Chapter 13: Synopsis of Auditory Function PDF

Synopsis of Auditory Function ----------------------------- 1. **What the Auditory System Does**: - The auditory system converts sound waves into specific patterns of *neural activity* (electrical signals in the brain). - These sound signals are then combined with information...

Synopsis of Auditory Function ----------------------------- 1. **What the Auditory System Does**: - The auditory system converts sound waves into specific patterns of *neural activity* (electrical signals in the brain). - These sound signals are then combined with information from other senses and brain areas involved in movement, attention, and alertness to help guide behavior. - For example, it helps us: - Turn toward a sound (like when someone calls your name). - Communicate with others (such as during a conversation). - Recognize whether a sound was made by us or something else (like distinguishing your own voice from background noise). 2. **First Stage: Collecting and Amplifying Sound**: - The first part of this process happens in the *external and middle ears*. - The *external ear* (what you see on the side of your head) collects sound waves. - The *middle ear* amplifies these sound waves, increasing their *pressure* so the sound energy can move from the air into the *fluid-filled cochlea* (part of the *inner ear*). 3. **Inner Ear: Transforming Sound**: - Inside the *inner ear*, the *cochlea* uses a series of mechanical processes to break down the sound wave into its parts: - *Frequency* (how high or low the sound is). - *Amplitude* (how loud or soft the sound is). - *Phase* (the timing of the sound wave). - Special cells in the cochlea, called *sensory hair cells*, convert (or *transduce*) this sound information into electrical signals. These signals are then passed along to the *auditory nerve*. 4. **Tonotopy**: - The cochlea organizes sound frequencies in a very structured way, known as *tonotopy*. - Different parts of the cochlea are sensitive to different sound frequencies. High-pitched sounds are processed in one area, while low-pitched sounds are processed in another. - This organized map of sound frequencies is an important feature that is preserved all the way through the brain\'s auditory pathways, helping us make sense of what we hear. **Starting Point: Cochlear Nucleus**: - The first stage of sound processing in the brain happens at the *cochlear nucleus*. This is where the information from the ear splits into several different pathways, each responsible for processing different aspects of sound. **Target 1: Superior Olivary Complex**: - One of the places the cochlear nucleus sends information is the *superior olivary complex*. This is the first place in the brain where information from *both ears* comes together. - The superior olivary complex is important for *sound localization*, which means figuring out where a sound is coming from in space. **Target 2: Inferior Colliculus**: - Another target is the *inferior colliculus*, located in the midbrain. This is an important hub where different sounds are integrated (combined) and where sound information can interact with the *motor system* (which controls movement). - The inferior colliculus sends the processed sound information to higher brain areas like the *thalamus* and the *cortex*. The cortex is crucial for things like *speech* and *music* perception. **Complex Pathways**: - There are many steps (or "stations") between the ear and the brain's cortex in the auditory system. This system is much more complex than those of other senses, like vision or touch. - The large number of processing stages suggests that understanding sounds---especially those important for communication and survival---requires a lot of brain power. **Tuning for Communication**: - Both the early (peripheral) and higher (central) parts of the auditory system are specially tuned to process the sounds made by other members of the same species (called *conspecific vocalizations*). - This suggests that the auditory system evolved to be highly specialized for recognizing and processing communication sounds, like human speech or animal calls. *External Ear* -------------- - The *external ear* is made up of the *pinna* (the outer part of the ear you can see), the *concha* (the hollow area next to the ear canal), and the *auditory meatus* (the ear canal). - Its job is to collect sound waves and direct them toward the *eardrum* (also called the *tympanic membrane*). **Sound Amplification**: - The way the ear canal is shaped causes certain sound frequencies, especially those around *3 kHz*, to be amplified naturally. This boosts the sound pressure by *30 to 100 times*. - Because of this amplification, humans are especially sensitive to sounds in the *2--5 kHz* range. **Hearing Loss Risk**: - This also means that our ears are more likely to suffer *hearing loss* in that frequency range when exposed to loud, continuous noise (like from heavy machinery or explosions). **Speech and 3 kHz**: - The *2--5 kHz range* is important for *speech perception*. Human speech covers many frequencies, but key sounds that help us tell words apart, like the consonants in \"ba\" and \"pa,\" are concentrated around *3 kHz*. - So, if someone has *hearing loss* in this frequency range, it can seriously affect their ability to *understand speech*. **Pinna and Concha's Second Function**: - Besides collecting sound, the *pinna* (outer ear) and *concha* (hollow part next to the ear canal) also help us figure out the *elevation* of a sound---meaning whether the sound is coming from above, below, or at the same level as our ears. **How This Works**: - The *shape* of the pinna is important. Its uneven curves and folds are designed so that sounds coming from above have more *high-frequency* components (higher-pitched sounds) than sounds coming from the same source at ear level. - This allows your ear to tell if a sound is coming from above, below, or straight ahead. **Demonstrating the Effect**: - This effect has been shown by using an *artificial* external ear (a model of the ear) to record sounds from different elevations. When these recorded sounds are played back through earphones, people perceive the recordings from higher elevations as coming from above, even though they are being played at the same level. *Middle Ear* ------------ 1. **Air vs. Fluid**: - *Sound* that hits your external ear is carried by *air*. But inside the *inner ear*, where sound is converted into signals for the brain, everything is filled with *fluid*. 2. **Middle Ear\'s Job**: - The main job of the *middle ear* is to make sure that sound waves traveling through air can be successfully transferred into the *fluid* in the inner ear. 3. **Impedance Mismatch**: - There\'s a problem when sound moves from *air* (which has low *impedance*, meaning it resists the sound waves less) into *fluid* (which has higher impedance and resists the sound waves more). - Normally, when sound moves from air to water (or fluid), *most* of the sound energy (over 99.9%) would be reflected, meaning very little sound would make it into the inner ear. 4. **How the Middle Ear Solves This**: - The middle ear has special mechanisms that *amplify* or *boost* the sound pressure. By the time sound reaches the inner ear, it has been increased by *200 times* so that enough sound energy can pass from the air into the fluid for you to hear properly. **1. How the Middle Ear Amplifies Sound:** The middle ear uses two main mechanical processes to *increase the pressure* of sound before it reaches the inner ear: - **First Process (Main Boost)**: - The *eardrum* (or *tympanic membrane*) is large compared to the *oval window* (the part of the inner ear where sound enters). - When sound hits the large eardrum, the force is concentrated onto the much smaller oval window, which increases the pressure of the sound by a lot. - **Second Process (Lever Action)**: - There are three small bones in the middle ear called the *ossicles* (the *malleus*, *incus*, and *stapes*). - These bones act like a lever, giving a mechanical advantage. This lever action also helps amplify the sound pressure. **2. Conductive Hearing Loss:** - If there\'s damage to the *external* or *middle ear*, it becomes harder for sound to be transferred to the inner ear. This is called *conductive hearing loss*. - An *external hearing aid* can help by boosting the sound pressure so that sound can still reach the inner ear efficiently. **3. How the Middle Ear Protects the Inner Ear:** - Two small muscles in the middle ear help regulate sound transmission: - The *tensor tympani* (controlled by *cranial nerve V*). - The *stapedius* (controlled by *cranial nerve VII*). - These muscles contract automatically when you hear loud sounds or make loud noises (like when you speak), which helps protect the inner ear by reducing the movement of the ossicles and limiting how much sound energy reaches the cochlea. **4. Hyperacusis:** - If either of these muscles becomes paralyzed (for example, if *cranial nerve VII* is damaged in a condition like *Bell's palsy*), it can lead to *hyperacusis*---a painful sensitivity to sounds that are normally not too loud. **5. Bone Conduction of Sound:** - Even if the middle ear isn't working properly (for example, if the eardrum or ossicles are damaged), sound can still get to the inner ear through the bones of your skull. - If you place a tuning fork against your head, the vibrations can travel through the bones directly to the inner ear, allowing you to hear the sound. **6. Weber Test:** - In clinics, doctors use the *Weber test*, where a tuning fork is placed on the scalp, to check for hearing loss. - This test helps determine if the hearing loss is due to *conductive problems* (like damage to the middle ear) or *sensorineural hearing loss* (which means damage to the inner ear or auditory nerve). *Inner Ear* ----------- **1. Cochlea\'s Function:** - The cochlea is the part of the inner ear where sound energy is converted into electrical signals that the brain can understand. - It also works like a *mechanical frequency analyzer*, breaking down complex sounds into simpler components. **2. Structure of the Cochlea:** - The cochlea is shaped like a spiral (hence its name, which comes from the Latin word for \"snail\"). If uncoiled, it would be about 35 mm long. - At the base of the cochlea, there are two important regions: the *oval window* and the *round window*. These areas help with sound transmission into the inner ear. - The cochlea has three fluid-filled chambers: - Scala vestibuli and scala tympani: These are on either side of the cochlear partition. - Scala media: A separate chamber within the cochlear partition. - The scala vestibuli and scala tympani are connected at the top of the cochlea by an opening called the *helicotrema*, which allows their fluids to mix. **3. How the Cochlea Responds to Sound:** - When sound reaches the cochlea, the *oval window* moves inward, causing the fluid inside the cochlea to move. This makes the *round window* bulge out slightly and deforms the *cochlear partition* (the flexible structure inside the cochlea). - The key to hearing is how the *basilar membrane* (part of the cochlear partition) vibrates in response to sound. **4. Frequency Tuning:** - The *basilar membrane* is different at each end: - It is *narrower and stiffer* at the base (near the oval window) and responds best to high-frequency sounds. - It is *wider and more flexible* at the apex (the top of the spiral) and responds best to low-frequency sounds. - A scientist named Georg von Békésy discovered that different parts of the basilar membrane vibrate most strongly at specific frequencies, creating a *traveling wave*. - High-frequency sounds cause the base of the membrane to vibrate. - Low-frequency sounds cause the apex to vibrate. - This creates a *tonotopic map*, where different frequencies activate different parts of the cochlea. **5. Breaking Down Complex Sounds:** - Complex sounds (like speech or animal noises) cause a pattern of vibrations across the cochlea, where the different frequencies making up the sound are separated and processed individually. **6. How Hair Cells Detect Sound:** - As sound waves move through the cochlea, they create a wave that vibrates the basilar membrane. This movement bends the stereocilia (tiny projections on the hair cell that stick out from the top), generating electrical signals that are sent to the brain as neural impulses. *Hair Cells and the Mechanoelectrical Transduction of Sound Waves* ------------------------------------------------------------------ **1. Types of Cochlear Hair Cells:** - There are **two types of hair cells** in the cochlea: - **Inner hair cells**: These are the main sensory receptors responsible for hearing. They send signals to the brain via the auditory nerve. 95% of auditory nerve fibers come from these cells. - **Outer hair cells**: These cells primarily receive signals from the brain and help modulate the movements of the basilar membrane. They play a role in amplifying sound and fine-tuning hearing. **2. Structure of Hair Cells:** - Hair cells are flask-shaped and have bundles of hair-like structures called **stereocilia** protruding from their tops into the scala media (the fluid-filled chamber within the cochlea). - Each bundle has anywhere from 30 to several hundred stereocilia, arranged in a graded pattern (shorter to taller). - There is also one **kinocilium**, a true cilium, but in humans, it disappears after birth, leaving only the stereocilia. **3. How Stereocilia Work:** - **Stereocilia** are connected by fine **tip links**, which are made of cell adhesion molecules (cadherin 23 and protocadherin 15). - These **tip links** are essential for converting the movement of the stereocilia into an electrical signal, known as a **receptor potential**. - When sound causes the basilar membrane to vibrate, the stereocilia are displaced: - **Towards the tallest stereocilia**: This stretches the tip links and **opens channels** that allow positive ions (cations) to flow into the hair cell, depolarizing it. - **Away from the tallest stereocilia**: This compresses the tip links and **closes the channels**, causing hyperpolarization (the opposite of depolarization). - The constant pivoting of the stereocilia back and forth modulates the ionic flow, creating a **graded receptor potential** that mirrors the movement of the basilar membrane. **4. Transmission of the Signal:** - The receptor potential causes the release of neurotransmitters from the hair cell, which then triggers **action potentials** in the fibers of the **auditory nerve (cranial nerve VIII)**. - These action potentials carry the sound information to the brain, matching the frequency of the sound at low frequencies. **5. Speed and Sensitivity:** - **Hair cell mechanotransduction** (the process by which mechanical forces are turned into electrical signals) is incredibly fast and sensitive. - The movement of the stereocilia at the **threshold of hearing** is as small as 0.3 nanometers (about the size of a gold atom). - The conversion of mechanical movement into an electrical signal happens in as little as **10 microseconds**, which is necessary for accurately detecting the source of sounds. - The direct mechanical gating of the channels (rather than slower chemical processes) allows for this speed. **6. Potential for Damage:** - The sensitivity of hair cells makes them vulnerable to damage, especially from **loud sounds**. - High-intensity sounds can break the tip links or destroy the hair bundles, leading to **irreversible hearing loss**, as human hair cells do not regenerate (unlike in some animals like fish or birds). - Since humans have only about **15,000 hair cells per ear**, any damage can have a significant impact on hearing. *The Ionic Basis of Mechanotransduction in Hair Cells* ------------------------------------------------------ **1. Resting Potential of Hair Cells:** - Hair cells have a **resting potential** of about **--45 to --60 mV** compared to the fluid around the base of the cell. - At rest, only a few of the ion channels responsible for sound transduction are open. **2. Depolarization Process:** - When the **hair bundle** (the stereocilia) moves towards the **tallest stereocilium**, more ion channels open, and **K+ (potassium) and Ca2+ (calcium)** ions flow into the cell, causing **depolarization**. - Depolarization opens more **voltage-gated calcium channels**, allowing more calcium to enter the hair cell. - This influx of calcium triggers the release of neurotransmitters from the **basal end** of the hair cell, which sends signals to the **auditory nerve**. **3. Biphasic Receptor Potential:** - Some ion channels remain open at rest, which makes the receptor potential **biphasic**: - Movement toward the tallest stereocilia **depolarizes** the hair cell (positive electrical change). - Movement away from the tallest stereocilia **hyperpolarizes** the hair cell (negative electrical change). - This allows the hair cell to generate a **sinusoidal receptor potential** (a smooth, wave-like electrical signal) in response to a sinusoidal stimulus (e.g., a steady tone). This ability to track the up-and-down movement of sound waves helps preserve **temporal information** in sound up to frequencies of about **3 kHz** **4. High-Frequency Sounds:** - At frequencies above 3 kHz, the hair cell can no longer follow the exact timing of sound waves, but it still signals their presence by creating a **tonic depolarization** (a constant depolarized state) that enhances neurotransmitter release. **5. Role of K+ in Depolarization and Repolarization:** - Hair cells use **K+ ions** for both **depolarization** (when K+ enters the cell) and **repolarization** (when K+ leaves the cell). - The K+ gradient in the hair cell is maintained largely by **passive ion movement**, making this process **energy-efficient**. **6. Ionic Environments:** - The apical end of the hair cell, including the stereocilia, is bathed in **endolymph**---a fluid rich in K+ and poor in Na+. - The basal end of the hair cell is surrounded by **perilymph**, a fluid that is low in K+ and high in Na+. - There is an **endocochlear potential**: the endolymph is about **80 mV more positive** than the perilymph, and the inside of the hair cell is about **125 mV more negative** than the endolymph. - This strong electrical gradient helps drive K+ into the cell when transduction channels open, allowing for **fast depolarization**. **7. Repolarization and Ionic Exchange:** - Depolarization from K+ entry opens **somatic K+ channels** in the basal membrane of the hair cell, which allows K+ to flow out of the cell into the perilymph, causing **repolarization**. - Additionally, **Ca2+** (calcium) entering the hair cell helps trigger neurotransmitter release and opens **Ca2+-dependent K+ channels**, further helping with K+ exit and repolarization. **8. Specialized Adaptations:** - The hair cell\'s ability to repolarize quickly and efficiently is due to the distinct **ionic environments** of the endolymph (rich in K+) and perilymph (poor in K+). This setup ensures that the hair cell can maintain its ionic gradients even during prolonged sound stimulation. - Disruptions to this balance, such as damage to **Reissner\'s membrane** or exposure to substances that harm the ion-pumping cells of the **stria vascularis**, can destroy the endocochlear potential and lead to **sensorineural hearing loss**. *The Hair Cell Mechanoelectrical Transduction Channel* ------------------------------------------------------ The **hcMET channel** is a critical part of the hair cells\' ability to convert mechanical sound vibrations into electrical signals **Challenges in Isolating the hcMET Channel:** - **Paucity of Material**: Each hair bundle (a collection of hair cell stereocilia) has only about **200 functional channels**, which makes up less than 0.001% of all the proteins in the hair bundle. This small amount makes **biochemical purification** of the hcMET protein extremely difficult. - **Complexity of the Transduction Apparatus**: The hcMET channel is part of a complex system that involves many **accessory proteins** that work together for mechanotransduction. This complexity further complicates efforts to isolate and identify the channel. **Genetic Research and Candidate Proteins:** - Genetic research into **heritable deafness** has identified several important **genes** related to hearing. Four promising candidates for the hcMET channel are: - **TMC1**, **TMC2, TMIE**, **LHFPL5** - These proteins are found at the **apical end of stereocilia** (the top part of the hair cells) and mutations in these genes can reduce or abolish mechanotransduction in hair cells, suggesting they play a key role in the hcMET channel. **Von Békésy's Model**: Von Békésy proposed that the cochlea works like a series of connected resonators (like tuning forks). In this model, the basilar membrane vibrates passively in response to sound. **Increased Vibration**: At low sound levels, the basilar membrane vibrates much more than expected from high sound levels. **Otoacoustic Emissions**: The ear can produce sounds on its own (called otoacoustic emissions), which are useful for testing hearing in newborns. These emissions can also be a source of tinnitus. **Outer Hair Cells**: The outer hair cells in the cochlea are crucial for this active process. Without them, the cochlea's sharp frequency tuning is lost. **HC-MET Channels**: Channels in hair cells might also contribute to the amplification of sound. *Tuning and Timing in the Auditory Nerve* ----------------------------------------- Hair cells in the cochlea can quickly follow vibrations of the hair bundle up to around 3 kHz (3,000 Hz). This means they can encode sounds and their changes in frequency and amplitude below this range through the pattern of their activity. - Beyond 3 kHz, hair cells and their nerve fibers cannot keep up with the sound frequency. Therefore, another mechanism is needed to handle higher frequencies. - The cochlea is organized such that different parts respond to different frequencies. This is called **tonotopic organization**. - **Labeled-Line Coding**: Each part of the cochlea (and the auditory nerve fibers connected to it) responds to a specific frequency range. This organization helps the auditory system preserve frequency information as it moves along the auditory pathway. - Each auditory nerve fiber connects to only one inner hair cell, though multiple fibers may connect to a single hair cell. - Fibers linked to the cochlea's apex (far end) respond to low frequencies, while those linked to the base (near the middle ear) respond to high frequencies. - **Tuning Curves**: These are graphs showing how different fibers respond to various frequencies. The lowest threshold on a tuning curve is called the characteristic frequency, which reflects the frequency the fiber is most sensitive to. **Cochlear Implants** use the tonotopic organization of the cochlea to mimic the patterns of activity that would normally occur in the auditory nerve. They help people with damaged hair cells by bypassing the faulty parts and directly stimulating the auditory nerve. **Phase-Locking**: - Hair cells respond to low-frequency sounds by releasing neurotransmitters only during the positive phases of the sound wave. This creates a pattern of nerve firing that is synchronized with the sound wave's phase. - This "phase-locking" is crucial for detecting interaural time differences (the time difference between when a sound reaches each ear), which helps with sound localization and understanding spatial aspects of sound. *How Information from the Cochlea Reaches Targets in the Brainstem* ------------------------------------------------------------------- **Parallel Organization**: - The auditory system, like the visual system, is organized in parallel. This means that different pathways process different aspects of auditory information simultaneously. **Auditory Nerve and Brainstem**: - When the auditory nerve enters the brainstem, it splits into branches that connect to three different parts of the cochlear nucleus. - The auditory nerve (along with the vestibular nerve, which is cranial nerve VIII) contains the central processes of bipolar spiral ganglion cells from the cochlea. These cells have: - **Peripheral Processes**: Contact inner hair cells in the cochlea. - **Central Processes**: Extend into the cochlear nucleus in the brainstem. **Cochlear Nucleus**: - **Branches of Auditory Nerve**: Within the cochlear nucleus, each auditory nerve fiber branches into: - **Ascending Branch**: Goes to the anteroventral cochlear nucleus. - **Descending Branch**: Goes to the posteroventral cochlear nucleus and the dorsal cochlear nucleus. **Tonotopic Organization**: - The tonotopic map from the cochlea (which maps different frequencies to different locations) is preserved in the cochlear nucleus. - Each part of the cochlear nucleus (anteroventral, posteroventral, dorsal) contains distinct cell populations with unique properties. **Information Transformation**: - The auditory nerve axons terminate in different densities and types in the cochlear nucleus, which allows for various transformations of the information coming from the hair cells. - This means the auditory information is processed and altered at this level before being sent further in the auditory pathway. *Integrating Information from the Two Ears* ------------------------------------------- **1. Parallel Pathways in the Auditory Brainstem:** - After entering the brainstem, the auditory nerve branches to different parts of the cochlear nuclei. These nuclei, in turn, send signals through multiple parallel pathways to different brain areas. - One important feature is **bilateral connectivity**---auditory signals go to both sides of the brain. Therefore, damage to central auditory structures doesn't cause hearing loss in just one ear (**monaural loss**). If there is a monaural hearing loss, it usually points to damage in the **middle ear, inner ear, or auditory nerve**. **2. Sound Localization Strategies:** - Humans use two strategies to determine where a sound is coming from along the horizontal plane (left-right axis), depending on the frequency of the sound: - **For low frequencies (below 3 kHz)**: The brain detects **interaural time differences** (tiny differences in the time it takes for sound to reach each ear). - **For high frequencies (above 3 kHz)**: The brain uses **interaural intensity differences** (differences in sound loudness between the two ears caused by the head casting a "shadow"). **3. Interaural Time Differences:** - **Sensitivity to Time Differences**: Humans can detect tiny differences in the arrival times of sound between the ears, as small as 10 microseconds. This allows precise sound localization (within 1 degree of accuracy). - **Medial Superior Olive (MSO)**: This brainstem structure computes interaural time differences. It gets inputs from both ears: - **Ipsilateral Input**: From the same-side ear. - **Contralateral Input**: From the opposite-side ear. - The **coincidence detector model** (by J. Jeffress) suggests that MSO neurons are specialized to respond when signals from both ears arrive at the same time. Different neurons are sensitive to different delays, which helps locate sound sources. - **Delay Lines**: Axons from the cochlear nucleus vary in length, creating a timing delay so that sounds arriving at slightly different times from each ear still reach the MSO simultaneously. This helps detect where the sound is coming from. **4. Interaural Intensity Differences:** - **Head Shadow Effect**: At higher frequencies (above 2 kHz), the sound waves are too short to bend around the head. This creates an "acoustic shadow" at the ear farther from the sound, resulting in a quieter sound in that ear. - **Lateral Superior Olive (LSO)**: This part of the brainstem processes interaural intensity differences: - The **ipsilateral ear** (same side as the sound) sends **excitatory signals** to the LSO. - The **contralateral ear** (opposite side) sends **inhibitory signals** via the **Medial Nucleus of the Trapezoid Body (MNTB)**. - The LSO compares these excitatory and inhibitory inputs to determine the sound's position. The LSO on the side of the sound will be more active, while the LSO on the opposite side will be inhibited. **5. Combining Information:** - Each LSO encodes sounds coming from the same side of the head (ipsilateral), so both LSOs work together to cover the full range of horizontal sound positions. - **MSO** (for time differences) and **LSO** (for intensity differences) pathways merge in the midbrain auditory centers. **6. Sound Elevation:** - Sound localization in the **vertical plane (elevation)** is determined by how the external ear (pinna) filters sounds. - The **Dorsal Cochlear Nucleus** detects spectral "notches" created by the shape of the pinna, helping identify the elevation of a sound source. - **Horizontal localization**: Two mechanisms---**interaural time differences** (processed in the MSO for low frequencies) and **interaural intensity differences** (processed in the LSO for high frequencies)---help localize sounds on the left-right axis. - **Vertical localization**: Spectral filtering by the pinna and processing in the dorsal cochlear nucleus helps determine sound elevation. Monaural Pathways from the Cochlear Nucleus to the Nuclei of the Lateral Lemniscus ---------------------------------------------------------------------------------- - The **binaural pathways** that help localize sound (from both ears) are just part of the cochlear nucleus\'s output. Auditory perception involves more than just identifying where sounds are coming from. - Another major set of pathways from the **cochlear nucleus** bypasses the **superior olive** and goes directly to the **nuclei of the lateral lemniscus** on the **opposite (contralateral)** side of the brainstem. - These **monaural pathways** respond to sounds arriving at only one ear (monaural sounds). - **Nuclei of the Lateral Lemniscus**: These nuclei are part of the auditory pathway located in the brainstem. They process **monaural sounds**, including the **onset** and **duration** of sounds. These nuclei are involved in processing the **temporal features** of sounds and contribute to aspects of hearing beyond sound localization. - **Onset Cells**: Some cells signal the start of a sound, no matter its **intensity** or **frequency**. - **Other Cells**: Some process the **duration** and other **temporal features** of sounds, though the specific roles of these pathways are not yet fully understood. - Like the outputs from the **superior olivary nuclei**, the pathways from the **nuclei of the lateral lemniscus** eventually converge in the **midbrain** auditory centers. - **Auditory pathways** from the **olivary complex** (which helps process binaural sound information), the **lemniscal complex** (which processes monaural sounds), and other projections from the **cochlear nucleus** all ascend to the **inferior colliculus** in the midbrain. This region is a key center for integrating sound information. - Experiments in the **barn owl** show how the **inferior colliculus** creates a **topographical map of auditory space**, combining information from both ears. Neurons in this map respond best to sounds coming from specific directions (with a preferred **elevation** and **azimuthal location**). ### Processing Complex Sounds in the Inferior Colliculus: - The **inferior colliculus** doesn't just process simple sounds (like pitch or loudness); it also responds to **complex temporal patterns** in sound. - Some neurons respond only to **frequency-modulated sounds** (where the pitch changes over time). - Other neurons respond to sounds of **specific durations** or arranged in **specific temporal sequences**. These patterns are common in important, biologically relevant sounds, such as: - Sounds made by predators. - **Intraspecific communication** (communication between members of the same species), like **human speech**. - In the **inferior colliculus**, multiple simpler sound cues (like timing, intensity, and frequency) converge and combine to form more **integrative and complex response properties**. This integration helps the brain create a **representation of auditory objects**---understanding not just where a sound is, but also recognizing what that sound is.  The Medial Geniculate Complex (**MGC)**, located in the **thalamus**, is the main relay point for auditory information on its way to the auditory cortex. All ascending auditory information passes through here, except for a few pathways that bypass the **inferior colliculus**. - **Ventral division**: Sends information to the **core region** of the auditory cortex. - **Medial and dorsal divisions**: Surround the ventral division and project to the **belt regions** around the core of the auditory cortex. The **MGC** shows: - **Selectivity for frequency combinations**: Cells respond to specific combinations of sound frequencies, likely due to the convergence of inputs from cochlear regions with different sensitivities. - **Selectivity for time intervals**: Cells also respond to specific time differences between sound frequencies, helping to measure things like distance in bats. - This process is like how **binaural neurons** in the **medial superior olive** (MSO) localize sounds based on time differences between the two ears, but in this case, it's about monaural signals with different frequencies. - In **humans**, speech sounds change rapidly over milliseconds. The ability of MGC neurons to integrate information over this timescale might be crucial for **speech perception**. - **spectral** (frequency-related) **and temporal** (time-related) **cues** are essential for processing communication sounds, including speech. The **auditory cortex** is essential for conscious sound perception and the processing of complex sounds like speech and music. It is organized in a hierarchical way, with **primary regions** mapping simple sound frequencies (tonotopy) and **secondary regions** processing more complex features like **pitch** and **temporal sequences**. Damage to this area can lead to deficits in both **speech recognition** and **temporal sound processing**, highlighting its role in human communication.