity: You now know that optimal auditory range is 20-20K and that even this range is found only among the young. By the time a person is 50 years old, the highest frequency they can perceive is often only 12K to 14K. This fact of life has been applied to the real world in a very creative way.
Store owners sometimes complain that teenagers loiter around the front door of an establishment without actually buying anything, and that their presence can drive away older customers who are a source of income. How then to move the teenagers to another place without calling the police and making a scene?
One company has turned the superior auditory capabilities of the youth into a disadvantage by marketing what is merely a loudspeaker that constantly emits a loud tone at 17.5KHz. The young individuals hear a piercing screech that is unbearably annoying while older patrons hear nothing at all.
Activity: Test your own ability to perceive tones by listening to these files. First adjust your speaker volume to a comfortable level and then play each sample in turn, Do they appear equally loud to you?
They say that turnabout is fair play, and so it is only reasonable that this same concept that is used to the disadvantage of the youth should be turned against the older generation. Some adolescents have learned to use a tone about 15K Hz as a ring tone for their cell phones when cell phone use is discouraged or not permitted. Can you guess why?
Activity: now that you have the concept that people differ in their frequency sensitivity and that age and exposure to loud sounds affects hearing, you can self-administer a hearing test to see how you perform.
Read the directions and start with blocks in the lower third of the display. Headphone might be useful. By clicking on blocks representing frequencies and amplitudes, the user can get a rough idea of their own audiometric response pattern.
Note to editor and reviewers: the IM suggests several activities using this free audiometry screening that can be done in class or assigned for an out of class activity that engage students in a meaningful way.
What is it like to have a hearing disability? Most people imagine that having a hearing disability simply means that the sounds around you are not loud enough. If that were the case, then simply amplifying the signal should cure many if not most hearing problems. Having read this chapter, you probably are able to see that the situation is generally more complex that that. Hearing disabilities are often not remediated by just amplifying the incoming signal. For many individuals suffering with a hearing loss, the cause is a loss of sensitivity to some frequencies but not others, or a difficulty separating the signal from background noise. Simply turning up the gain on the signal is not helpful in such cases.
[Your instructor / the multimedia companion / the course web page] contains directions for this activity as well as audio files that have been filtered to remove selected frequency ranges. All of these files were recorded at the same intensity level. The first file is a calibration file; adjust the volume of your speakers to a comfortable level using this file as a guide. With the activity response sheet in front of you, play the rest of the files in order.
Each audio file was recoded at the same physical amplitude level; were they all equally loud? If not, what does that tell you about our auditory system?
We are not equally sensitive to all frequency ranges.
The remainder of the files have some frequency range filtered out. Was the content of each audio file easy to comprehend or difficult?
Note to editor and reviewers: I provide the instructor and the student with several wav files of a human voice; there is a normal control sample followed by several files which have been filtered to remove specific frequency ranges. This drop out more accurately simulates what hearing impaired individuals actually experience and so provide the student with a better grasp of the implications of hearing loss.
I am providing you with 3 samples.
segment 1 (the calibration file)
Given the difficulties in speech perception that occur when the basilar membrane or auditory nerve is damaged, it seems reasonable that we would search for solutions to those difficulties. You will see that your understanding of auditory anatomy informs the discussion of remedies such as cochlear implants.
Cochlear implants. Back in the day, there was not much help that science could offer to the hearing impaired. One could use a simple ear trumpet to amplify sounds, but that passive technology was of only moderate help
By the 1990s, the technology to micro-miniaturize electronics and batteries had arrived and hearing aids could fit into the ear canal itself. This meant for the first time that a hearing impaired individual would not have to show evidence that they were using a hearing aid.
While each advance in technology led to better performance and also better aesthetics, these devices were inherently limited for many individuals as their mode of operation was to amplify the original auditory signal outside the ear. For many auditory problems, this was not a satisfactory solution.
In the 1960’s, scientists and engineers began to work on devices that would bypass the outer and middle ears and feed a signal directly into the cochlea. For those patients with intact hair cells who suffered from a significant or profound hearing loss, such a device could be of great help. By the late 1980,a workable cochlear implant was available and since then, they have gotten smaller and more powerful. Modern cochlear implants consist of an external microphone that is attached to the skull, a processor which analyzes and processes the signal which is then transmitted through the skull to a receiver and stimulator attached to bone and finally an array of electrodes that are woven into the cochlea. The basic concept is to reproduce at the level of the auditory nerve as much of the natural input as possible. The auditory nerve follows the form of the cochlea and is tonotopically organized (that is, the portion of the nerve that is near the base of the cochlea provides information about low frequency tones while the portion of the nerve that is near the tip provides information about higher pitched tones). The surgeon implants the tiny electrodes into the nerve. You might immediately recognize that more electrodes spaced out over the length of the auditory nerve translate into better resolution, especially of speech sounds. Given the size of the cochlea and the difficulty of inserting multiple electrodes into the nerve, at present, about 24 electrodes is the limit.
You can hear simulations of what speech sounds like to a cochlear implant patient at this site:
Hear a simulation of what listening to music is like for an individual wearing a cochlear implant :
Note to editors and reviewers: I have a collection of professionally created simulations of what a cochlear implant wearer would hear of speech and music; they come in 1, 4 8, 12 and 20 input leads. These are permed for my use in the text.
Audition beyond the ear: just as visual processing begins at the retina and extends deeper into the brain, so it is with audition. Quite a bit of processing occurs downstream from the auditory nerve. The path to comprehension proceeds from the auditory nerve into the cochlear nucleus, superior olivary nucleus, inferior colliculus, the medial geniculate body and finally the primary auditory cortex in the temporal lobe. .
One point about the primary auditory cortex worth mentioning is that there is a physical relationship between location on the PAC and tone perception. Signals from the base of the cochlea report to posterior regions of the PAC and are perceived as high notes, while signals from the apex of the cochlea report to the anterior PAC and are experienced a low notes.
Auditory localization: our auditory system can inform us of the identity of objects in our surroundings; we can distinguish a tiger from a house cat, a warning signal from a whistling teapot and the rustling leaves in an approaching breeze. One easily overlooked skill of the auditory system is the ability to create an auditory scene that includes information about the location of an object in relation to the perceiver. With vision, we can easily tell if a car is to our left or out right by simply looking at it continuously; with sound, the stimulus reaches our ear and often is then gone forever. How then do we use sound to provide information about the location of an object in relation to the perceiver?
The answer is quite interesting. Figure 8.14 depicts an everyday event; an object which is off to one side of the perceiver is creating a sound. The pressure wave created by that source is traveling literally at the speed of sound, or approximately 768 miles per hour. You can see from Fig. 8.14 that the pressure waves traveling from the source arrive at the perceiver’s left ear just miliseconds before it reaches the right ear. Each ear processes the signal but at the level of the medial superior olivary nuclei, it appears that the 2 structures compare the arrival times of the two signals and, based on that data, create an auditory image of the location of the source in relation to the perceiver.
Activity: You can demonstrate this remarkable human ability by asking a friend to sit down and close their eyes; you stand in front and create an auditory signal by snapping your fingers or clicking your fingernails together. Ask your friend to point to the sound source and then move the source around their head. Their accuracy at source location without sight is remarkable. Now look at Figure 8.14 again. Can you predict which 2 locations for the sound source are easily confused and why?
We leave our discussion of the front end of auditory perception by discussing exceptional cases of sound location. Some individuals have developed the ability to navigate effectively via echolocation. Such individuals are generally blind or severely visually impaired and so they emit mouth clicks and then use the echo from objects in their surroundings to create an auditory image of the world. Such individuals may be able to distinguish chairs from tables, a car from a tree, etc. Furthermore, when recordings that simulate what each ear of an echolocator was hearing as they navigated through an environment were played through headphones to other blind echolocators, those participants who were exposed to the recordings perceived the same objects in the environment as the original participants perceived. MRI studies on human echolocators suggest that during echolocation, the areas of the brain normally reserved for analyzing visual stimuli are enlisted by these blind individuals to create an auditory scene.
Activity: Search the internet to find out more about human echolocators such as Daniel Kish, Ben Underwood, Tom De WItt., Dr. Lawrence Scadden and Juan Ruiz.
We will return to the general topic of auditory perception in chapter X when we discuss language and music, but we can conclude our discussion of the basic sensory mechanisms involved in audition by noting that there is much more to the story.
Chemosenses: Olfaction and Gustation
Joan grew up in a family that appreciated good food. Her mother Florence prepared meals for her extended Iowa family and her cuisine could be described as delicious home cooking. Florence was famous of her habit of always having several varieties of freshly baked pie on hand, one of which always happened to be the one a guest seemed to favor at their last visit. Joan learned from her mother how to prepare such tasty fare. Joan’s mother-in-law Stephanie had catered elegant parties for wealthy families on Chicago’s Gold Coast and from Stephanie, Joan added the ability to prepare meals that could grace any table. Holidays at Joan’s house were always events that involved appetizing meals served in a graceful fashion.
However, when Joan reached middle age, she began to notice that food was less tasteful. It had lost much of its pungency. Spicy meals were less spicy; sours were less biting and even salt had lost much of its flavor. Joan realized that food had lost its zest. A visit to her physician led to a surprising conclusion: Joan had lost her sense of smell. She had developed anosmia. Some rare individuals are anosmic from birth but most develop anosmia later in life as a result of disease, head trauma, or damage to the temporal lobe. Some famous anosmics are Ben Cohen of Ben and Jerry’s ice cream, musicians Stevie Wonder, Michel Hutchence and Justin Hayward, actors Cynthia Nixon and Bill Pullman, and poet William Wordsworth
If I asked you to describe your favorite vacation day, you are likely to relay heavily upon your sense of vision and hearing. You might describe the beautiful sunsets, the soaring mountains or the beautiful beaches and somewhere in your description you would likely mention music, the sounds of nature or the interesting dialects you encountered. However, it would not be long before you would enlist your sense of smell and of taste to flesh out your narrative. What did the ocean smell like? What mood did the scent of the flowers create in you? How was the food that you ate distinctive and memorable? Our sense of smell and taste are more important than we realize.
30,000 foot view
Vision and Audition rely on a transformation of physical energy in the form of a photon or a pressure wave into a neural signal. Olfaction and gustation are chemosenses in that a chemical molecule creates the stimulus that we code into smell and taste. The chemosenses help us to locate palatable food, avoid dangerous foods and they may even play a role in mate selection and reproduction.
Evolution of Chemosenses: There is a reasonable case to be made that the chemosenses made an early appearance in the history of life on earth. An organism that could accept or reject an item as food prior to fully ingesting it would have an advantage over those organisms that could not. In any case, the chemosenses play an important if often unrecognized role in dealing with the world.
Let’s begin by comparing our olfactory abilities with those of other organisms.
Comparative olfaction: not all organisms have a sensitive sense of smell. Birds generally do not, although vultures which feed on carrion use their sense of smell to aid them in finding their next meal. Mammals typically do have a good sense of smell and this is especially true of predators who use their sense of smell to detect prey. Dogs have a sense of smell that is 100,000 to 1,000,000 times more sensitive than our own and bloodhounds have a sense of smell that is 10 to 100 million times more sensitive than our ability; they can detect a scent that is days old. Bears have an even more sensitive olfactory system than bloodhounds and can smell food at a distance of 18 miles.
Perceptual characteristics of Olfaction: we have verbal descriptors of our sensory experience that permit us to communicate our internal perceptual state to another person. We could describe a sunset as blood red and a conversation as screeching. Terms that are often used to convey olfactory perceptions to another are musky, putrid, pungent, camphor-like (mothballs), ethereal (solvents), floral and pepperminty.
Anatomy and physiology of olfaction: Our sense of smell is integrated into the nose, a structure whose job is to filter the outside air. Inside the nose are a series of ridges known as the turbinates that tumble the air for better reception by the olfactory system.
Figure 8. 15 the human olfactory system. Image courtesy of Ignacio Icke.
Legend 1 = olfactory bulb. 2 = mitral cells 3 bone 4 nasal epithelium 5 glomerulus 6 olfactory receptor cells
The olfactory bulb sits at the base of the skull just below eyebrow level, atop the nasal cavity and underneath the brain. This area of the skull is known as the cribiform plate and its tiny perforations allow projections of the olfactory bulb to extend outside the skull and into the world. The receptors for odorants lie in a layer of mucosa; they report back to glomerulus cells that are then polled by mitral cells that then signal to the brain via the olfactory nerve.
Compared to our visual and auditory senses, our sense of smell is relatively poorly understood. Part of the problem has to do with the mechanical difficulties of doing odorant research; it is much easier to present stimuli at defined intensities and durations orf visual and auditory stimuli than it is for olfactory stimuli. In addition, the location of the mechanism makes it somewhat problematic to investigate. That being said, researchers have uncovered some of natures mysteries of olfaction.
For example, it appears that humans have receptors for approximately 339 different molecules. Out of this alphabet of odorants, we create complex smells in much the same way that a symphony creates a complex sound out of the combination of many different instruments playing at once. It is estimated that the typical young adult can reliably recognize 10,000 different odorants.
How do olfactory receptors work?We always wish for a neat and tidy story in science and sometimes nature cooperates. Unfortunately, that is not the case with olfaction. The simple answer to our question would be a lock and key model, where odorant molecules fit into specific receptor sites and when they do, that receptor fires. If that were the case, then molecules with similar configurations should express similar odors and dissimilar molecules should not smell the same. For example, beta-cyclocitral (C10H13O) and p-anisaldehyde (C8H8O2) have dissimilar molecular shapes yet both are described as having a minty odor. Thymol and Menthol have almost identical molecular structures yet the former has an off-putting odor while the latter is appealing.
Two other theories are often invoked to explain olfaction. The odotope or weak shape theory proposes that rather than a single key-lock model of olfaction, receptors respond to small but individual physical features of a molecule and so a single odorant might stimulate several receptors simultaneously. While this model explains the data better than the simple lock and key model, it also falls short at fully predicting the odorant-odor relationship.
A more recent and intriguing theory of olfaction changes the focus of perception from molecular shape to the vibrational frequency of the actual atoms themselves. The evidence to support vibration theory is contradictory. For example, vibrational theory predicts that molecules that are identical except for being mirror images should smell the same. Chemists refer to these as enantiomers (your left and right hands can be considered enantiomers)and since they would have identical vibrational frequencies they should smell the same. In some cases they do but in others they do not; for instance, the ‘right-handed’ enantiomere of Carvone smells like spearmint and its mirror image smells like caraway.
Genetics and olfaction: In 2004, neuroscientists Linda Buck and Richard Axel were awarded the Nobel Prize in medicine for their identification of genes that control olfaction. Their work shed new light on how genes can affect even our sense of smell. We now believe that each of us smells the world in a slightly different way; most people detect almost every odorant but not necessarily at the same sensitivity and perhaps each of us is anosmic for one or two of the roughly 10,000 odorants we are aware of. For example androstenone (a substance produced in boar’s testicles and some people’s perspiration) is not detected by half the population, is powerfully aversive for 35% and has a flowery musky odor for 15%. While not everyone has the opportunity to be exposed to a boar’s odor, most but not all of us have noticed that after eating asparagus, one’s urine has an unpleasant odor. In both cases, olfactory sensitivity to the odoranthas a generic basis.
The original articles on the genetic basis of androstenone and asparagus perception are available on line at these URLs.
Secondary Olfactory system: most mammals have a secondary olfactory system that is sensitive to pheromones, or chemical messengers whose function is to convey social information between members of a species, for example, information about clan membership or sexual receptivity. The receptor site for the secondary olfactory system is thought to be the vomeronasal organ (VNO) which reports to the amygdala. The question is: do humans have a VNO and do humans communicate via phermones?. The answer to the first question is unclear. Some anatomists believe that they can identify a VNO in humans, other believe that the VNO is vestigial and disappears in utero and some anatomists can find no evidence of a VNO in humans. While evidence for a VNO in humans is not strong, there is considerable evidence that we do communicate to an extent via pheromones. Chemical communication will be covered in greater detail in Chapter X, Sex and Reproduction.
Conclusion: we still have much to learn about the olfactory system in humans. For this reason, scientists who work in industry to create odorants for such uses as perfumes, foods and laundry products struggle to create the chemistry
Activity: Go to the web site of the Monell Center, a world class research center for the study of taste and smell.
Our olfactory and gustatory systems are linked; one enhances the other. Cold and flu sufferers with blocked nasal passages and heavy smokers all know that the enjoyment of food is diminished for these individuals.
Note to editor and reviewers: There is a discussion on pheromones, the McClintock effect and the general question of olfactory cues for mating that is currently sited in the chapter on sex.
Gustation: the biological basis of taste
- Taste, like smell, is a chemosense.
- Taste, like smell, helps us avoid harmful foods and also helps us favor foods high in energy value.
- Most but not all of taste is in the tongue
- There are 4 basic tastes; unless there are 5
- Some individuals have very unusual taste abilities
Anatomy and Physiology of the tongue: the human tongue is an organ consisting of muscle and tissue; it is unique in that its musculature is only connected at one end. The tongue is the primary organ of gustation or taste, but there are some taste receptors in the roof of the mouth, the oral mucosa and even the top of the esophagus. Our sense of taste comes from structures called papillae. To maintain our distinction between sensory structure and site of coding into the nervous system, the eye, ear, nose and tongue are all sensory structures while the retina, basilar membrane, olfactory receptor cells and papillae are sites where sensory coding takes place.
Our sense of taste is produced by the action of chemicals upon receptors which are mostly located on the dorsal surface of the tongue. These receptors are referred to as taste buds, as they are structures that protrude above the surface of the tongue. In fact, each taste bud consist of a pore or opening in the tongue epithelium which leads down to a receptor. These receptors are called papillae and there are typically between 8000 and 10,000 papillae of four varieties distributed across the tongue.
- Fungiform papillae are mushroom shaped and are found mostly on the dorsal surface and the sides of the tongue.
- Filiform papillae and thin and have hair-like structures on top. They do not contribute directly to taste; instead they act mechanically like a grater to abrade food into smaller particles; not surprisingly, they are located right in the middle of the dorsal surface.
- Foliate papillae are vertical folds found on the edges of the tongue especially towards the rear.
- Circumvallate pappilae : are the least common type; most people have only 10 to14 int total and they are situated at the back of the tongue.
Figure 8.16 A typical taste bud. Image permed.
Basic dimensions of taste: Just as sight has the attribute of color and hearing has the characteristic of pitch or tone, taste has its own underlying dimensions. The basic tastes are thought to be sweet, sour, bitter and salty. More recently, evidence has suggested that a 5th basic taste known as umami also exists. Umami can be described as ‘savory’ or the sensation of broth or meatiness.
How do taste receptors work? The answer is complicated and not completely understood, but it appears that receptors can be characterized as more narrow-field than broad field. This means that taste receptors are not tuned for only one taste, nor do they respond to a wide range of taste dimensions. Rather, papillae appear to be mostly tuned to a particular dimension but respond slightly to other characteristics. This explains why artificial sweeteners such as saccharine and aspartame have both a sweet and a slightly bitter characteristic; finding a molecule that has no calories and stimulates only the sweet receptors is more difficult than it might appear at first glance.
In addition to science creating molecules that have a specific taste, nature has its own tricks. There are two substances that temporarily block specific gustatory receptor sites, thereby creating a false taste sensation. The berries of the plant synsepalum dulcificum (also known as the miracle fruit) contains a molecule known as miraculin which binds to sweet receptors on the tongue. When stimulated by foods with a low pH (i.e sour foods), miraculin stimulates the sweet receptors thereby leading to the paradoxical perception of sweetness while ingesting sour foods. There are also herbs that partially block the sweet receptors, thereby reducing the taste of sweets.
It would be very neat and tidy if each type of papillae were responsible for a specific taste dimension and if each papillae variety was assigned its own patch of real estate n the tongue. It was thought for many years that this was the case; a paper published in 1901 made this claim and generations of scientists and physicians were taught this ‘fact’. Generations of students wondered why their own tongue didn’t seem to fit the facts; but in the 70’s we realized that the ‘tongue map’ presented in Figure 8.17 which so many students had been taught was incorrect.
Figure 8.17. The classical tongue map. 1 = bitter, 2 = sour, 3 = salt 4 = sweet. Perm requested.
Taste buds are continuously being replaced; if you slightly burn you tongue with a hot beverage, you will lose some taste sensitivity for a few days but it will return. Severe burns permanently damage the tongue epithelium and in that case, there is a permanent loss of taste sensitivity.
Blocking taste receptors: now that you know the basics of how your papillae work, you might have wondered if nature or science can play tricks on our tongue, and the answer is yes. Synsepalum dulcificum, also known as the miracle fruit, has the rather unusual property of making sour foods sweet. S. dulcificulum contains a protein known as miraculin, which binds to sweet receptors and causes sour foods to taste sweet and sweet foods to taste sickeningly sweet. The effects lasts for about an hour but can be halted by hot liquids that clear the receptors. Miraculin has been suggested as an aid to dieters who have a sweet tooth.
The IM contains instructions for a lab using miracle fruit.
A second example of using the science of gustation in a practical fashion comes through what is known as sugar blocker gum. This gum appears to block sugar receptors, causing sweet foods to lose their appeal.
Clinical issues: as with other sensory systems, deficits do occur. Some individuals experience dysgeusia which is a condition where the receptors produce an incorrect response. Some medical conditions and some drugs such as Lunesta, Flagyl and some anticancer agents produce parageusia, in which the individual experiences a metallic taste. More serious is ageusia which is the complete lack of a sense of taste. Ageusic patients are at risk for food-born illnesses as they cannot detect through taste the off flavor of spoiled food.
Genetic influences on taste: Our papillae code for tastes based on the proteins that are expressed in that taste bud and it is our genes that control the expression of those proteins. Given this arrangement, it is not surprising that there would be genetic differences in taste sensitivity. A early insight into the link between genes and gustation was the discovery that the chemical PTC is perceived as bitter to about two-thirds of tasters and tasteless to the rest. As PTC is toxic in large doses, the related chemical PROP is used in research and demonstrations due to its lower toxicity. This relationship between genotype and behavior has more meaning than just classroom demonstrations of PROP sensitivity and an example of this is the possible link between POPR sensitivity and alcoholism.
Genetics, taste and alcoholism: For example, the gene TAS2R38 expresses a protein that encodes the bitter sensation. Different genotypes of this gene lead to varying abilities to perceive bitter substances; individuals with some variants are quite sensitive to bitter while other genotypes attenuate the experience. A heightened sensitivity to bitterness also means that ethanol is experienced as being bitter. PROP sensitive individuals find the taste of alcohol to be less pleasant. Alcoholism certainly has many causes; some cultural, some environmental and genetic aspects that relate to impulsivity, and inability to control craving, a genetic trait that codes the taste of alcohol to be unpleasantly bitter is one of the many roots causes of this disease.
Supertasters: While deficits are clinically interesting, understanding how some individuals have unusual gustatory abilities also helps us understand how taste works. The term ‘supertaster’ was coined by Linda Bartoshuk, a scientist whose career was devoted to investigating how genes shape the perception of taste. Barkoshuk and her colleagues found individuals whose sensitivity to tastes was unusually high. Such individuals were not processing taste with the same sensitivity as normals but with a lower criterion for response as a person who lived in an earthquake zone might be no better at detecting ground vibrations but might have a lower threshold for anxiety if they occur), Barkoshuk’s supertasters had considerably lower thresholds for detecting certain tastes. For example, supertasters find PROP to be considerably more bitter than the rest of us.
If Bartoskuk were right that supertesters are atypical, then supertasters should differ from most individuals in some anatomical or physiological aspect of gustation and confirmatory evidence was soon coming. Supertasters have many more taste buds than normals and this abundance of receptors certainly plays a role in the phenomenon.
Are you a supertaster? The IM contains a set of simple instructions for using common materials to identify supertasters among students.