Lecture 11 From ear to thalamus

Interaural Time Difference (ITD): The difference in time that sound reaches each ear, which aids in localization.
Interaural Level Difference (ILD): Variability in sound pressure between the ears, helping to identify sound direction.
Head-Related Transfer Function (HRTF): Affects sound frequencies based on the listener's body shape, crucial for identifying vertical sound location.

Microanatomy: Specialized cells that detect sound waves.
Mechano-electrical Transduction: The process through which hair cells convert mechanical vibrations into electrical signals.

Auditory Nerve: Carries auditory signals from the cochlea to the brain.
Cochlear Nucleus: The initial hub for processing auditory signals.
Superior Olive: Analyzes sound direction using binaural (two-ear) cues.
Inferior Colliculus: Integrates various auditory information for further processing.
Medial Geniculate Nucleus: The thalamic relay center that channels auditory signals to the cortex.

Definition: Sound involves rapid fluctuations in pressure within a medium such as air.
Sound Pressure Level (SPL): Measures the magnitude of these pressure variations in decibels.
Frequency: Indicates how often pressure fluctuations occur (1 Hz represents one cycle per second).
Human Hearing Range: Ranges from 20 Hz to 20,000 Hz, with speech frequencies between 200 Hz to 2,000 Hz.
Speed of Sound: Approximately 340 m/s (760 mph) in dry air at sea level.

Interaural Time Difference (ITD): The time delay experienced by sounds arriving at each ear, influenced by the sound source's location.
Interaural Level Difference (ILD): The disparity in sound pressure levels between the ears, influenced by head shadow effects.
Head-Related Transfer Function (HRTF): Shapes sound characteristics for location identification, particularly in the vertical plane.

Description: The time difference in sound arrival that occurs when the sound source is not directly in front or behind.

Difference: Sound pressure levels vary by ear; higher frequency sounds are more significantly impacted.
Head Shadow: The effects of the head diminishing sound intensity on one side, creating a shadow in the sound wave.

Helps to shape the frequency response of sounds based on their source location, allowing for detailed spectral analysis.

Low Frequencies: Primarily utilize ITD for localization.
High Frequencies: Mainly rely on ILD cues.
Experimentation: Research by Wightman and Kistler examined control of ITD/ILD within virtual auditory environments.

Outer Ear: Consists of the pinna.
Middle Ear: Comprises the ossicles (malleus, incus, stapes) and the tympanic membrane.
Inner Ear: Includes the cochlea and auditory-vestibular nerve.

Mechanism: The tympanic membrane vibrates, with ossicles transmitting these vibrations to the oval window, effectively amplifying sound.

Cochlear Fluids: Involves perilymph (low potassium) and endolymph (high potassium).
Basilar Membrane Movement: Responds to various sound frequencies, critical for sound detection.
Hair Cells: Inner hair cells convey sound information to the brain, while outer hair cells play a role in amplifying sound signals.

Sound Wave Mechanism: Variations in pressure lead to traveling waves that affect specific sections of the basilar membrane based on frequency.
Frequency Sensitivity: The base reacts to high frequencies while the apex responds to low frequencies.

Cochlear Components: Comprised of one row of inner hair cells and three rows of outer hair cells, featuring stereocilia that form hair bundles.
Mechanism of Action: Depolarization occurs when stereocilia bend toward the kinocilium, while hyperpolarization results from bending away.

Transmission: Hair cells transmit auditory signals through the auditory nerve from spiral ganglion cells.
Frequency Information: The auditory nerve is organized tonotopically, reflecting the frequency of sound signals.

Low Frequencies (<4 kHz): Phase-locking does not happen for these frequencies, relying on tonotopic organization for signal processing.

Dorsal Cochlear Nucleus: Involved in complex spectral analysis before projecting to the inferior colliculus.
Superior Olive: The first site to receive binaural input for sound direction analysis.
Inferior Colliculus: Plays a role in integrating varied auditory information.
Medial Geniculate Nucleus (MGN): A thalamic relay station crucial for auditory information processing.

Functionality: React to sound location through ITD and ILD input.
Mechanism: Various neurons are responsive to differing ITDs and ILDs, aiding in spatial hearing.

Cues: Utilize ITD and ILD for horizontal sound localization; HRTF aids in vertical direction.
Frequency Analysis: The cochlea serves as a frequency analyzer, responding dynamically to traveling sound waves.
Hair Cell Function: Converts mechanical movement into neural signals for auditory processing.
Auditory Nerve: Transports organized frequency data to cerebral auditory centers.
Sound Localization: The superior olive plays a key role in processing binaural sound cues.