Audition, the Body Senses, and the Chemical Senses - Audition

Audition

  • Hearing (audition) has three main roles:
    • Detecting sounds.
    • Locating sound sources.
    • Recognizing sound sources and their meaning.

The Stimulus

  • Sounds are created by vibrating objects that move air molecules.
  • These vibrations create alternating compressions and expansions of air, forming waves that travel at approximately 1,200 km/h.
  • Humans perceive these waves as sound if the vibration ranges between 30 and 20,000 times per second.
  • Sounds vary in:
    • Pitch: determined by the frequency of vibration (measured in Hertz, Hz – cycles per second).
    • Loudness: a function of intensity, reflecting the degree of compression and expansion of air.
    • Timbre: provides information about the nature of the sound (e.g., oboe vs. train whistle), determined by the mixture of different frequencies.

Anatomy of the Ear

  • The ear is divided into the outer, middle, and inner ear.

Outer Ear

  • Pinna (external ear) funnels sound through the ear canal to the tympanic membrane (eardrum).
  • The eardrum vibrates with the sound.
  • Damage to the eardrum impairs hearing, especially for low frequencies.
  • Pinna shape aids in sound localization; altering it disrupts sound localization ability, but auditory adaptation can occur over several days.

Middle Ear

  • Small, hollow region behind the tympanic membrane.
  • Contains ossicles (middle ear bones): malleus (hammer), incus (anvil), and stapes (stirrup).
  • The malleus connects to the eardrum and transmits vibrations to the incus and then to the stapes, which presses against the oval window (opening in the cochlea).
  • Ossicles efficiently transmit energy; the stapes makes smaller, more forceful excursions against the oval window compared to the eardrum's movement against the malleus.

Inner Ear

  • Cochlea: snail-shaped structure filled with fluid.
  • Incoming sound energy is transferred into the liquid medium of the cochlea.
  • The chain of ossicles serves as an efficient means of energy transmission, overcoming the inefficiency of airborne sound directly impinging against the oval window (99.9% energy loss).
  • Divided into three sections: scala vestibuli, scala media, and scala tympani.
  • Organ of Corti: the receptive organ consisting of the basilar membrane, hair cells, and tectorial membrane.
  • A flexible membrane-covered opening, the round window, allows the fluid inside the cochlea to move back and forth.

Auditory Hair Cells and the Transduction of Auditory Information

  • Inner and outer auditory hair cells are the auditory receptors located on the basilar membrane.
  • The human cochlea contains approximately 3,500 inner hair cells and 12,000 outer hair cells.
  • Hair cells have cilia (fine, hairlike projections) arranged in rows according to height.
  • Hair cells form synapses with dendrites of bipolar neurons, sending auditory information to the brain.
  • Inner hair cells are essential for normal hearing.
  • Outer hair cells act as effector cells, modifying the mechanical characteristics of the basilar membrane, influencing sound vibrations on inner hair cells.
  • The bases of the cilia are attached to the basilar membrane.
  • The tips of the cilia of outer hair cells are attached to the tectorial membrane above.
  • Sound waves cause both the basilar membrane and the tectorial membrane to flex up and down.
  • These movements bend the cilia of the hair cells in one direction or the other.
  • The cilia of the inner hair cells do not touch the overlying tectorial membrane, but the relative movement of the two membranes causes the fluid within the cochlea to flow past them, making them bend back and forth, too.
  • Cilia contain actin and myosin filaments, making them rigid.
  • Adjacent cilia are linked by elastic filaments known as tip links, attached to insertional plaques.
  • Tip links are slightly stretched, creating tension.
  • Bending of cilia bundle causes receptor potentials.
  • The fluid surrounding auditory hair cells is rich in potassium.
  • Each insertional plaque contains a cation channel (TRPA1).
  • When the cilia bundle is straight, the probability of an ion channel being open is approximately 10 percent.
  • Small amounts of K^+ and Ca^{2+} diffuse into the cilium.
  • Movement toward the tallest cilium increases tension, opening all ion channels, increasing cation flow, and depolarizing the membrane, increasing neurotransmitter release.
  • Movement toward the shortest cilium relaxes tip links, closing ion channels, ceasing cation influx, hyperpolarizing the membrane, and decreasing neurotransmitter release.

The Auditory Pathway

  • The auditory pathway consists of the structures of the ear, cochlear nerve, subcortical structures, and auditory cortex.

Afferent Connections with the Cochlear Nerve

  • The organ of Corti sends auditory information to the brain via the cochlear nerve.
  • The cochlear nerve is a bundle of axons of bipolar neurons that send auditory information to the brain.
  • The cell bodies of these bipolar neurons reside in the cochlear nerve ganglion.
  • Excitatory postsynaptic potentials in auditory nerve axons trigger action potentials, which form synapses with neurons in the medulla.

Efferent Connections with the Cochlear Nerve

  • The cochlear nerve contains efferent axons originating from the superior olivary complex in the medulla (olivocochlear bundle).
  • These fibers synapse directly on outer hair cells and on dendrites of inner hair cells.
  • Acetylcholine, an inhibitory neurotransmitter, is secreted by efferent terminal buttons, inhibiting hair cells.
  • Functions of this inhibitory pathway include protecting the cochlea from noise-induced damage.

Subcortical Structures

  • Axons enter the cochlear nucleus of the medulla and synapse there.
  • Most neurons in the cochlear nucleus send axons to the superior olivary complex (medulla).
  • Axons then pass through the lateral lemniscus to the inferior colliculus (dorsal midbrain).
  • Neurons then send axons to the medial geniculate nucleus of the thalamus, which projects to the auditory cortex of the temporal lobe.
  • Each hemisphere receives information from both ears, primarily the contralateral one.
  • Auditory information also reaches the cerebellum and reticular formation.

Auditory Cortex

  • The frequency map of the basilar membrane is preserved through processing in the subcortical structures and mapped in the primary auditory cortex.
  • The basal end (highest frequencies) is represented most medially, and the apical end (lowest frequencies) is represented most laterally.
  • This relationship is referred to as tonotopic representation.

Hierarchical Organization in the Auditory Cortex

  • Auditory cortex has a hierarchical arrangement similar to the visual cortex.
  • The primary auditory cortex is located on the upper bank of the lateral fissure and consists of three regions, each receiving a tonotopic map.
  • The belt region, the first level of auditory association cortex, surrounds the primary auditory cortex and receives information from it as well as from subcortical regions.
  • The parabelt region, is the highest level of auditory association cortex, receives information from the belt region and from the medial geniculate nucleus.

Two Streams in the Auditory Cortex

  • Similar to the visual cortex, the auditory cortex has dorsal and ventral streams.
  • The anterior stream is involved with analysis of complex sounds.
  • The posterior stream is involved with sound localization.

Perception of Pitch

  • Pitch corresponds to the physical dimension of frequency.
  • The cochlea detects frequency by place coding (moderate to high frequencies) and rate coding (low frequencies).

Place Coding

  • Different frequencies cause different parts of the basilar membrane to flex back and forth.
  • The frequency of sound is coded by the specific neurons that are active.
  • Evidence comes from:
    • Antibiotic drugs that cause degeneration of hair cells, starting at the basal end of the cochlea.
    • High-frequency hearing loss caused by exposure to loud sounds.
    • Cochlear implants: devices that restore hearing by stimulating different parts of the basilar membrane.

Rate Coding

  • Lower frequencies are detected by neurons firing in synchrony with the movements of the apical end of the basilar membrane.
  • Evidence comes from studies of people with cochlear implants: stimulation of a single electrode with pulses of electricity produced sensations of pitch proportional to the frequency of the stimulation.

Perception of Loudness

  • Louder sounds produce more intense vibrations, leading to a higher rate of firing by cochlear nerve axons.
  • For place coding, pitch is signaled by which neurons fire, and loudness is signaled by their rate of firing.
  • For low-frequency sounds, loudness is signaled by the number of active axons arising from neurons at the apex of the basilar membrane.

Perception of Timbre

  • Timbre is the quality of sound that allows us to distinguish different instruments or voices.
  • The clarinet note possesses a fundamental frequency, which corresponds to the perceived pitch of the note.
  • Different instruments produce overtones with different intensities.
  • When the basilar membrane is stimulated, different portions respond to each of the overtones.
  • Dynamic sounds have different beginnings, middles, and ends.
  • The auditory cortex analyzes a complex sequence of multiple frequencies that appear, change in amplitude, and disappear.
  • The auditory cortex must analyze a complex sequence of multiple frequencies that appear, change in amplitude, and disappear.

Perception of Spatial Location

  • The auditory system determines whether the source of a sound is to the right or left of us.
  • Three physiological mechanisms detect sound source location:
    • Phase differences for low frequencies (less than approximately 3,000 Hz).
    • Intensity differences for high frequencies.
    • Timbre analysis to determine the height of the source.

Localization by Means of Arrival Time and Phase Differences

  • Neurons respond selectively to different arrival times of sound waves at the left and right ears.
  • Phase differences refer to the simultaneous arrival, at each ear, of different portions of the oscillating sound wave.
  • Some auditory neurons respond only when the eardrums are at least somewhat out of phase.
  • Neurons in the superior olivary complex in the brain are able to use the information they provide to detect the source of a continuous sound.
  • Neurons receive information from two sets of axons coming from the two ears.
  • Each neuron serves as a coincidence detector; it responds only if it received signals simultaneously from synapses belonging to both sets of axons.

Localization by Means of Intensity Differences

  • High-frequency stimuli to the right or left of the midline stimulate the ears unequally.
  • The head absorbs high frequencies, producing a "sonic shadow."
  • Some neurons respond differentially to binaural stimuli of different intensity in each ear, providing information that can be used to detect the source of tones of high frequency.

Localization by Means of Timbre

  • People’s external ears contain several folds and ridges.
  • Depending on the angle at which the sound waves strike these folds and ridges, different frequencies will be enhanced or attenuated.
  • The timbre of sounds changes along with elevation of the source of the sound.
  • Each individual must learn to recognize the subtle changes in the timbre of sounds that originate in locations in front of the head, behind it, above it, or below it.

Perception of Complex Sounds

  • Hearing has three primary functions: to detect sounds, to determine the location of their sources, and to recognize the identity of these sources.
  • The auditory system must recognize patterns of constantly changing activity that belong to different sound sources.

Perception of Environmental Sounds and Their Location

  • The auditory system must recognize that particular patterns of constantly changing activity belong to different sound sources.

Processing Streams for Complex Sounds

  • Pattern recognition of complex sounds appears to be accomplished by circuits of neurons in the auditory cortex.
  • Neurons convey information to the auditory cortex contain special features that permit them to conduct this information rapidly and accurately.
  • Their axons contain special low-threshold voltage-gated potassium channels that produce very short action potentials.
  • Terminal buttons are large and release large amounts of glutamate, and the postsynaptic membrane contains neurotransmitter-dependent ion channels that act unusually rapidly; thus, these synapses produce very strong EPSPs.
  • The terminal buttons form synapses with the somatic membrane of the postsynaptic neurons, which minimizes the distance between the synapses and the axon’s delay in conducting information to the axon of the postsynaptic neurons.
  • The auditory cortex is organized into an anterior stream (perception of complex sounds) and a posterior stream (perception of sound location).
  • Damage to the auditory association cortex can impair various aspects of auditory perception.
  • Inhibiting structures in these pathways results in specific deficits in perceiving “what” and “where” for auditory stimuli.
  • There is plasticity in the auditory processing pathways.
  • For example, the superior auditory abilities of blind individuals have long been recognized: Loss of vision appears to increase the sensitivity of the auditory system.

Perception of Music

  • Consists of sounds of various pitches and timbres played in a particular sequence with an underlying rhythm.