Auditory System and Hearing: Ear Anatomy, Sound Localization, and Disorders

Eustachian tube

Connects ear to throat to equalize pressure on either side of the eardrum.

Acoustic Reflex

Muscles (tensor tympani & stapedius) attached to ossicles contract, reducing ossicular movement about 25-50 ms following intense and/or low-frequency sounds.

Spontaneous Otoacoustic Emission

When there is cochlear movement, the ear can emit sound, rarely heard by the person whose ear it's coming from but can be heard by others nearby.

Grommet

A drain/tube.

Cochlea

Fluid (perilymph) filled tube divided into 3 canals: Vestibular canal, Middle canal, Tympanic canal.

Basilar Membrane

Tissue dividing cochlea into upper/lower sections; fluctuations in pressure produced by stapes movement create a traveling wave along it.

Organ of Corti

Receptor organ for sound that transforms vibrations into neural impulses; sits on the basilar membrane inside the middle canal.

Tectorial Membrane

Small membrane that arches over specialized cells called hair cells (stereocilia).

Inner Hair Cells

Single row of cells near tectorial membrane base (~7000 fewer than outer hair cells) that detect sound.

Outer Hair Cells

3-5 rows of cells on the opposite side of the tectorial membrane that contain muscle-like filaments that contract upon stimulation.

Place Code Theory

Sounds of different frequencies affect different places along the basilar membrane, generating signals to the brain via the auditory nerve.

Psychoacoustics

The examination of psychological responses to auditory stimuli.

Tinnitus

Hearing sound in the absence of external stimulation, usually results from neural activity but sometimes caused by cochlear movement.

Conductive Hearing Loss

Problems in the outer or middle ear that prevent sound conduction to the cochlea, such as wax, infection, or other blockages.

Sensorineural Hearing Loss

Problems in cochlea/auditory nerve, e.g., traumatic sound, stroke, blood clots.

Auditory Adaptation

When you overwork the auditory system, it becomes less efficient at detecting sounds.

Habituation

Attention to signal decreases, leading to paying less attention to it.

Auditory Fatigue

Hearing some sounds over time can create temporary or permanent threshold changes.

Acoustic Trauma

Damage to hair cells resulting from excessive noise and/or very loud sounds.

Bone Anchored Hearing Aid (BAHA)

Embedded in bone near the pinna, transmits sound by vibrations.

Recruitment

Sensorineural damage causing specific ranges of hearing loss.

Cochlear Implant

External mic connected to microelectrodes attached to the cochlea; electronically stimulates afferent neurons.

Sound

The perceptual experience of acoustic energy.

Sound Stimulus

Movement through an elastic medium (air, water, bone) brought about by vibration of an object.

Vibrations

Transmitted best when molecules are more densely packed (water faster than air).

Speed of Sound in Air

343 m/s

Speed of Sound in Water

1,500 m/s

Amplitude

Pressure change; perceptual experience of loudness (larger = louder).

Hertz (Hz)

Measure of sound frequency (cycles/second).

Low-frequency sound

Has long wavelengths (~1 Hz).

High-frequency sound

Has short wavelengths (~3 Hz).

Decibels (dB)

Measure of intensity of sound.

Human range of hearing

Ranges up to 140 dB.

0 dB

Can't hear.

140 dB

Painful → permanent damage.

Pure Tone

Sounds of a particular temporal frequency (can't be broken down anymore).

Complex Sounds

Most of our auditory experiences are made up of combinations of sounds.

Phase Angle

Where the sound wave begins in its cycle.

Inverting the Phase of Sound Waves

Add inverse ones together → possible no sound (how noise-canceling headphones work).

Outer Ear (Pinna)

Collects sound; its shape helps us locate sound.

External Auditory Canal

Small passageway leading to tympanic membrane.

Tympanic Membrane (Eardrum)

End of the auditory canal; sound waves hit this → causes it to vibrate.

Ossicles (Ossicular Bridge)

Hammer (Malleus), Anvil (Incus), Stirrup (Stapes); transmit sound to the oval window (of cochlea).

Cone of confusion

Any point that keeps equal distance between ears.

Sound localization

Helps to guide our eyes to a location of interest.

Pinnae

Angled and contoured so sounds are reflected into auditory canals in a unique way depending on location.

Location

A perceptual feature we perform on sounds heard relative to the positioning of our ears.

Interaural Time Differences (ITD)

Relative arrival times of sounds to each hemisphere give two pieces of information: onset difference (as small as 0.3 msec) and phase difference (good for locating continuous sounds with no specific onset difference).

Interaural Level Differences (ILD)

The source of a sound is based on the relative intensity of sound at each ear; the ear closer to the source receives greater intensity.

Sound Shadow

The head produces a 'sound shadow' when sound arrives from the side — one ear gets direct sound while the other is shaded by the head.

Medial Geniculate Nucleus (MGN)

Relays auditory information to the primary auditory cortex.

Reverberation Time

The delay between direct and reflected sound, affecting spatial perception.

Auditory System Pathways

The auditory system is divided into pathways that process 'What' and 'Where' information.

Auditory Stream Segregation

The brain groups sounds by source or location to form perceptual streams.

Parabelt Area

Part of the auditory cortex for higher-level processing.

Cochlear Nucleus

The first brainstem structure to receive auditory input from inner hair cells.

Biosonar Systems

Most effective when animals emit sounds of very high frequency.

Doppler Shifts

Help estimate the speed of objects relative to the animal.

Tonotopic Frequency Organization

Cells in the primary auditory cortex (A1) are organized according to specific frequencies.

Sound Localization Underwater

Difficult because sound travels faster, reducing interaural differences.

Pitch

The perceptual correlate of frequency.

Hearing Loss Intensity Level

Exposure to sounds above 150 dB can cause immediate and permanent hearing damage.

Normal Hearing Range

Humans hear roughly between 20 Hz and 20,000 Hz, with high-frequency loss common in aging.

Harmonics

Frequencies that are integer multiples of the fundamental frequency, adding richness and defining timbre in sound.

Fundamental Frequency

The lowest frequency component determining pitch.

Tympanic Membrane Function

To vibrate in response to sound waves entering the ear canal.