Chapter 6 - Hearing

Audition

Audition

A sensory system specialized to sense and perceive sound waves

Human Auditory System

The sensory system responsible for hearing, adapted for physically hearing and psychologically understanding many kinds of sound information

Two Dimensions of Sound

1. Physical

2. Psychological

Physical Dimension of Sound

The sense of hearing depends on our physical ability to detect sound waves

Sound Waves

Vibrations that travel through the air, periodically compressing air, water, or other media. They also vary in amplitude and frequency.

Amplitude

The intensity of sound waves; loudness is the perception of intensity; however loudness can also be attributed to frequency

Frequency

The number of compressions per second, measured in hertz (Hz) of a sound

Human Audible Sound Wave Range

15-20,000 Hz

(other species do better, like below 15 or above 20,000 Hz)

Pitch

A kind of perceiving frequency of sound (also called tone). Determined by different frequencies

Timbre

The quality or complexity of a tone; can be demonstrated by people singing the same note but sound different

Prosody

Conveying emotional information by tone of voice

Three Regions for the Ear

1. Outer ear

2. Middle ear

3. Inner ear

The Outer Ear

Comprised of The Pinna and The Auditory Canal. The pinna helps us locate the source of a sound by altering reflections of sound waves

The Middle Ear

Comprised of The Tympanic Membrane which is attached to three tiny bones: Malleus, Incus, and Stapes (hammer, anvil, and stirrup) which are responsible for Sound Conduction

The Tympanic Membrane

Also known as the eardrum, located in the middle ear and is responsible for converting sound waves into mechanical vibrations by vibrating at the same frequency as sound waves which strike it

The Inner Ear

A complex system comprised of 1) The Oval Window and 2) The Cochlea and altogether is responsible for hearing and balance

1) oval window receives vibrations from the three tiny bones in the middle ear and 2) The Cochlea that contains three fluid-filled (endolymph) tunnels

The Oval Window

A membrane-covered opening in the inner ear that receives vibrations from the three tiny bones in the middle ear

The Cochlea

A spiral-shaped structure that contains three fluid-filled (endolymph) tunnels: The Scala Vestibuli, Scala Media, and Scala Tympani divided by tectorial and basilar membranes

Responsible for converting mechanical vibrations into electrical impulses

Describe the Process of Hearing

The stirrup causes the oval window to vibrate, setting in motion all the fluid in the cochlea; when fluid in the cochlea vibrates, a shearing action occurs to stimulate hair cells (receptors) that stimulate the auditory neurons to fire action potentials along the 8th cranial nerve

Auditory Receptors

Hair cells that lie between the Basilar Membrane and the Tectorial Membrane in the Cochlea; their axons run in the basilar membrane

Three Theories of Sound Perception

1. Place Theory

2. Frequency Theory

3. The Current Pitch Theory

Place Theory

A theory good for explaining high frequency sounds (above 5,000 Hz)

Each place along the basilar membrane of the cochlear is tuned to a specific frequency and vibrates whenever that kind of frequency is present

→ each frequency activates some set of hair cells at only one place along the basilar membrane and the brain distinguishes frequencies by what neurons are activated

Frequency Theory

A theory good for explaining low frequencies

When the basilar membrane vibrates in synchrony with the frequency of the sound, its causes axons of the auditory nerve to fire action potentials at the same frequency

e.g., a sound at 50 Hz causes 50 action potentials in the auditory nerve

The Current Pitch Theory

Combines modified version of both previous theories: high frequency sounds best explained by place theory and low frequencies sounds best explained by frequency theory

Frequency theory (low frequency explanation)

Due to the refractory period of the neuron in his axon, as sound exceeds 100 Hz, it becomes harder for the neuron to continue to fire action potentials in synchrony with a sound waves;

→ so at higher frequencies, an individual neuron might fire every 2nd, 3rd, 4th, or later wave so action potentials from each cell are phase-locked to a peak of the sound waves

Volley Principle of Pitch Discrimination

A principle believed to be important for pitch perception below 4000 Hz.

There are no individual axons that approach high frequencies alone rather the auditory cortex as a whole can process volleys of impulses up to 4000 Hz resulting from a large number of auditory neuron activity at any moment.

Place Theory (high frequency explanation)

The cochlea is it curl-like organ in the basilar membrane in the cochlea varies from stiff at its base to floppy at the apex → so the highest frequency sounds vibrate hair cells near the base and lower frequency sounds vibrate hair cells near the apex.

The Primary Auditory Cortex (A1)

Located in the superior temporal cortex and is the FIRST destination of those cortices at which auditory information arrives (auditory information is processed and decoded here) and receives most (not 100%) of auditory information from the opposite ear

The Primary Auditory Cortex (A1) (where center?)

Located posterior to the A1 at the superior temporal cortex and contains the visual MT and MST-like area

The Primary Auditory Cortex (A1) (what center?)

Located anterior to the A1 and also in the Parietal cortex and is sensitive to patterns of sound

Auditory Information Travels (where?)

Between the superior olive and the inferior colliculus

Tonotopic Map

Each set of neurons in the A1 cortex responds best to one tone; neurons preferring a given tone in the A1 cluster together to provide a map of the sounds

Auditory System Development (impaired?)

Like the visual system, the auditory system needs experience to develop normally; either constant noise or lack of exposure to sound will impair the development

Primary Auditory Cortex (A1) (damage?)

Damage to the A1 doesn't necessarily cause complete deafness; people can still hear simple sounds but their main deficit is in the ability to recognize combinations or sequences of sounds like music or speech

This shows that A1 isn’t necessary for all hearing

Cells Outside the A1

Mostly belong to the secondary auditory cortices and respond to objects

e.g., “what is the object” or “where is the object” → so this surrounding area plays the role that is identical to the ventral stream of the visual cortex (or the what center)

Two Categories of Hearing Impairment

1. Conductive or middle ear deafness

2. Nerve deafness or inner ear deafness

Conductive Deafness

Occurs if the three tiny bones of the middle ear failed to transmit sound waves properly to the Cochlea

Caused by disease, infections, or tumorous bone growth, but can be corrected by surgery or hearing aids that amplify stimulus

Nerve Deafness

Results from damage to the cochlea (e.g., damage to the hair cells or the auditory nerves) which often produces tinnitus

This damage can be confined to one part of the cochlea in this case people may only hear certain frequencies

Damage can be inherited or caused by prenatal problems or early childhood disorders, and also by drug-induced toxicity

Tinnitus

Frequent or constant ringing in the air that occurs due to damage to the cochlea

→ due to the damage if a part of the A1 no longer gets its normal input axons representing other parts of the body may invade a brain area previously responsive to sounds especially high frequencies (similar to the mechanisms of the phantom limb)

Sound Localization

Can be done by comparing responses of the two ears in direction and distance, reflecting the differences in intensity → mechanism is based on three cues

Three Cues of Sound Localization

1. Detecting difference in time

2. Sound shadow

3. Phase difference

Sound Localization (Difference in Time)

Localizes sounds with sudden onset.

The sound coming directly from the side reaches the closer ear about 600 µs before reaching the other ear. So, sounds coming from intermediate location reach the two ears at delays between zero and 600 µs.

Sound Localization (Sound Shadow)

Comparing the difference in intensity between two ears. Mainly for high frequency sounds with a wavelength shorter than the width of the head. This makes the sound louder for the closer ear. (up to 3000 Hz)

Sound Localization (Phase diference )

Detecting the difference in phases which provides information for localizing sound with low frequencies about 1500 Hz in humans

Every sound wave has phases with two consecutive peaks 360° apart waves are in phase when peak 360° when a sound originates to the side of the head the sound wave strike the two ears out of phase that can be detected. (how much out of phase depends on the frequency of sound, the size of the head and the direction of the sound)

Three Mechanical Senses

1. Proprioceptive Sensation

2. Tactile

3. Mechanical-Produced Pain

Proprioceptive Sensation

Provides head and body position that helps coordinate movements, joint position and gravity, and are regulated by vestibular organs (whose sensations we are seldom aware of) and several proprioceptors

Tactile Sensations

Includes light (fine) touch and pressure

Mechanical-Produced Pain

Includes pinching, squeezing and stretching (distortions of a mechanical receptor activates this)

Vestibular Sensations

Sensations mediated by a vestibular organ and detects the direction of tilt and the amount of acceleration of the head or even the body (these are sensations we are seldom aware of)

Vestibular Sense

The system that detects the movement of the head and body in their position to direct compensatory movement of the eyes (helps maintain balance of the body)

Vestibular Organ

Located in the ear and is adjacent to the cochlea and consist of two otolith organs: the saccule and utricle and three semicircular canals

Otoliths

Stone-like calcium carbonate particles floating in the endolymph and lie next to sensory hair cells and excite hair cells by a shearing action when the head tilt LINEARLY in different directions, and detect position

Three Semicircular Canals

Oriented in perpendicular planes and are also filled with endolymph and lined with hair cells. ANGULAR acceleration of the head at any angle causes this substance to push against hair cells (shearing action)

Movement of Otolith and the Three Semicircular Canals (what does it do?)

Those types of movements, fire action potentials from the vestibular system to travel via the 8th cranial nerve to the brainstem, spinal cord and cerebellum for controlling your posture into higher cortex for guiding your locomotor orientation

Mechanical senses (caused by?)

Light touch (discriminative touch), deep pressure, and mechanical pain (pinching, squeezing, or stretch)

Four Types of Touch Receptors

1. Simple bare neuron’s axonal terminals

2. A modified dendrite-like ending

3. An elaborated neuron’s axonal ending

4. A bare ending

Simple bare neuron’s axonal terminals

A touch receptor with a free nerve ending where many are pain receptors response to intense mechanical stimuli like pinching or squeezing; they have a higher threshold for being activated

Near base of hairs and elsewhere in skin

Free Nerve Endings

A sensory receptor that detects and transmit information about the stimuli to the spinal cord; the most abundant type of nerve ending

The Pacinian Corpuscle

A type of touch (onion-like) receptor that detects SUDDEN displacement or HIGH-frequency vibrations on the skin and conveys information about instant changes in tissue; resists gradual or constant pressure changes, instead is a RAPIDLY adapting receptor

Reacts to sudden or vibrating stimulus that bends the membrane and increases the flow of sodium ions to trigger an action potential

In both hairy and hairless skin

Merkel Disks

Modified dendrite-like receptors responding to LIGHT touch - SLOWLY adapting

Men and women generally have the same number but since women tend to have smaller fingers, they have a higher density leading to more sensitivity

Sensory Input (In the head)

Somatosensory information from touch receptors in the head enters the CNS through a few pairs of the cranial nerves

Sensory Input (In the body)

Information from receptors below the head enters the spinal cord → passes toward the brain through the 31 pairs of spinal nerves

Innervation (what is it? what are the components?)

The process by which nerves connect to a limited area of the body (i.e., tissue: muscles, organ, skin)

Contains a 1) sensory component and 2) motor component

Dermatome

An area where each sensory spinal nerve innervates a limited area (stripe-like) of the body

Sensory Information (path: from spinal cord to?)

Sent to the thalamus before traveling to the somatosensory cortex in the parietal lobe

Somatosensory Cortex

Part of the brain responsible for receiving (responds) and processing (decodes) various types of sensory information (i.e., tactile, vibration, pressure, proprioception, thermal, and pain) primarily from the contralateral side of the body

Localizes where the sensory stimulus is taking place

Information entering the spinal cord travels in different __ and __ pathways

Well-defined; distinct

The Tactile Path (what is it? pathway?)

A system that conveys information about fine and pressure, as well as proprioception and contains LARGE sized axons (thick myelinated axons)

Begins at mechanical receptors → their axons makes a cross at the level of the brainstem to the contralateral cortex

The Pain Path (what is it? pathway?)

A system that conveys sharp pain, slow dull or burning pain, and painfully cold and has two sets of smaller sized axons (thinly myelinated and unmyelinated axons)

Begins at the pain receptors in free nerve-endings, and crossing at the level of the spinal cord and ascends on the contralateral side of the spinal cord to the cortex

A patient whose myelinated axons are destroyed but unmyelinated axons are spared would __

Still feel temperature, pain, and itch, but loses sense of touch

Somatosensory Cortex (damage?)

Can result in the impairment of body perceptions (e.g., touch, body position, pain, ete.,) depending on the location affected and types of pathways affected

An Alzheimer’s patient with damage to the somatosensory cortex who can’t correctly point to their own elbow has a loss of what?

Proprioceptive sensation

Pain (caused by what three things?)

1. Mechanical stimuli

2. Thermal stimuli

3. Chemical stimuli

Pain (sensed by?)

The least specialized pain sensors, i.e., Free-nerve endings

Pain Sensors

Mechanically sensitive to intense stimuli like pinchin

Chemically sensitive to acids or other irritants

Activated by extreme temperatures

Nociceptive Receptor (nociceptors)

Specialized sensory receptors that detect potentially harmful stimuli like extreme temperature, pressure, or chemicals

Transient Potential Vanilloid-1 (TRPV1) Receptor

A nociceptive receptor that is sensitive to capsaicin, an ingredient in hot pepper that produces burning or stinging sensations

Pain Axons

Free-nerve endings that carry pain messages and are thinly or unmyelinated axons containing TRPV1 receptors with slower conduction velocity

Thinly myelinated axons are relatively faster at __

Conveying sharp pain (early pain)

Pain Axons (release what two neurotransmitters?)

1. Glutamate (GLU) for mild pain

2. Glutamate and substance P for stronger pain

Substance P

A neuromodulator or co-transmitter, released with GLU that results in the increased, prolonged intensity of pain

Opioid Mechanism System

A system that puts the brakes on prolonged pain; contains opioid receptors that are sensitive to (exogenously) opiate-like drugs (like morphine) and internally (endogenously) released opioid substances (neuropeptides)

Opioids

Also called endorphins, are endogenously released morphine-like substances that’s responsible for reducing prolonged pain by blocking the release of substance P in the spinal cord through activation of its receptors (locks dull pain but not sharp pain)

Endogenous Analgesia

The body’s natural pain control system; non-pain stimuli via 1) touch receptors and 2) descending opioid axons from the brain can modify (gate) the intensity of pain

e.g., if you stub your toe, a natural and effective reaction is to rub the site of injury for a minute, this would effectively alleviate the pain

Placebo (what is it? underlying mechanisms?)

A drug (or therapeutic procedure) with no pharmacological processes involved

Partially increases the release of opioids and dopamine

Evidence for Placebo

Patient testimony of pain decrease and brain images that show decreased response to pain

Decreases response in the cingulate cortex (limbic brain) not in the somatosensory cortex, so the placebo effects are mainly on emotional response to pain, not on the sensation itself

Analgesia

The way to alleviate pain by exogenous analgesic drugs (not placebo) whose pharmacological basis is to reduce (or block) pain signaling (pain path)

Cannabinoids

(chemical related to marijuana) block certain kinds of pain by mainly acting on cannabinoid receptors expressed in sensory neurons to inhibit pain transmission

Capsaicin

The active ingredient in peppers that produces a painful burning sensation by releasing substance P; BUT faster than neurons are able re-synthesize it, leaving the cells less able to send further pain messages

e.g., rubbing capsaicin on a painful area produces a temporary burning sensation followed by a longer period of decreased pain

Anti-depressant and Anxiolytics (alleviate?)

help alleviate the emotional-associated pain

Sensitization (what is it; how is it exacerbated? neurochemically?)

The pathological pain due to injury; the CNS has mechanisms to exacerbate pain after tissue has been damaged and inflamed

Neurochemically, it is a result of the body release histamine, nerve growth factor and other chemical that are necessary to repair the body → these are bioactive irritants that sensitize the responses of pain axons (pain from physiological → pathological)

Underlying Mechanisms for Pain Sensitization

Pain receptors become potentiated after an intense barrage of painful stimuli due to injury (i.e., increased sensory sensitivity) that leads to 1) allodynia and 2) hyperalgesia

Allodynia

Normally non-painful stimulus causing pain

Hyperalgesia

Slightly painful stimulus is now perceived as significantly more painful

Itch (what is it; what is it caused by?)

A separate kind of sensation from touch, or pain with at least two different variation, which activates distinct pathway that tends to be slower than other tactile sensations (slower than touch and pain); once spinal cord is reached, it activates gastrin-releasing peptide

Two Variations of Itch

1. Mild tissue damage (e.g., a cut) causing the skin to (in effort to heal) release histamines that dilate blood vessels and produce an itching sensation

2. Contact with certain plants also produces itch

Gastrin-Relasing Peptide

A chemical produced by a neuron that is activated by itch which evokes the scratch response

Differentiating Itch from Pain

Scratching produces mild pain, which inhibits itch; also opiates reduce pain but increase itch (itch is not a type of pain)

Gustatory System

A system by which chemical senses are mediated; including taste and smell that are done by chemical coding

Two Theories of Sensory Coding at Sensory Receptors

1. The Labeled-line principle

2. Across-fiber pattern