PSB CH6 - AUDITION

I. Audition

A. Sound and the Ear

Physics and Psychology of Sound

Sound waves are periodic compressions of air, water, or another medium.

Sound waves vary in amplitude and frequency.

Amplitude: Intensity of a sound wave. In general, sounds of greater amplitude sound louder, but exceptions occur.

Frequency: Number of compressions per second, measured in hertz (Hz) of a sound.

Pitch: The perception of frequency (the higher the frequency of a sound, the higher its pitch).

Most adult humans can hear sounds ranging from 15 to almost 20,000 Hz.

Children hear higher frequencies than adults, because the ability to perceive high frequencies decreases with age and exposure to loud noises.

The third aspect of sound is timbre, meaning tone quality or tone complexity.

Structures of the Ear

Auditory canal → tympanic membrane → oval window

• The anatomy of the ear is described in terms of three regions: the outer ear, the middle ear, and the inner ear.

  • The outer ear includes the pinna (structure of flesh and cartilage attached to the side of the head) and the auditory canal.

  • The pinna helps us locate the source of a sound by altering reflections of sound waves.

• The middle ear is comprised of the tympanic membrane (eardrum), which vibrates at the same frequency as the sound waves that strike it.

  • Sound waves reach the tympanic membrane through the auditory canal.

  • The tympanic membrane is attached to three tiny bones (hammer, anvil, and stirrup).

• The inner ear consists of the oval window, which receives vibrations from the tiny bones of the middle ear, and the cochlea, which contains three fluid-filled tunnels: the scala vestibuli, scala media, and scala tympani.

  • The stirrup causes the oval window to vibrate, setting in motion all the fluid in the cochlea.

  • The auditory receptors (hair cells) lie between the basilar membrane and the tectorial membrane in the cochlea.

  • When fluid in the cochlea vibrates, a shearing action occurs, which stimulates hair cells; these cells then stimulate the auditory nerve cells (eighth cranial nerve).

Pitch Perception

Frequency and Place – your ability to understand speech or enjoy music depends on your ability to differentiate among sounds of different frequencies.

Place Theory: Each area along the basilar membrane is tuned to a specific frequency and vibrates whenever that frequency is present.

Each frequency activates hair cells at only one place along the basilar membrane, and the brain distinguishes frequencies by which neurons are activated. This theory has a downfall in that various parts of the basilar membrane are bound too tight for any part to resonate like a piano string.

Frequency Theory: We perceive certain pitches when the basilar membrane vibrates in synchrony with a sound, causing the axons of the auditory nerve to produce action potentials at the same frequency.

The current theory is a modification of both place and frequency theories:

For low frequency sounds (below 100 Hz), the basilar membrane does vibrate in synchrony with the sound wave in accordance with frequency theory. The pitch of the sound is identified by the frequency of impulses and the loudness is identified by the number of firing cells.

Volley principle of pitch discrimination: The auditory cortex as a whole can have volleys of impulses up to about 4000 per second, even though no individual axon approaches this frequency alone.

The volley principle is believed to be important for pitch perception below 4000 Hz, although it is unclear how the brain uses this information.

For high frequency sounds (above 4000 Hz), we use a mechanism similar to place theory. High frequency vibrations strike the basilar membrane, causing a traveling wave. This causes displacement of hair cells near the base (where the stirrup meets the cochlea). Low frequency sounds produce displacement farther along the membrane.

Tone deafness or amusia: A disorder where individuals are seriously impaired at detecting small changes in frequency.

Many relatives of those with amusia have the same condition. It is associated with a thicker than average auditory cortex in the right hemisphere but fewer than average connections from the auditory cortex to the frontal cortex.

Absolute pitch or perfect pitch: The ability to hear a note and identify it accurately. It is somewhat determined by genetic predisposition. The main determinant is extensive musical training.

The Auditory Cortex

Auditory information passes through several subcortical structures with an important crossover in the midbrain that enables each hemisphere of the forebrain to get its major auditory input from the opposite ear.

Primary auditory cortex (area A1): Ultimate destination of auditory information is located in the superior temporal cortex.

Area A1 also is important for auditory imagery. Similar to the visual system, the auditory system needs experience to develop normally. Both constant noise and lack of exposure to sound will impair the development of the auditory system.

Damage to the A1 does not leave someone deaf; it may hinder the ability to recognize combinations or sequences of sounds, like music or speech.

In the primary auditory cortex, cells respond preferentially to certain tones. Cells preferring a given tone in the auditory cortex cluster together providing a map of the sounds referred to as a tonotopic map. Thus, the cortical area with the greatest response indicates what sound or sounds are heard.

Cells outside area A1 respond best to auditory “objects” such as animal cries, machinery noise, music, etc.

Hearing Loss

Deafness

Conductive or middle-ear deafness: Failure of the bones of the middle ear to transmit sound waves properly to the cochlea.

Conductive deafness can be caused by diseases, inheritance, infections, or tumorous bone growth near the ear. This deafness can be corrected by surgery or hearing aids.

Nerve or inner-ear deafness: Damage to the cochlea, hair cells, or auditory nerve that causes a permanent impairment in hearing in one to all ranges of frequencies. Nerve deafness can be inherited or caused by prenatal problem and early childhood disorders.

Tinnitus: Frequent or constant ringing in the ear. Tinnitus is often produced by nerve deafness. It is a phenomenon similar to phantom limb, where axons corresponding to other parts of the body may invade the brain area previously responsive to sounds, especially high-frequency sounds.

Heating, Attention, and Old Age

Many older people continue to have hearing problems despite wearing hearing aids.

Part of the explanation is that the brain areas responsible for language comprehension have become less active.

The rest of the explanation relates to attention. Many older people have a decrease in their inhibitory neurotransmitters in the auditory portions of the brain. As a result, they have trouble suppressing the irrelevant sounds and attending to the important one. Also, instead of making a quick, crisp response to each sound, the auditory cortex has delayed, spread-out responses to each sound, such that the response to one sound partly overlaps the response to another.

Sound Localization

Determining the direction and distance of a sound requires comparing the responses of the two ears.

One method is the difference in time of arrival at the two ears.

Another cue for location is the difference in intensity between the ears. For high-frequency sounds, with a wavelength shorter than the width of the head, the head creates a sound shadow, making the sound louder for the closer ear.

Adult humans are accurate at localization for frequencies above 2000 to 3000 Hz, and less accurate for progressively lower frequencies.

A third cue is the phase difference between the ears. Every sound wave has phases with consecutive peaks 360 degrees apart.

II. The Mechanical Senses

A. The mechanical senses respond to pressure, bending, or other distortions of a receptor.

B. Vestibular Sensation

1. The vestibular organ monitors head movements and directs compensatory

movements of the eyes. It is critical for eye movements and maintaining balance.

2. The vestibular organ consists of the saccule, utricle, and three semicircular canals.

3. Calcium carbonate particles (otoliths) lie next to hair cells excite them when the

head tilts in different directions.

4. The three semicircular canals are filled with a jellylike substance and lined with

hair cells. Acceleration of the head causes this substance to push against hair cells,

which in turn causes action potentials from the vestibular system to travel via part

of the eighth cranial nerve to the brainstem and cerebellum.

C. Somatosensation

1. The somatosensory system involves the sensation of the body and its movements,

including discriminative touch, deep pressure, cold, warmth, pain, itch, tickle, and

the position and movements of joints.

2. Somatosensory Receptors

a. Examples of touch receptors are pain receptors, Ruffini endings, Meissner’s

corpuscles, and Pacinian corpuscles.

b. Stimulation of touch receptors opens sodium channels in the axon, possibly

starting an action potential if the stimulation is strong enough.

c. Pacinian corpuscle detects sudden displacements or high-frequency vibrations

on the skin.

d. Receptors for heat and cold can be stimulated by certain chemicals as well as

mechanical stimulation. Capsaicin, a chemical found in hot peppers such as

jalapeños, stimulates the receptors for painful heat.

3. Tickle

a. The sensation of tickle is interesting but poorly understood.

b. Why can’t you tickle yourself? When you touch yourself, your brain compares

the resulting stimulation to the “expected” stimulation and generates a weaker

somatosensory response than you would experience from an unexpected touch

4. Somatosensation in the Central Nervous System

a. Information from touch receptors in the head enters the CNS through the

cranial nerves. Information from touch receptors below the head enters the

spinal cord through the 31 spinal nerves and passes toward the brain.

b. Each spinal nerve has a sensory component and a motor component. Each

sensory spinal nerve innervates a limited area of the body called a dermatome.

c. Sensory information from the spinal cord is sent to the thalamus before

traveling to the somatosensory cortex in the parietal lobe.

d. The somatosensory cortex receives information primarily from the

contralateral side of the body.

e. Damage to the somatosensory cortex impairs body perceptions. A patient with

Alzheimer’s who exhibited such damage had trouble putting her clothes on

correctly.

D. Pain

1. Pain, the experience evoked by a harmful stimulus, directs our attention towards

danger.

2. Stimuli and Spinal Cord Paths

a. Pain sensation begins with the least specialized of all receptors, a bare nerve

ending.

b. The axons carrying pain information have little or no myelin and therefore

conduct impulses relatively slowly, in the range of 2 to 20 meters per second

(m/s).

c. Thicker and faster axons convey sharp pain. The thinner ones convey duller

pain.

d. Mild pain causes the release of the neurotransmitter glutamate, whereas

stronger pain also releases several neuropeptides including substance P and

CGRP (calcitonin gene-related peptide).

e. The pain-sensitive cells in the spinal cord relay information to several sites in

the brain.

i. One path extends to the ventral posterior nucleus of the thalamus and then

to the somatosensory cortex, which responds to painful stimuli, memories

of pain, and signals that warn of impending pain.

ii. The pain pathway crosses immediately from receptors on one side of the

body to a tract ascending the contralateral side of the spinal cord.

iii. Touch information travels up the ipsilateral side of the spinal cord to the

medulla, where it crosses to the contralateral side.

iv. Pain and touch reach neighboring sites in the cerebral cortex.

3. Emotional Pain

a. Painful stimuli also activate a path that goes through the reticular formation of

the medulla and then to several of the central nuclei of the thalamus, the

amygdala, hippocampus, prefrontal cortex, and cingulate cortex.

b. These areas react not to the sensation but to its emotional associations.

c. Hurt feelings can be like real pain (You can relieve hurt feelings with pain-

relieving drugs such as acetaminophen (Tylenol®)!

4. Ways of Relieving Pain

a. Insensitivity to pain is dangerous. People with a gene that inactivates pain axons

suffer repeated injuries and generally fail to learn to avoid dangers.

b. Opioids and Endorphins

i. Opioid Mechanisms: released by the brain to dull prolonged pain after you

are alerted of danger.

ii. Opioids bind to receptors in the spinal cord and periaqueductal gray area

to block the release of substance P and decrease prolonged pain.

iii. Endorphins: the transmitters that attach to the same receptors as morphine.

Different endorphins (naturally released by the brain) relieve different

types of pain.

iv. Gate Theory: Information not related to pain travels to the spinal cord and

closes the “gates” for each pain message, thereby modulating the subjective

experience of pain. Although gate theory turned out to be wrong, the

general principle is valid: nonpain stimuli modify the intensity of pain.

c. Placebos

i. A placebo is a drug or other procedure with no pharmacological effects.

ii. In medical research, an experimental group receives a potentially active

treatment and the control group receives a placebo.

d. Cannabinoids and Capsaicin

i. Cannabinoid (chemical related to marijuana): Blocks certain kinds of pain

through the periphery of the body rather than the CNS.

ii. Capsaicin: Stimulates receptors for heat. When rubbed onto a sore

shoulder, an arthritic joint, or other painful area produces a temporary

burning sensation followed by a longer period of decreased pain. High

doses, or low doses for a prolonged period, can damage pain receptors.

Eating it will not relieve your pain—unless your tongue hurts.

5. Sensitization of Pain

a. The body also has mechanisms to increase pain after tissue has been damaged

and inflamed.

b. Pain sensitization is a result of the body releasing histamine, nerve growth

factor, and other chemicals that are necessary to repair the body.

c. Nonsteroidal anti-inflammatory drugs decrease pain by reducing the release of

chemicals from damaged tissue.

E. Itch

1. Exists in two forms

a. In response to tissue damage, due to release of histamine.

b. In response to contact with certain plants.

2. A particular spinal cord path conveys itch sensation.

a. Itch activates neurons in the spinal cord that produce a chemical called gastin-

releasing peptide.

3. Itch is useful because it directs you to scratch the itchy area and remove whatever is

irritating your skin.

4. Vigorous scratching produces mild pain, and pain inhibits itch. Opiates reduce pain

and increase itch. The inhibitory relationship between pain and itch is evidence that

itch is not a type of pain.

III. The Chemical Senses

A. Chemical Coding

1. Labeled-line principle: Receptors of a sensory system that respond to a limited

range of stimuli and send a direct line to the brain.

2. Across-fiber pattern principle: Receptors of a sensory system respond to a wide

range of stimuli and contribute to the perception of each of them.

3. Vertebrate sensory systems probably do not have any pure labeled-line codes. Taste

and smell stimuli excite several kinds of neurons, and the meaning of a particular

response by a particular neuron depends on the responses of other neurons.

B. Taste

1. Taste results from the stimulation of taste buds. Taste differs from flavor, which is

the combination of taste and smell. Taste and smell axons converge into many of

the same cells in an area called the endopiriform cortex.

2. Taste Receptors

a. Taste receptors are actually modified skin cells that last only about 10-14 days

before being replaced.

b. Mammalian taste receptors are located in taste buds, which are located in

papillae (structures on the surface of the tongue). A given papillae may contain

from 0 to 10 taste buds and each taste bud contains about 50 receptor cells.

c. In adult humans, taste buds are located mainly on the outside edge of the

tongue.

3. How Many Kinds of Taste Receptors?

a. We have long known of the existence of at least four types of “primary” tastes:

sweet, sour, salty, and bitter. Chemicals that alter one receptor but not others

have been used to identify taste receptor types.

b. Adaptation: Decreased response to a stimulus as a result of recent exposure to

it (e.g., if the tongue is soaked in two sour solutions, one after the other, the

second solution will not taste as sour as the first).

c. Cross-adaptation: A reduced response to one taste because of exposure to

another. There is little cross-adaptation in taste.

d. Umami: A taste associated with glutamate. Researchers have found a glutamate

taste receptor responsible for this fifth type of taste.

e. Different chemicals not only excite different receptors, they also produce

different rhythms of action potentials.

4. Mechanisms of Taste Receptors

a. Saltiness receptors work by allowing salt to cross the membrane. The higher

the concentration of salt, the greater the response of the receptors (i.e., the

larger the receptor potential).

b. Sourness receptors detect acids.

c. Sweetness, bitterness, and umami receptors work by activating a G-protein that

releases a second messenger within the cell.

d. To identify the wide range of chemicals that have a bitter taste, which are

usually toxic, we have not one bitter receptor but a family of about 25 bitter

receptors.

5. Taste Coding in the Brain

a. The perception of taste depends on a pattern of responses across taste fibers.

b. Taste information from the anterior two-thirds of the tongue travels to the brain

via the chorda tympani, a branch of the seventh cranial nerve (facial nerve).

Information from the posterior tongue and throat is carried to the brain along

branches of the ninth and tenth cranial nerves.

c. These three nerves project to the nucleus of the tractus solitarius (NTS) in

the medulla. The NTS relays information to the pons, lateral hypothalamus,

amygdala, thalamus, and two areas of the cerebral cortex (the insula is

responsible for taste, and the somatosensory cortex is responsible for the sense

of touch on the tongue).

d. Each hemisphere of the cortex receives input mostly from the ipsilateral side of

the tongue.

6. Variations in Taste Sensitivity

a. Phenythiocarbamide (PTC) is a chemical whose taste is controlled by a single

dominant gene. Some people hardly taste PTC, others taste it as bitter, and

some taste it as extremely bitter.

b. The prevalence of nontasters of PTC varies across cultures and is not

obviously related to spiciness of traditional cuisine in those cultures.

c. People who are insensitive to the taste of PTC are less sensitive to other tastes

as well.

d. People who taste PTC as extremely bitter are supertasters and have the

highest sensitivity to all tastes.

e. Supertasters have the largest number of fungiform papillae (the type of papillae

near the tip of the tongue).

C. Olfaction

1. Olfaction: The sense of smell; the detection and recognition of chemicals that come

in contact with membranes inside the nose.

2. Continued stimulation of an olfactory receptor produces adaptation. This adaptation

is more rapid than that of sight or hearing.

3. Olfactory Receptors

a. Olfactory cells: Neurons that line the olfactory epithelium and are responsible

for smell. In mammals, each olfactory cell has cilia (threadlike dendrites)

where receptor sites are located.

b. Olfactory receptors are made up of a family of proteins that traverse the cell

membrane seven times and respond to chemicals outside the cell by causing

changes in a G-protein inside the cell. The G-protein provokes chemical

activities that lead to an action potential.

c. It is estimated that humans have hundreds of different types of olfactory

receptor proteins. Rats and mice are believed to have a thousand types.

4. Implication for Coding

a. In the olfaction system, the response of one receptor can identify the

approximate nature of the molecule and the response of a larger population of

receptors enables more precise recognition. This is possible because of the

large number of olfactory receptors.

5. Messages to the Brain

a. Axons of olfactory receptors carry information to the olfactory bulb. Each

odorous chemical excites only a limited part of the olfactory bulb. Olfaction is

coded in terms of which area of the olfactory bulb is excited.

b. The olfactory bulb sends its axons to several parts of the cortex. The

connections are precise, as all receptors sensitive to a given group of chemicals

send information to the same small cluster of cells in the cortex. The

organization of the olfactory cortex is almost identical from one individual to

another.

c. In contrast to receptors for vision or hearing, olfactory receptors survive for

just over one month and then are replaced by new cells that have the same odor

sensitivities as the original cells.

6. Individual Differences

a. On average, women detect odors more readily than men, and the brain’s

responses to odors are stronger in women.

b. Young adult women exposed repeatedly to a faint odor will gradually become

more sensitive to the odor. This ability is not found in males, girls before

puberty, or women after menopause.

D. Pheromones

1. The vomeronasal organ (VNO): A set of receptors located near, but separate from,

the olfactory receptors.

2. Pheromones: Chemicals released by an animal that affect the behavior of other

members of the same species, especially sexually.

3. The receptors in the VNO are specialized to respond only to pheromones. Each

VNO receptor responds to just one pheromone and does not show adaptation after

continued exposure.

4. Unlike most mammals, the VNO is small in adult humans. Moreover, no receptors

have been found in the human VNO.

5. Humans do respond to pheromones and have at least one type of pheromone

receptor located in the olfactory mucosa.

6. Pheromones play a role in human sexual behavior similar to that in other mammals.

Pheromones can synchronize the menstrual cycles of women who spend a lot of

time together and enhance the regularity of the menstrual cycle of a woman who is

in an intimate relationship with a man.

E. Synesthesia

1. Synesthesia is the experience some people have in which stimulation of one sense

evokes a perception of that sense and another one also.

a. Someone might perceive the letter J as green or say that each taste feels like a

particular shape on the tongue.

2. One hypothesis is that axons from one cortical area branch into another cortical

area.