Auditory Cognitive Neuroscience: Sound and Hearing

• Auditory Cognitive Neuroscience focuses on sound and hearing

Announcements

• The syllabus should be reviewed.
• Activity 1 is a thought paper on the Dahary et al. (2023) paper that’s due next week.
• Search for a project partner; a partner quiz is available online.

Anatomy of the Ear

• The ear guides incoming music signals to the brain.
• Auditory transduction is the route sound takes to get to the brain through the ear.

Sound

• Sound is a mechanical disturbance of a medium (gas, liquid, or solid).

Speed of Sound

• The speed of a disturbance depends on the density of the medium, and strength of intermolecular forces
• Heavier molecules take longer to start and stop moving
• Stronger forces will push faster and cause acceleration
• To calculate the speed of sound the following formula is used: c = square root of E / ρ
• c represents the speed in meters per second (ms⁻¹)
• ρ is the density of the material (in kg m⁻³)
• E is the Young's modulus of the material (in N m⁻²)
• The Young's Modulus applies to solids; the adiabatic bulk modulus applies to air.

Sound Properties

• Some sounds originate from a single, "aperiodic" disturbance, like a thud or clap.
• The compressions and refractions of some sounds are periodic, meaning they recur in certain intervals.
• This is how pitch is perceived, which is present in both music and speech.

Sine Waves

• Sine Waves contain compressions and rarefactions
• Sine waves have three parameters: amplitude, frequency, and phase

Sine Waves - Amplitude

• Amplitude is the sounds maximum displacement of molecules from one extreme to resting position
• Amplitude is perceived as volume
• Decibels (dB) are the units for perceived volume.
• Amplitude envelope refers to changes to amplitude over time.
• Changes to the amplitude envelope are perceived as timbre, or the quality of a sound.

Sine Waves - Frequency

• Periodic vibration explains how frequently the wave repeats itself.
• Changes in frequency are perceived as changes to pitch
• Frequency (F) refers to the number of cycles per second, measured in Hertz (Hz).
• Period (T) Measures time (sec) to complete one cycle
• Frequency = 1 / Period

Sine Waves - Phase

• The starting position is the phase, defined as the progression of wave through one cycle, and measured in degrees
• Changes to the phase of a sound wave are perceived as changes to the quality or timbre of a sound.
• Combining 2 waves together results in twice the amplitude

Anatomy of the Ear

• There are 3 main components to the ear that each contribute to hearing: outer, middle and inner ear.
• The outer ear is important for locating sound and enhancing high frequencies
• The pinna is a flap of skin and cartilage with grooves, ridges and depressions but is individually unique
• The concha is a depression at the entrance of the ear, and functions as an acoustic resonant cavity
• The pinna and concha help determine if a sound is coming from the front or back
• The Auditory Canal is a 25-35 mm path between the concha and the tympanic membrane and amplifies the sounds between 2-4 kHz
• The Tympanic Membrane or ear drum, is a light, elastic structure. It transforms acoustic pressure variations from the environment into mechanical vibrations in the middle ear

Middle Ear - Ossicles

• The middle ear has 2 functions:
• Transmits movement of the tympanic membrane to the fluid which fills the cochlea without significant energy loss.
• Since sound travels through different media (air and fluid), there is lots of energy loss. To compensate for energy loss, the ear has several structural modifications that create impedance matching.
• Tympanic membrane to stapes footplate ratio = 13:1; Malleus to stapes ratio = 1.3:1
• Pressure at the stapes footplate is about 33.8 times larger than the pressure at the tympanic membrane.
• Protect the ear from loud sounds Acoustic reflex describes 2 key muscles (stapedius muscle and tensor tympani) that contract automatically in response to sounds >75 dB. The contraction reduces the efficiency of the vibrations transmitted by the tympanic membrane - the stapedius pulls the stapes away from the oval window, and the tensor tympani pulls the malleus away from the ear drum The ear provides roughly 12-14 dB of attenuation for frequencies under 1000 Hz.
• Reflex contraction is not always fast enough and takes approximately 60-120 ms

Inner Ear

• Converts mechanical vibrations from the stapes into nerve firings that are processed by the brain via the cochlea

• Cochlea: a tube coiled into a spiral with one end at the base and the other at the apex

• Reissner's membrane and the basilar membrane create three tube sections:

• Scala tympani (T)

• Scala vestibuli (V)

• Scala media (M)

• Scala tympani and vestibuli are filled with perilymph (incompressible fluid)

• Scala vestibuli ends at the oval window, and the scala tympani ends at the round window.

• The helicotrema is a small hole where the perilymph can flow through.

• Acoustic vibrations cause the stapes bone to move at the oval window, and move the perilymph in the cochlea.

• The membrane covering the round window moves to compensate for the movement in the oval window

• The perilymph is incompressible.

• Stapes moves in = round window membrane moves out

• Stapes moves out = round window membrane moves in

• This movement causes travelling waves in the scala vestibuli, displacing Reissner's membrane and the basilar membrane.

Basilar Membrane

• The basilar membrane carries out the frequency analysis of sounds by decomposing complex sounds into pure tones, which is essentially a Fourier transformation.
• The following describes the base of the basilar membrane:
• Higher frequencies
• Smaller, stiffer structures respond better to higher frequencies.
• The following describes the apex of the basilar membrane:
• Lower frequencies
• Larger, more flexible or "floppier" less stiff structures respond better to lower frequencies.
• Humans hear in the range of 20 - 20000 Hz.

Organ of Corti

• Movements of the basilar membrane are converted into nerve firings by the Organ of Corti.
• The organ consists of hair cells that trigger nerve firings when they are bent due to the wave movement.
• The nerve is a bundle of auditory nerves that go the brain for processing.

Place Theory

• The Place Theory of Pitch Perception asserts that the basilar membrane is a set of independently tuned resonators.
• Sounds/tones of single frequencies cause a specific place on basilar membrane to vibrate at the most (maximum displacement).
• If a pure tone is input into the ear, only one specific area of the basilar membrane would vibrate.
• If a complex tone is input, the basilar membrane response is basically the sum of responses of a component.
• This theory works because the auditory cortex is organized according to a tonotopic or cochleotopic map.

Volley Theory

• The basilar membrane vibrates to match the frequency.
• Nerve fibers cannot encode high frequencies over 2000 Hz because of the refractory period
• Nerve fibers work together to encode high frequencies..

Cochlear Implants

• Cochlear implants are surgical devices used by profoundly or completely deaf individuals
• They bypass the damaged ear or hair cells and send electrical signals to the auditory nerve to stimulate the cochlea
• They do not work for people with cortical deafness, which is very rare
• The microphone picks up sounds, and an audio processor converts sounds into digital code through channels that each correspond to a different frequency
• An implant transforms the digital code into an electrical signal and sends it to an electrode array positioned on the cochlea
• Lastly the cochlea sends that information to the brain via the auditory nerve.
• People with normal hearing have thousands of hair cells, each responding to different frequencies.
• Individuals with implants have implanted channels of about 22 "hair cells" -This makes it more difficult to relay information about pitch & timbre. -Individuals may have a difficult time understanding information about music, emotions and tone in languages.
• Those with implants can train their brains using music therapy, which will help efficiently interpret sounds.