Sound pressure wave
- Sound is a mechanical wave that results from the back and forth vibration of the particles of the medium through which the sound wave is moving.
- If a sound wave is moving from left to right through air, then particles of air will be displaced both rightward and leftward as the energy of the sound wave passes through it.
- The motion of the particles is parallel (and anti-parallel) to the direction of the energy transport.
- A sine wave can be used to encode information about the compression and rarefaction (expansion) of a sound pressure wave.
Increases of the physical property of _______ from low to high are associated
- with increases in the perceptual experience of ______.
- frequency; pitch
Increases of the physical property of _______ from small to large (blue arrow)
- are associated with increases in the perceptual experience of _______.
- amplitude; loudness
The _______ stays constant over distance, but the _______ of the sound
- pressure wave decreases.
- wavelength; amplitude
- Basically, the energy contained in the sound pressure wave is lost as the sound pressure wave propagates through the atmosphere.
- As a result, the amount of compression and rarefaction (amplitude of the wave) decreases over distance, but the timing of the compression and rarefaction (frequency) does not.
a graph that depicts the relationship between the loudness of a pure tone, expressed in decibels, sound-pressure level (dB SPL), and the frequency of the tone.
- includes the pinna (AKA auricle) and auditory canal and ends at the tympanic membrane.
- The pinna consists of cartilage covered by skin and is shaped to capture sound waves and funnel them through the ear canal to the tympanic membrane.
- The pinna is important in localizing sound from front vs. back and helps with vertical sound localization (i.e., is sound higher or lower).
- The ear canal directs the sound pressure wave onto the tympanic membrane and also amplifies sounds that are between 3 and 12 kHz.
(AKA eardrum) is a thin membrane that separates the external ear from the middle ear whose function is to transmit sound from the air to the ossicles inside the middle ear
from tympanic membrane to oval window; includes the ossicles and drains out through the eustachian tube to the back of the throat.
- The ossicles (also called auditory ossicles) are the three smallest bones in the human body. They are contained within the middle ear space and serve to transmit and amplify sounds from the air to the fluid-filled cochlea.
- The absence of the auditory ossicles would constitute a moderate-to-severe hearing loss.
The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum
- The incus or anvil is the anvil-shaped small bone or ossicle in the middle ear.
- It connects the malleus to the stapes.
- The stapes or stirrup is the stirrup-shaped small bone or ossicle in the middle ear which is attached to the incus and oval window
- the bottom of the stapes on the oval window is called the footplate
The oval window is a membrane-covered opening which leads from the middle ear to the vestibule of the inner ear
Conductive hearing loss
Mechanical hearing loss, resulting from blockage in the ear canal, a ruptured eardrum, or restriction of the movement of the tiny bones in the middle ear, which prevents sound vibrations being transferred to the cochlea. Seen in otosclerosis
- a form of conductive hearing loss
- a condition in which there is abnormal growth of bone of the middle ear which can result in hearing loss.
- Seen in 0.5%-10% of population, usually starts in middle age.
- Exact cause is unclear – genetic factors play a role, viruses like measles may be involved as well.
- Treated with hearing aids and/or surgery to remove the stapes.
- from oval window to auditory nerve
- includes oval window, round window, cochlea, auditory nerve fibers, and the semicircular canals of the vestibular system.
- The coiled and channeled main structure of the inner ear, which contains three fluid-filled canals that run along its entire convoluted length
- the fluid-filled canals are separated by membranes, one of which is the basilar membrane, on which thousands of hair cells (auditory receptors) are arranged and are stimulated by the vibration of the stapes.
The basilar membrane within the cochlea of the inner ear is a stiff structural element that separates two liquid-filled tubes (the scala – you don’t need to know these) that run along the coil of the cochlea, forming a base for the hair cells to transduce the sound waves in the cochlear fluid to electrochemical signals in the brain
- map of tones:
- Each section of the basilar membrane responds to a preferential frequency and the sections are organized from high to low.
- Tonotopic organization is also seen in the cortex as tonotopic gradients (organized cortical representations of tones).
Inner hair cells
- the sensory receptors of the auditory system located on the basilar membrane in the cochlea that convert sound waves to nerve signals by having their hairlike stereocilia being physically moved by sound waves in the cochlear fluid.
- Hair cells are columnar cells, each with a bundle of 100-200 specialized stereocilia at the top, for which they are named.
- These cilia are the mechanosensors for hearing.
- Lightly resting atop the longest cilia is the tectorial membrane, which moves back and forth with each cycle of sound, tilting the cilia and allowing electric current into the hair cell.
- Hair cells, like the photoreceptors of the eye, show a graded response, instead of the spikes typical of other neurons.
- Loud noise can damage and destroy hair cells, which do not regrow. Continued exposure to loud noise causes progressive damage, eventually resulting in hearing loss and sometimes ringing in the ears (tinnitus).
Stereocilia and Kinocilium
- stereocilia are projections at the top of the hair cell that are attached to one another by structures which link the tips of one cilium to another.
- Stretching and compressing the tip links may open an ion channel and produce the receptor potential in the hair cell
- The kinocilium is one larger, more stable cilium to which the stereocilium attach at the tips.
Outer hair cells
- hair-like cells on basilar membrane that are involved in amplifying sounds and improving frequency selectivity; only found in mammals.
- Although there are nearly 3x more outer hair cells than inner hair cells, outer hair cells do not directly transduce sound pressure waves to neural signals.
Organ of Corti
- The organ of Corti is the organ in the inner ear of mammals that contains the hair cells (the auditory sensory cells).
- Transduction occurs through vibrations of structures in the inner ear causing displacement of cochlear fluid and movement of hair cells at the organ of Corti to produce electrochemical signals that activate the auditory nerve fibers synapsing on the inner hair cells.
Sensorineural hearing loss
hearing loss caused by damage to the sensory cells and/or nerve fibers of the vestibulocochlear nerve (auditory nerve / Cranial nerve VIII), the inner ear, or central processing centers of the brain. Seen in many forms of congenital and acquired deafness
- is a surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf or severely hard of hearing.
- Most commonly, a cochlear implant is used when the hair cells of the patient are damaged/developed with a genetic abnormally affecting action potentials. In order for a cochlear implant to work, the auditory nerve fibers (along the basilar membrane) must still be intact, as the electrodes of the implant serve to activate these auditory nerve fibers.
- (If the auditory nerve is damaged, a cochlear implant will not work.)
a group of cell bodies in the lower section (medulla) of the brainstem that receives the inputs from all the auditory nerve fibers coming from the cochlea
a small group of cell bodies (nucleus) in the middle section (pons) of the brainstem involved in the localization of sound by determining differences in the timing (medial superior olive) and intensity/level (lateral superior olive) of neural responses from each ear for a particular sound.
Medial superior olive
Interaural time difference – the time difference of arrival of sounds between the ears
Lateral superior olive
Interaural level difference – the difference of the intensity level of sounds between the ears
Inferior colliculus (IC)
- Located just below the visual processing centers known as the superior colliculus.
- It also contains neurons that are tonotopically organized, and it likely integrates information regarding sound localization.
- The IC projects to the thalamus (MGN) and cortex.
Medial geniculate nucleus (MGN)
- section of the thalamus that the auditory pathway connects through prior to reaching primary auditory cortex (A1).
- (Similar to how visual input connects through the lateral geniculate nucleus [LGN] of the thalamus before reaching primary visual cortex [V1]).
- The lateral sulcus (also called Sylvian fissure or lateral fissure; fissure = large sulcus) is the sulcus that divides the frontal and temporal lobes of the brain.
- Primary auditory cortex (A1) is located within the lateral sulcus.
Superior temporal gyrus
The most superior gyrus in the temporal lobe, situated just below the lateral sulcus, on which is much of auditory cortex
Superior temporal sulcus
The most superior sulcus in the temporal lobe, situated just below the superior temporal gyrus
Primary auditory cortex (A1)
- the main area of cortex which first processes auditory information in the brain, situated on the inferior surface of the lateral sulcus
- contains core, belt, and parabelt subdivisions.
- Each subdivision contains multiple auditory field maps.
- is a cortical map of sound frequency (single tones)
- is a cortical map of sound time duration (periodicity)
is a rare form of sensorineural hearing loss caused by bilateral cortical lesions in the primary auditory cortex located in the temporal lobes of the brain (although it is likely actually damage to primary and/or neighboring regions of auditory cortex).
Cortical deafness is
- an auditory disorder where the patient is unable to hear sounds but has no apparent damage to the anatomy of the ear, which can be thought of as the combination of auditory verbal agnosia and auditory agnosia.
- Patients with cortical deafness cannot hear any sounds, that is, they are not aware of sounds including nonspeech, voices, and speech sounds.
- Although patients appear and feel completely deaf, they can still exhibit some reflex responses such as turning their head towards a loud sound.
- is a rare form of agnosia that manifests itself primarily in the inability to recognize or differentiate between sounds.
- It is not a defect of the ear or "hearing", but rather a neurological inability of the brain to process sound meaning.
- It is caused by bilateral damage to the anterior superior temporal gyrus, which is part of the auditory pathway responsible for sound recognition, the auditory "what" pathway.
- Persons with auditory agnosia can physically hear the sounds and describe them using unrelated terms, but are unable to recognize them.
- They do not tend to report ‘feeling deaf.’
Pure word deafness (auditory verbal agnosia)
- the selective inability to comprehend the spoken word, in the absence of aphasia or defective basic hearing.
- Perception of environmental sounds and other complex, non-speech sounds is generally normal.
- Pure word deafness is usually caused by bilateral damage to temporal lobes (often including white matter connecting temporal and frontal lobes).
- Patients can still read and write.
Non-verbal auditory agnosia
the selective impairment in nonverbal (e.g., environmental sounds) auditory comprehension, , in the absence of verbal comprehension deficits, other aphasias, or defective basic hearing; may arise from lesions in/near Wernicke’s area
- the selective inability to recognize musical tones or to reproduce them (agnosia for music).
- It involves loss of the ability to recognize musical notes, rhythms, and intervals and the inability to experience music as musical.
- Patients often describe music as being indistinguishable from the sound of pots and pans banging together.
- Amusia can be congenital (present at birth) or be acquired sometime later in life (as from brain damage).
- is a comprehension deficit specific to the auditory modality.
- Written comprehension is unimpaired. It is distinct from pure word deafness in that the ability to repeat is intact.
- (See similar selective retention of repetition in language aphasias next.)