3.2 Auditory pathways

The auditory pathway is a complex system that transforms sound waves into neural signals our brains can interpret. From the outer ear to the auditory cortex, each structure plays a crucial role in processing auditory information, enabling us to hear and understand sounds in our environment.

This topic explores the anatomy of the ear, sound transduction, and the various neural structures involved in auditory processing. Understanding these pathways helps us grasp how we perceive sound, localize its source, and process complex auditory information like speech and music.

Anatomy of the ear

• The ear is the organ responsible for detecting sound waves and converting them into neural signals that the brain can interpret
• It is divided into three main sections: the outer ear, middle ear, and inner ear
• Each section plays a crucial role in the process of hearing and has unique anatomical features

Outer ear

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• Consists of the pinna (visible part of the ear) and the ear canal
• Pinna helps to collect and funnel sound waves into the ear canal
• Ear canal is a tube-like structure that directs sound waves towards the eardrum (tympanic membrane)
• Outer ear also helps to protect the more delicate structures of the middle and inner ear from damage

Middle ear

• Air-filled cavity located between the outer ear and inner ear
• Contains three tiny bones called ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup)
• Ossicles form a chain that transmits and amplifies sound vibrations from the eardrum to the oval window of the inner ear
• Eustachian tube connects the middle ear to the back of the throat, helping to equalize pressure between the middle ear and the outside environment

Inner ear

• Consists of the cochlea, a snail-shaped structure filled with fluid and lined with sensory hair cells
• Vestibular system, responsible for balance and spatial orientation, is also located in the inner ear (includes the semicircular canals, utricle, and saccule)
• Sound vibrations from the middle ear are transmitted to the fluid in the cochlea, causing the basilar membrane to vibrate and stimulate the hair cells
• Hair cells convert the mechanical energy of sound into electrical signals that are sent to the brain via the auditory nerve

Transduction of sound

• Transduction is the process by which sound energy is converted into electrical signals that the brain can interpret
• Occurs primarily in the cochlea of the inner ear
• Involves the specialized sensory cells called hair cells and their associated structures

Hair cells in the cochlea

• Two types of hair cells: inner hair cells (IHCs) and outer hair cells (OHCs)
• IHCs are the primary sensory cells responsible for transmitting auditory information to the brain
• OHCs amplify and fine-tune the sound vibrations, enhancing sensitivity and frequency selectivity
• Hair cells have stereocilia (hair-like projections) on their apical surface that bend in response to fluid movement in the cochlea

Frequency coding

• The cochlea is tonotopically organized, meaning that different frequencies are processed at different locations along the basilar membrane
• High frequencies are processed at the base of the cochlea, while low frequencies are processed at the apex
• Hair cells and their associated nerve fibers are tuned to specific frequencies, allowing for the discrimination of different pitches

Intensity coding

• The intensity (loudness) of a sound is coded by the rate of firing of auditory nerve fibers
• Louder sounds cause hair cells to depolarize more, leading to an increased firing rate in the associated nerve fibers
• The brain interprets the increased firing rate as a louder sound

Auditory nerve

• The auditory nerve (cranial nerve VIII) transmits electrical signals from the hair cells in the cochlea to the cochlear nucleus in the brainstem
• It is responsible for relaying auditory information from the inner ear to the central auditory pathway

Spiral ganglion cells

• Cell bodies of the auditory nerve fibers are located in the spiral ganglion, which is situated in the modiolus (central axis) of the cochlea
• Bipolar neurons with one end connected to the hair cells and the other end projecting to the cochlear nucleus

Tonotopic organization

• The tonotopic organization of the cochlea is maintained in the auditory nerve
• Nerve fibers originating from the base of the cochlea (high frequencies) are located in the center of the auditory nerve
• Fibers from the apex (low frequencies) are located on the periphery of the nerve

Cochlear nucleus

• The first relay station in the central auditory pathway, located in the brainstem
• Receives input from the auditory nerve and processes the incoming auditory information
• Divided into several subdivisions with distinct functions

Dorsal vs ventral regions

• The cochlear nucleus is divided into dorsal and ventral regions
• Dorsal cochlear nucleus (DCN) is involved in processing complex sounds and integrating auditory and somatosensory information
• Ventral cochlear nucleus (VCN) is primarily involved in processing temporal and spectral features of sound

Parallel processing pathways

• The cochlear nucleus gives rise to multiple parallel processing pathways that carry different aspects of auditory information
• These pathways include the dorsal acoustic stria (to the contralateral inferior colliculus), the intermediate acoustic stria (to the superior olivary complex), and the trapezoid body (to the contralateral superior olivary complex)
• Parallel processing allows for the simultaneous analysis of different sound features (e.g., timing, intensity, and frequency)

Superior olivary complex

• A group of nuclei located in the brainstem that plays a crucial role in binaural hearing and sound localization
• Receives input from both the ipsilateral and contralateral cochlear nuclei

Lateral vs medial nuclei

• The superior olivary complex consists of several nuclei, including the lateral superior olive (LSO) and the medial superior olive (MSO)
• LSO is involved in processing interaural level differences (ILDs) for high-frequency sounds
• MSO is involved in processing interaural time differences (ITDs) for low-frequency sounds

Interaural time differences

• ITDs are the differences in the arrival time of a sound at the two ears
• Used for localization of low-frequency sounds (below ~1.5 kHz)
• MSO neurons are sensitive to ITDs and help the brain determine the location of a sound source

Interaural level differences

• ILDs are the differences in the intensity of a sound at the two ears
• Used for localization of high-frequency sounds (above ~1.5 kHz)
• LSO neurons are sensitive to ILDs and help the brain determine the location of a sound source

Inferior colliculus

• The principal midbrain nucleus in the auditory pathway
• Receives input from the cochlear nuclei, superior olivary complex, and contralateral inferior colliculus
• Integrates information from various auditory nuclei and plays a role in sound localization and auditory attention

Central nucleus

• The central nucleus of the inferior colliculus (ICC) is the primary auditory processing center in the midbrain
• It is tonotopically organized and processes information about sound frequency, intensity, and timing
• Neurons in the ICC are sensitive to specific frequencies and respond best to sounds coming from particular locations in space

Lateral cortex

• The lateral cortex of the inferior colliculus (LCIC) is involved in multisensory integration
• It receives input from the auditory, visual, and somatosensory systems
• Neurons in the LCIC respond to complex sounds and are modulated by visual and somatosensory stimuli

Dorsal cortex

• The dorsal cortex of the inferior colliculus (DCIC) is involved in auditory learning and plasticity
• It receives input from the auditory cortex and is thought to play a role in the descending modulation of auditory processing
• Neurons in the DCIC show experience-dependent plasticity and are important for auditory learning and memory

Medial geniculate nucleus

• The principal auditory nucleus in the thalamus
• Receives input from the inferior colliculus and sends output to the primary auditory cortex
• Serves as a relay station for auditory information and is involved in auditory attention and gating

Ventral vs dorsal divisions

• The medial geniculate nucleus is divided into ventral and dorsal divisions
• The ventral division (vMGN) is the primary relay for auditory information and is tonotopically organized
• The dorsal division (dMGN) is involved in processing complex sounds and is modulated by inputs from other sensory systems and higher-order cortical areas

Auditory thalamus

• The medial geniculate nucleus is part of the auditory thalamus, which also includes the nearby suprageniculate nucleus and the posterior thalamic nucleus
• The auditory thalamus is involved in gating and modulating the flow of auditory information to the cortex
• It plays a role in auditory attention, learning, and memory

Primary auditory cortex

• The main cortical area responsible for processing auditory information
• Located in the temporal lobe of the brain, within the superior temporal gyrus
• Receives input from the medial geniculate nucleus of the thalamus

Heschl's gyrus

• The primary auditory cortex is located in Heschl's gyrus, a ridge on the superior temporal gyrus
• Heschl's gyrus is the first cortical area to receive auditory input from the thalamus
• It is essential for the initial processing of sound features such as frequency, intensity, and timing

Tonotopic maps

• The primary auditory cortex is tonotopically organized, with different frequencies represented in a systematic spatial arrangement
• Low frequencies are represented anteriorly, while high frequencies are represented posteriorly
• This tonotopic organization is inherited from the cochlea and maintained throughout the auditory pathway

Columnar organization

• The primary auditory cortex is organized into functional columns, with neurons in each column responding to similar sound features
• Columns are arranged perpendicular to the surface of the cortex and extend through the cortical layers
• This columnar organization allows for the efficient processing of different aspects of sound in parallel

Auditory association areas

• Regions of the cortex that are involved in higher-order processing of auditory information
• Located adjacent to the primary auditory cortex in the superior temporal gyrus
• Responsible for the perception and interpretation of complex sounds, such as speech and music

Planum temporale

• A cortical area located posterior to Heschl's gyrus on the superior temporal plane
• Involved in the processing of complex sounds, particularly in the left hemisphere
• Plays a role in language processing and is larger in the left hemisphere in most individuals

Superior temporal gyrus

• A gyrus located in the temporal lobe, above the superior temporal sulcus
• Contains the primary auditory cortex and several auditory association areas
• Involved in the processing of complex sounds, speech, and music

Wernicke's area

• A language-processing area located in the posterior superior temporal gyrus, typically in the left hemisphere
• Plays a crucial role in language comprehension and the interpretation of spoken and written language
• Damage to Wernicke's area can result in receptive aphasia, characterized by difficulty understanding language

Auditory processing streams

• Two main pathways for the processing of auditory information in the cortex: the ventral "what" pathway and the dorsal "where" pathway
• These pathways are analogous to the visual processing streams and are involved in different aspects of auditory perception

Ventral "what" pathway

• Extends from the primary auditory cortex anteriorly along the superior temporal gyrus
• Involved in the recognition and identification of sounds, such as speech and environmental sounds
• Processes the semantic content of auditory information and is important for language comprehension

Dorsal "where" pathway

• Extends from the primary auditory cortex posteriorly and dorsally towards the parietal lobe
• Involved in the spatial processing of sounds and the localization of sound sources
• Integrates auditory information with other sensory modalities to create a coherent representation of the auditory environment

Subcortical feedback loops

• Neural circuits that allow higher-order cortical areas to modulate the processing of auditory information at lower levels of the auditory pathway
• Enable top-down control of auditory processing and play a role in auditory attention, learning, and plasticity

Corticofugal system

• A system of descending projections from the auditory cortex to subcortical auditory nuclei
• Allows the cortex to modulate the processing of auditory information at earlier stages of the auditory pathway
• Plays a role in shaping the response properties of neurons in the thalamus, midbrain, and brainstem

Descending auditory pathways

• Neural pathways that carry information from higher-order auditory areas to lower-level auditory nuclei
• Include projections from the auditory cortex to the medial geniculate nucleus, inferior colliculus, superior olivary complex, and cochlear nucleus
• Enable top-down control of auditory processing and are important for auditory attention, learning, and plasticity

Binaural hearing

• The ability to process and integrate auditory information from both ears
• Crucial for sound localization and the perception of auditory space
• Involves the comparison of timing, intensity, and spectral cues between the two ears

Sound localization

• The process of determining the location of a sound source in space
• Relies on the comparison of interaural time differences (ITDs) and interaural level differences (ILDs) between the two ears
• ITDs are used for localizing low-frequency sounds, while ILDs are used for localizing high-frequency sounds

Binaural integration

• The neural processing of auditory information from both ears
• Occurs at various levels of the auditory pathway, including the superior olivary complex, inferior colliculus, and auditory cortex
• Allows for the extraction of spatial cues and the creation of a unified representation of the auditory environment

Binaural beats

• An auditory illusion that occurs when two tones with slightly different frequencies are presented separately to each ear
• The brain perceives a third tone with a frequency equal to the difference between the two presented tones
• Binaural beats have been studied for their potential effects on brain function and mental states, although the evidence for their efficacy is mixed

Hearing loss

• A reduction in the ability to detect and process auditory information
• Can be caused by a variety of factors, including aging, exposure to loud noise, genetic factors, and certain medications
• Hearing loss can have significant impacts on communication, social interaction, and quality of life

Conductive vs sensorineural

• Conductive hearing loss occurs when there is a problem with the transmission of sound through the outer or middle ear (e.g., earwax blockage, middle ear infection, or damage to the ossicles)
• Sensorineural hearing loss occurs when there is damage to the inner ear (cochlea) or the auditory nerve (e.g., due to aging, noise exposure, or genetic factors)
• Mixed hearing loss is a combination of both conductive and sensorineural hearing loss

Central auditory processing disorders

• A group of disorders characterized by difficulties in the processing of auditory information in the central nervous system
• Individuals with central auditory processing disorders may have difficulty understanding speech in noisy environments, following complex instructions, or discriminating between similar sounds
• These disorders can occur despite normal hearing sensitivity and may be associated with other neurodevelopmental conditions, such as attention deficit hyperactivity disorder (ADHD) or specific language impairment