Auditory System: Introduction Sound: a tiny pressure wave • Sound: Physics; Salient features of perception. • Waves of compression and expansion of the air – Weber-Fechner laws, as in touch, vision – (Imagine a tuning fork, or a vibrating drum pushing the • Auditory Pathway: cochlea – brainstem – cortex air molecules to vibrate) – Optimal design to pick up the perceptually salient features • Tiny change in local air – Coding principles common to other sensory systems: � sensory or “place” maps, pressure: � receptive fields, – Threshold (softest � hierarchies of complexity. sounds): 1/10 10 – Coding principles unique to auditory system: timing Atmospheric pressure – Physiology explains perception – Loudest sounds • fMRI of language processing (bordering pain): 1/1000 • Plasticity (sensory experience or external manipulation). Atmospheric pressure • Diseases: • Mechanical sensitivity – Hearing impairment affects ~ 30 million in the USA + range Pitch (Frequency): heard in Octaves Pitch (Frequency): heard in Octaves • PITCH: our subjective perception is a LOGARITHMIC FUNCTION • PITCH: our subjective perception is a LOGARITHMIC FUNCTION of the physical variable (frequency ). Common Principle of the physical variable (frequency ). Common Principle • Pitch perception in OCTAVES: “Equal” intervals actually • Pitch perception in OCTAVES: “Equal” intervals actually MULITPLES.Sound “Do” in musical scales: MULITPLES.Sound “Do” in musical scales: C1. 32.703 Hz. C1. 32.703 Hz. C2. 65.406. C2. 65.406. C3. 130.81. C3. 130.81. C4 . 261.63. (middle C) C4 . 261.63. (middle C) C5 . 523.25. C5 . 523.25. C6 . 1046.5. C6 . 1046.5. C7. 2093. C7. 2093. • Two-tone discrimination: like two-point discrimination in the Pressure somatosensory system. Proportional to the frequency (~ 5%). • Weber-Fechner Law • WHY? Physiology: “place” map for frequency coding from the cochlea up to cortex; sizes of receptive fields. Just like Tim somatosensory system e Complex sounds: Multiple frequencies Loudness: Huge range; logarithmic Pressure 140 dB S cale • Why DECIBELS ? T hreshold of pain 130 • LOUDNESS perception: 120 Jet takeoff (200 ft) also LOGARITHM of the Tim R iveting machine 110 physical variable (intensity). (operator's position) e “wa” 100 P neumatic hammer (6 ft) Pressure – Fechner (1860) noticed: “equal” steps of perceived loudness S ubway train (20 ft) 90 P rinting press plant actually multiples of each other 80 V acuum cleaner (10 ft) in intensity. Logarithmic Tim – Defined: log scale (Bel) 70 Near freeway (busy traffic) e – 10 log 10 (I / I th ) Decibels: S peech (1 ft) 60 L arge store – Threshold: 0 dB: (1/10 10 • Natural sounds : atmospheric pressure) 50 Average residence – multiple frequencies (music: piano chords, hitting keys R esidential area – Max: 5,000,000 larger in at night 40 P erson's own heartbeat simultaneously; speech). We hear it as a “whole” not parts. amplitude, 10 13 in power and breathing S oft whisper (5 ft) 30 – constantly changing (prosody in speech; trills in bird song) – HUGE range. Inside S ound-proofed • Hierarchical system, to extract and encode higher • Encodes loudness 20 movie studio features (like braille in touch, pattern motion in vision) • Adapts to this huge range 10 (like light intensity) 0 Hearing threshold 1
Timing: Used to locate sound sources Auditory System: Demands • Frequency (logarithmic, octave scale) • Not PERCEIVED directly, but critical for LOCATING sources of sound in space: • Complex sounds: multiple & changing frequencies. – Interaural Time Difference (ITD) as a source moves away from the • Loudness (logarithmic scale; extending over a range of midsaggital plane. 5,000,000 in amplitude, i.e. 2.5 x 10 13 in intensity) – Adult humans: maximum ITD is 700 • (properties analogous to touch and vision) microseconds. – We can locate sources to an accuracy of a few degrees. This means we can measure • Timing, to 10 microsecond accuracy ITD with an accuracy of ~ 10 microseconds – Thus, auditory system needs to keep track of time to the same accuracy. – Unique to auditory system (vs. visual or touch) Auditory System: Ear Middle Ear: Engineering; diseases • Perfect design to transmit tiny vibrations from air to fluid inside cochlea • Stapedius muscle: damps loud sounds, 10 ms latency. • CONDUCTIVE (vs. SENSORINEURAL) hearing loss – Scar tissue due to middle-ear infection (otitis media) – Ossification of the ligaments (otosclerosis) • Rinne test: compare loudness of (e.g.) tuning fork in air vs. placed against the bone just behind the auricle. Principles of Neural Science (PNS) Fig 30-1 • Surgical intervention usually highly effective Principles of Neural Science, Chapter 30 Inner ear: Cochlea Basilar Membrane • Incompressible • 3 fluid-filled fluid, dense bone cavities (temporal). • Traveling wave • Transduction: (vibrations) IN THE organ of Corti: FLUID 16,000 hair cells, basilar • Basilar membrane: membrane to Individual elements tectorial (vibraphone, not membrane didgeridoo). PNS, Fig 30-3 PNS Fig 30-2 2
Basilar Membrane: tonotopy, octaves Basilar Membrane: tonotopy, octaves • Thick & taut near base • Thick & taut near base • Thin & floppy at apex • Thin & floppy at apex • Couples with vibrating • Couples with vibrating fluid to give local peak fluid to give local peak response. response. • Tonotopic PLACE map (...homunculus) • LOGARITHMIC: 20 Hz -> 200 Hz -> 2kH -> 20 kHz, each 1/3 of the membrane • Two-tone discrimination • Timing PNS Fig 30-3 PNS Fig 30-3 Organ of Corti Auditory System: Hair Cells Auditory system AND Vestibular system (semicircular canals) PNS Fig 31-1 Auditory System: Hair Cells Auditory System: Hair Cells • Force towards kinocilium opens • Force towards kinocilium opens channels & K + , Ca 2+ enter, channels & K + , Ca 2+ enter, depolarizing cell by 10s of mV. depolarizing cell by 10s of mV. Force away shuts channels. Force away shuts channels. • Tip links (em): believed to • Tip links (em): believed to connect transduction channels connect transduction channels (cation channels on hairs) (cation channels on hairs) • Cell depolarized / hyperpolarized – frequency: basilar membrane – timing: locked to local vibration – amplitude: loudness • Neurotransmitter (Glu?) release • Very fast (responding from 10 Hz – 100 kHz i.e.10 µ sec accuracy). PNS Fig 31-2, 31-3 PNS Fig 31-2 3
Hair Cells: Tricks to enhance response Hair Cells: Tricks to enhance response • Inner hair cells: MAIN SOURCE of afferent signal in • Inner hair cells: MAIN SOURCE of afferent signal in auditory (VIII) nerve. (~ 10 afferents per hair cell) auditory (VIII) nerve. (~ 10 afferents per hair cell) • Outer hair cells: primarily • Outer hair cells: primarily get EFFERENT inputs. get EFFERENT inputs. Control stiffness, amplify Control stiffness, amplify membrane vibration. membrane vibration. (5,000,000 X range) (5,000,000 X range) • To enhance frequency tuning: – Mechanical resonance of hair bundles: Like a tuning fork, hair bundles of cells near base of cochlea are short and stiff, vibrating at high frequencies; hair bundles near the tip of the cochlea are long and floppy, vibrating at low frequencies. – Electrical resonance of cell membrane potential (in mammals?) – An AMAZING feat of development. • Synaptic transmission speed (10 µ s): PNS Fig 30- 10 – Synaptic density: for speed? (normal synapse: 1 to 100s of ms) Ear: a finely tuned machine Cochlear prosthesis Optimally engineered to: • Most deafness: • pick up the very faint vibrations of sound & SENSORI-NEURAL • extract perceptually relevant features hearing loss. – pitch • Primarily from loss of cochlear hair cells, which – loudness do not regenerate. – complex patterns • Hearing loss means – timing problems with language acquisition in kids, social isolation for adults. • When auditory nerve unaffected: cochlear prosthesis electrically stimulating nerve at correct tonotopic site. PNS Fig 30-18 Auditory Nerve (VIII cranial nerve) Auditory Nerve (VIII): Receptive fields • Neural information from • Receptive fields: TUNING inner hair cells: carried by CURVE from hair cell cochlear division of the • “Labeled line” from “place” VIII Cranial Nerve. coding. T uning curves for single auditory fibres • Note: bandwidths equal (guinea pig) on log frequency scale. 120 ) L Determines two-tone P PNS Chapter 30 S 100 hreshold intensity (dB discrimination. 80 • Bipolar neurons, cell 60 bodies in spiral ganglion, 40 proximal processes on 20 hair cell, distal in cochlear nucleus. T 0 0.1 1 10 50 T one frequency (kHz) 4
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