Auditory Pathway

Series of pressure wave: compression/decompression of air molecules; three qualities:

• Frequency or pitch: number of cycles/second, (Hz) 

• Amplitude or intensity: expressed on a logarithmic scale of decibels (dB)

• Direction of travel 

Three parts → Outer, middle, and inner ear 

Sound Transmission: Outer Ear, Pinna 

• Visible part of the ear 

• Collects sound waves and funnels it towards auditory canal

• Provides some amplification of the sound (approximately 5-6 dB)

• Along with the head, provides important sound shadows – sounds from different positions in space can be recognized by their characteristics frequency signatures 

Tympanic Membrane

• Thin membrane (eardrum) 

• Forms boundary between outer and middle ear 

• Vibrates in response to sound waves

• Changes acoustical energy into mechanical energy 

Sound Transmission: Middle Ear

• Airborne pressure waves cause motion of tympanic membrane 

• Compression → membrane pressed in

• Decompress → membrane bulges out 

Three bones of the middle ear (Ossicles: malleus, incus, and stapes) 

• Translate pressures waves into motion of fluid in inner ear

• To enlarge sensitive hearing, the area of the tympanic membrane is larger than stapes footplate, as a result, low amplitude vibrations are concentrated into large amplitude motions of stapes (oval window)  

Sound Transmission: Inner Ear

• The inner ear consists of a coiled tube called the cochlea, contains three fluid filled membranous compartments: 

  • Scala Vestibuli: Filled with perilymph (Low K+, high Na+) 

  • Scala Media: Filled with endolymph (High K+, Low Na+) 

  • Scala Tympani: Filled with perilymph (low K+, high Na+ )

• There is a voltage difference between endolymph and perilymph (endo 80 mV more positive) provides a driving force for K+

Hair Cells

• Sensory receptor of auditory signals

• Deflection of stereocilia (hairs) generate differential potentials in the cell 

  • Tip-link: strand from one hair to adjacent hair 

• Ion channels are pulled open by tip links, deflection toward tallest hair (kinocilium) opens gates, away closes

• High K+ in endolymph causes K+ flow into the cell when gate opens

• Release: Mechanical Stimulus → K+ comes in → Depolarization → Open voltage gated Ca 2+ channels → Ca2+ comes in → Neurotransmitter Released 

• No Release: Mechanical Stimulus → K+ stops coming in →  Hyperpolarization→ Close voltage gated Ca 2+ channels → Ca2+ stops coming in  → Neurotransmitter not released 

Organ of Corti 

• The hair cells arranged between the basilar and tectorial membranes 

• Basilar membrane vibration displaces the mechanosensory bundles 

• Inner hair cells form one row

  • Connected to afferent nerves 

• Send sensory information to the brian

• Outer hair cells form several rows 

  • Connected to few afferent fibers

  • Influence sensitivity of inner hair cells

  • 3x as many outer than inner hair cells 

• Upward movement of basilar membrane pushes cilia against tectorial membrane, shearing them

• Downward movement of basilar membrane relieves shear force and reverses its direction 

Sound Transmission: Inner Ear

• Useful to consider the cochlea uncoiled, the stapes movement of the oval window creates waves within the cochlear fluid. Positions along the basilar membrane have a frequence for maximum vibration  

• Frequency of vibration/sensitivity depends on membrane position: 

  • Base: membrane is stiff and thin (more responsive to high Hz) 

  • Apex: membrane is wide and floppy (more responsive to low Hz) 

Basilar Membrane

• There is a tonotopic map (a location/place code) along the basilar membrane

• Different frequency sounds excite different basilar membrane regions 

Transduction of Sound Energy to Electrical Signals 

• Airborne pressure waves in external ear vibrate the eardrum

• The ossicles convert pressure waves into (cochlea) fluid waves 

• Fluid waves displace basilar and tectorial membranes

• Shear between membranes bends hair cells ciliary bundles

• Ciliary bending leads to alternate depolarization and hyperpolarization of the hair cells membrane potential 

• Leads to increased and decreased rates of transmitter release

• Chemical transmitter causes depolarization of afferent axon

• Afferent neurons sends actional potential to the  auditory neurons in the brainstem and onto cortex 

Auditory Pathway: Axons in the auditory nerve connect to single points on the basilar membrane

• Several groups of nucleus between cells and cortex: 

  • Cochlear Nucleus

    • Dorsal (DCN) and Ventral (VCN) 

  • Superior Olivary Nucleus

    • Inferior Colliculus

  • Medial Geniculate Nucleus (MGN) 

• Number of cross connections between the two sides: binaural information

• Tonotopic maps at each level of pathway 

• Auditory information reaches the cochlear nuclei of the medulla (DCN, VCN) 

• From these, nuclei information travels inferior calculus, then the thalamus medial geniculate nucleus (MGN) 

• Ultimate destination is the primary auditory cortex 

Temporal Codes of Population of Neurons 

• Auditory neurons can phase-lock to low frequencies

• Phase-locking in this example is the ability to fire at the precise time in a cycle of a sine wave, cell fires roughly same time of every cycle of the sinusoidal stimulus

• You can decode the frequency by looking at the timing between action potentials

• At higher frequencies, the action potentials occur at random phases of the sound wave; the cycle is too fast for the action potentials of single neurons to accurately represent their timing 

• Phase-locking provides temporal information form the two ears (interaural time differences) up to 3 kHZ, differences is a cue for sound localization and the perception of auditory “space” 

• The cochlea cannot represent space directly 

Sound Localization

• Unlike vision, auditory system must commute map of spatial locations 

• Interaural Time Differences

  • Sound coming from the right arrive at the right ear a little earlier than the at the left ear

  • This small time difference is used by the medial superior olive 

• Neurons in the medial superior olive (MSO) operate as coincidence detector using interaural timing difference

  • MSO neurons have bipolar dendrites – extend medial and lateral 

  • Lateral dendrites receive input from the ipsilateral cochlear nucleus

  • Medial dendrites receive input from contralateral cochlear nucleus

  • MSO neurons respond when both excitatory signals arrive at the same time. Different neurons are sensitive to different delay times 

• Unlike vision, auditory system must compute map of spatial locations 

• Interaural Time Difference (ITD) 

  • A cell in the superior olive responds with an increase in firing rate to  time difference in the aerial of sound to the ears

  • This cue to the sound’s location is conveyed up to the inferior colliculus and onto cortex

  • Best at low frequencies (<3000 Hz) 

  • Differences between the intensity of the sounds in the two ears

  • Best at higher frequencies: head is a sound barrier at these frequencies (sound at opposite ear is less loud)

  • Head forms a sound shadow, so the sound reaching the other ear is softer. The interaural 

• Neurons in the lateral superior olive (LSO) and the medial nucleus of the trapezoid body (MNTB) compute the position based on intensity differences

  • Stronger stimulus ear excites left LSO

  • Stimulus also inhibits right LSO via MNTB interneuron 

  • Excitation from left ear is greater than inhibition from right ear, results in net excitation to higher processing centers 

  • Inhibition from left side if greater than excitation from right side results in net inhibition on right side, and no signal to higher processing centres 

• Arrangement of excitation-inhibition makes LSO neurons fire most strongly in response to sounds arising directly lateral to the listener on the same side as the LSO 

• LSO neurons are paired bilaterally symmetrical. Each LSO neurons only encodes the location of sounds from the ipsilateral side → both LSO neurons to encode full range of horizontal positions

• Auditory system uses both timing and intensity difference between two ears to compute location

• Two sources are eventually merge din the midbrain auditory centers

• Two mechanisms are fairly accurate for right/left discrimination, but cannot tell whether sound is in front or behind, above or below 

• Elevation of sound sources is determined by spectral filtering mediated by the outer ear

• Vertically asymmetric convolutions of the outer ear are shaped so that more high-frequency components are transmitted from an elevated source than form the same source at ear level

• Evidence that this spectral information created by outer ear is detected by dorsal cochlear nucleus 

Medial Geniculate Nucleus

• The thalamic relay of the auditory system

• Organized tonotopically similar to earlier stages of auditory processing, each layer contains neurons with the same characteristic frequency

• Most cells are responsive to stimulation through either ear and also sensitive to interaural timing and intensity differences 

Auditory Cortex (A1) → In temporal lobe

• Receives auditory information form both sides of the head

• Maintains tonotopic map established at basilar membrane

• Cells respond to: 

  • Combinations of tones

  • Binaural and monaural sounds 

Lateralization

• Natural sounds are complex and their representation tends to be asymmetric across the two hemisphere

• Bold contrast signal changes show activation elicited by speech, environment, and music

• Music is highly lateralized to the right regions of A1, speech in lect

Auditory Receptive Fields

• Represent the frequency tuning properties of auditory nerves

• Plots of threshold loudness for producing a response from the neuron at different frequencies

• Each neuron is most sensitive at a particular frequency, which corresponds to the neuron’s innervation along the basic membrane

• Characteristic frequency: frequency at which neuron is most responsive - from cochlea to cortex 

Hearing Loss

• Conductive Hearing Loss: Sound waves are unable to travel efficiently from external → inner ear

  • Occurs due to damage to the tympanic membrane or middle ear ossicles 

  • Treatment: hearing aids

  • Problems: lack of directionality, flat amplification curves, so problem in noisy environments

• Sensorineural Hearing Loss: From nerve damage, outer and middle ear okay, but inner ear are affected

  • Treatment: cochlear implants if auditory nerve is intact

  • Problems: invasive

Cochlear Implants

• By knowing where frequency is represented along the cochlea, it is possible to bypass the translation of the air pressure

• Implants fitted to patients who suffer from deafness due to different cause

• A number of electrodes are inserted to the cochlea, terminating at different places

• Electrical current derive from filtering sound stimulates selective auditory nerve fibers near the electrode, giving a crude place coding of frequency components 

• Sound → Microphone → Analyzer: Breaks down sound into individual frequency components → Stimulator: High frequencies activate electrodes at high-frequency (basal) end, low frequencies at apical end