Hearing: Physiology and Psychoacoustics - Notes

Hearing Physiology and Psychoacoustics

Overview

• This section covers the physiology and psychoacoustics of hearing.
• Topics include the nature of sound, the auditory system's anatomy and physiology, loudness and pitch perception, and hearing impairments and their amelioration.

The Function of Hearing

• Sounds originate from object vibrations which create pressure changes in a surrounding medium.

What Is Sound?

• Sound waves travel at a specific speed dependent on the medium (e.g., 340 meters/second in air, 1,500 meters/second in water).
• Physical qualities of sound waves:
  • Amplitude (Intensity): The magnitude of displacement of a sound pressure wave, perceived as loudness.
  • Frequency: The number of times per second a pressure pattern repeats, perceived as pitch.
• Units for measuring sound:
  • Hertz (Hz): Frequency unit; 1 Hz = 1 cycle per second.
  • Decibel (dB): Physical intensity unit; defines the ratio between two sound pressures.
    • Each 10:1 sound pressure ratio equals 20 dB, and a 100:1 ratio equals 40 dB.
• Psychological qualities of sound:
  • Loudness: Psychological aspect related to perceived intensity (amplitude).
  • Pitch: Psychological aspect mainly related to perceived frequency.
• Low-frequency sounds correspond to low pitches, while high-frequency sounds correspond to high pitches.
• Human hearing uses a limited range of frequencies (Hz) and sound pressure levels (dB).
• The ratio between the faintest and loudest sounds humans can hear is more than 1:1,000,000.
• Sound levels are measured on a logarithmic scale in decibels (dB).
  • A small decibel change (e.g., 6 dB increase) corresponds to a doubling of pressure.
• Sine waves (pure tones) are simple sounds but are not common in everyday life.
  • Sine Wave: Waveform for which variation as a function of time is a sine function.
• Most sounds are complex and can be described as combinations of sine waves using Fourier Analysis.
• Complex sounds are described as a spectrum, displaying energy distribution across frequencies.
• Harmonic Spectrum: Spectrum of a complex sound with energy at integer multiples of the fundamental frequency.
  • Typically caused by a simple vibrating source (e.g., guitar string, saxophone reed).
  • Fundamental Frequency: The lowest-frequency component of a complex periodic sound.
• Timbre: Psychological sensation allowing listeners to judge dissimilarity between sounds with the same loudness and pitch. It's conveyed by harmonics and other high frequencies.

Basic Structure of the Mammalian Auditory System

• The sense of hearing evolved over millions of years, with different animals having varying hearing capabilities.
• Outer Ear:
  • Pinnae collect sounds from the environment.
  • Sound waves are funneled by the pinnae into the ear canal.
  • The ear canal's length and shape enhance certain sound frequencies.
  • Purpose of the ear canal: collect sound waves, funnel them to the tympanic membrane and insulate/protect the tympanic membrane.
  • Tympanic Membrane (Eardrum): A thin sheet of skin at the end of the outer ear canal that vibrates in response to sound. Puncturing it will likely heal itself, but it’s still possible to damage it beyond repair.
• Middle Ear:
  • Consists of three tiny bones (ossicles) that amplify and transmit sounds to the inner ear.
  • Ossicles: Malleus, Incus, and Stapes.
  • Malleus: Receives vibrations from the tympanic membrane and is attached to the incus.
  • Incus: The middle ossicle.
  • Stapes: Connected to the incus on one end and the oval window of the cochlea on the other (border between middle and inner ear).
  • Amplification by ossicles is essential for hearing faint sounds; hinged joints act like levers, and the stapes concentrates sound energy due to its smaller surface area.
  • Tensor tympani and stapedius muscles in the middle ear decrease ossicle vibrations when tensed to muffle loud sounds and protect the inner ear. The acoustic reflex follows the onset of loud sounds by 200 ms, so it cannot protect against abrupt sounds.
• Inner Ear:
  • Fine changes in sound pressure are transduced into neural signals; analogous to the retina's function in vision.
  • Cochlea: Spiral structure containing the organ of Corti and filled with watery fluids in three parallel canals.
• Cochlear Canals and Membranes:
  • Vestibular Canal: Extends from the oval window (base) to the helicotrema (apex); pressure waves move through it first.
  • Tympanic Canal: Extends from the helicotrema (apex) to the round window (base).
  • Middle Canal: Sandwiched between the vestibular and tympanic canals and contains the cochlear partition.
  • Reissner’s Membrane: Separates the vestibular and middle canals.
  • Basilar Membrane: Forms the base of the cochlear partition, separating the middle and tympanic canals.
  • Vibrations transmitted through the tympanic membrane and ossicles cause the stapes to push/pull the oval window, moving fluid in the vestibular canal. Any remaining pressure from intense sound is transmitted through the helicotrema back to the cochlear base through the tympanic canal, then released through the round window.
• Organ of Corti:
  • Structure on the basilar membrane, composed of hair cells and auditory nerve fiber dendrites.
  • Movements of the cochlear partition are translated into neural signals by structures in the organ of Corti.
  • Hair Cells: Support stereocilia and transduce mechanical movement into neural activity sent to the brainstem and some receive input from the brain. Arranged in four rows along the basilar membrane.
  • Tectorial Membrane: Gelatinous structure attached on one end, extending into the middle canal, floating above inner hair cells, and touching outer hair cells. Vibrations displace the tectorial membrane, bending stereocilia and releasing neurotransmitters.
  • Stereocilia: Hairlike extensions on hair cells that initiate neurotransmitter release when flexed. Tips are connected to neighbors by tip links.
• Coding of Amplitude and Frequency in the Cochlea:
  • Place Code: Different parts of the cochlea are tuned to different frequencies; information about frequency is coded by the location of greatest mechanical displacement along the cochlear partition.
  • Inner Hair Cells: Convey almost all information about sound waves to the brain (using afferent fibers).
  • Outer Hair Cells: Receive information from the brain (using efferent fibers) and are involved in a feedback system.
• Auditory Nerve (AN):
  • Responses of individual AN fibers to different frequencies depend on their place along the cochlear partition.
  • Frequency Selectivity: Clearest when sounds are very faint.
  • Threshold Tuning Curve: Graph plotting thresholds of a neuron or fiber in response to sine waves with varying frequencies at the lowest intensity that will give rise to a response.
  • Outer hair cells receive feedback from the brain and can make parts of the cochlear partition stiffer, which makes the responses of inner hair cells more sensitive and more sharply tuned to specific frequencies.
  • Two-Tone Suppression: Decrease in the response (firing rate) of one auditory nerve fiber to one tone when a second tone is presented at the same time.
  • Rate Saturation:
    • Occurs when a nerve fiber is firing as rapidly as possible, and further stimulation cannot increase the firing rate.
    • Isointensity Curves: Chart measuring an AN fiber’s firing rate to a wide range of frequencies, all presented at the same intensity level.
  • Rate-Intensity Function: Graph plotting the firing rate of an auditory nerve fiber in response to a sound of constant frequency at increasing intensities.
• Temporal Code for Sound Frequency:
  • Phase Locking: Firing of a single neuron at one distinct point in the period (cycle) of a sound wave at a given frequency, providing a temporal code.
  • Temporal Code: Tuning of different parts of the cochlea to different frequencies, in which information about a particular frequency of an incoming sound wave is coded by the timing of neural firing as it relates to the period of the sound.
  • Volley Principle: Multiple neurons can provide a temporal code for frequency if each neuron fires at a distinct point in the period of a sound wave but does not fire on every period.
• Auditory Brain Structures:
  • Cochlear Nucleus: First brainstem nucleus where afferent auditory nerve fibers synapse.
  • Superior Olive: Early brainstem region where inputs from both ears converge.
  • Inferior Colliculus: Midbrain nucleus in the auditory pathway.
  • Medial Geniculate Nucleus: Part of the thalamus that relays auditory signals to the temporal cortex and receives input from the auditory cortex.
  • Primary Auditory Cortex (A1): First area within the temporal lobes responsible for processing acoustic organization.
  • Belt Area: Region of cortex adjacent to A1, with inputs from A1, where neurons respond to more complex characteristics of sounds.
  • Parabelt Area: Region of cortex lateral and adjacent to the belt area, responding to more complex characteristics of sounds and input from other senses.
  • Tonotopic Organization: Neurons that respond to different frequencies are organized anatomically in order of frequency, starting in the cochlea and maintained through the primary auditory cortex (A1).
• Comparison of Auditory and Visual Systems:
  • Auditory system does a large proportion of processing before A1; visual system does a large proportion of processing beyond V1; differences may be due to evolutionary reasons.

Basic Operating Characteristics of the Auditory System

• Psychoacoustics: Branch of psychophysics studying the psychological correlates of physical dimensions of acoustics to understand the auditory system's operation.
• Intensity and Loudness:
  • Audibility Threshold: Lowest sound pressure level reliably detected at a given frequency.
  • Equal-Loudness Curve: Graph plotting sound pressure level (dB SPL) against the frequency for which a listener perceives constant loudness.
  • Frequency composition is the determinant of how we hear sounds.
• Temporal Integration: Process by which a constant-level sound is perceived as louder when it is of greater duration.
• Masking: Using a second sound, frequently noise, to make the detection of another sound more difficult.
  • White Noise: Noise consisting of all audible frequencies in equal amounts; analogous to white light in vision.
  • Critical Bandwidth: The range of frequencies conveyed within a channel in the auditory system.

Hearing Loss

• Hearing can be impaired by blockage or damage to auditory processing structures, such as obstruction of the ear canal (e.g., earplugs) or excessive ear wax (cerumen).
• Conductive Hearing Loss: Caused by problems with the bones of the middle ear.
  • Otosclerosis: Abnormal growth of middle ear bones; can be remedied by surgery.
• Sensorineural Hearing Loss: Due to defects in the cochlea or auditory nerve, often from hair cell injury (e.g., infection or ototoxic antibiotics/cancer drugs). Common cause is damage to hair cells due to excessive noise exposure.
• Presbycusis: Age-related hearing loss.
  • Young people: range of 20–20,000 Hz
  • By college age: 20–15,000 Hz
• Hearing Aids:
  • Electronic hearing aids compress sound intensities into a range the user can hear.
• Cochlear Implants:
  • Flexible coils with miniature electrode contacts threaded through the round window toward the cochlea apex.
  • A microphone transmits radio signals to a receiver in the scalp.
  • Signals activate miniature electrodes at appropriate positions along the cochlear implant.