Hearing PDF

Sound Sound: pressure changes in the air Sound waves: waves of pressure changes in air caused by vibrations of a source Initiated by movement that disturbs air molecule causing them to collide with others, resulting in air pressure change outward from the source ◦ Travel faster under water ◦...

Sound Sound: pressure changes in the air Sound waves: waves of pressure changes in air caused by vibrations of a source Initiated by movement that disturbs air molecule causing them to collide with others, resulting in air pressure change outward from the source ◦ Travel faster under water ◦ Travel slower than light: ~1500m/s Vs light ~340m/s ◦ Prolonged exposure to intense sound can cause irreversible hearing loss ◦ Human can hear limited range (vary with age) 20Hz to 20kHz -5dB to 130dB Compression: increase in air pressure in the space that air molecules created by moving in a specific direction ◦ Positive peak Rarefaction: decrease in air pressure in the space that air molecules created by moving in a specific direction ◦ Negative peak Cycle: repeating segment of air pressure changes Periodic sound waves: waves in which the cycles of compression and rarefaction repeat in a periodic fashion Hertz (Hz): number of cycles per second Decibels (dB): decibels dB SPL = 20log(p/p0) Audibility curve: min amplitude at which sounds can be detected at each frequency ◦ From 20 Hz to 20kHz ◦ From -5dB to 130 dB ◦ Varies with age Equal loudness contour: curve showing amplitudes of tones at different frequencies that seem equally loud Phon: unit of loudness ◦ Loudness of a tone in phons = amplitude of 1k Hz tone that sounds equally loud Dimensions of Sounds 1. Frequency: number of cycles per second ◦ Measured in hertz (Hz) ◦ Perceived as pitch of the sound 2. Amplitude/intensity: difference between the max and min sound pressure in a sound wave (highest and lowest level of oscillation) ◦ Perceived as the intensity of sound 3. Waveform: more complex shape than sine wave (complex sounds), sum of multiple pure tones ◦ Fourier analysis: mathematical procedure for decomposing a complex waveform into a collection of sine waves with varies frequencies and amplitudes Pure tone: sinusoidal (sine) wave; simplest periodic sound wave  ◦ Harmonic Spectrum: depiction of the amplitudes at all frequencies Fundamental frequency: lowest frequency component of complex waveform; dominant frequency ◦ 100 Hz ◦ Determines perceived pitch ◦ Harmonic: each component of the complex waveform ◦ First harmonic is fundamental frequency ◦ Overtones: higher frequencies that are multiples of the fundamental frequency ◦ Timbre (sound quality): difference in sound quality between 2 sounds with the same pitch and loudness or differences in relative amplitudes of the overtones The Ear 1. OUTER EAR PINNA collects sounds from environment ◦ Funnels into ear canal ◦ Helps localize sounds EAR CANAL enhances certain frequencies (~2k-5k Hz) ◦ Terminates in the tympanic membrane TYMPANIC MEMBRANE thin sheet of skin at the end of ear canal ◦ Sound makes it vibrate ◦ Vibrations transmitted to middle ear 2. MIDDLE EAR Amplification and transmission Vibrations from tympanic membrane amplified by OSSICLES (3 tiny bones) ◦ MALLEUS (hammer): transmits vibrations from the tympanic membrane ◦ INCUS: transmits vibrations from malleus to stapes ◦ STAPES: vibrates the oval window of the cochlea 2 muscles (tensor tympani & stapedius) contract in response to high intensity ◦ Attenuation reflex: protects the inner ear from damage due to loud sounds Not effective for sudden high intensity sounds due to delays EUSTACIAN TUBE: allows for equalization of pressure in the middle ear ◦ Connects middle ear and top part of throat 3. INNER EAR Signal transduction Vibrations are transducted into a neural signal COCHLEA: contains fluid-filled chambers 1. Vestibular canal REISSNER'S MEMBRANE: separates the vestibular canal from the cochlear duct Filled with perilymph 2. Cochlear duct  BASILAR MEMBRANE: separates the cochlear duct from the tympanic membrane ◦ Filled with endolymph Contains ORGAN OF CORTI: sits on the basilar membrane and contains the hair cells responsible for signal transduction 3. Tympanic canal Filled with perilymph Vestibular and tympanic canals are connected at the helicotrema, which provides an open pathway for the perilymph to carry vibrations ◦ Round window: membrane covered opening that serves as a relief valve for the pressure waves traveling through the perilymph Frequency tuning BASILAR MEMBRANE Basilar membrane is narrow and thick at the base and wide at the apex Characteristic frequency: frequency to which each location on the basilar membrane responds most readily to ◦ high frequency vibrations cause maximum displacement at the base ◦ low frequency vibrations cause maximum displacement at the apex Auditory signal transduction ORGAN OF CORTI has hair cells that generate a neural signal from the vibration of the basilar and tectorial membranes There are 3 rows of outer hair cells + 1 row of inner hair cells Inner hair cells ~5% of auditory hair cells Innervated by type 1 auditory neurons Responsible for auditory transduction; from mechanical energy into APs STEREOCILIA: hairlike extensions on the hair cells ◦ Tip connected to the side of its neighbour by a tip link ◦ Shear forces on it pull on the tip links, which open mechanic-gated ion channels and depolarize the hair cell Outer hair cells ~95% of auditory hair cells Innervated by type 2 auditory afferent neurons Tip links on OHCs depolarize the cell when there's tension: opens ion channels Primary function: amplify and sharpen the inner hair cells response; not to send that signal to the CNS ◦ They dance: contract and change their length to amplify the movement of basilar membrane ◦ Motile response: amplifies and sharpens the movements of the basilar membrane Cochlear amplifier  Also receive efferent innervation that allows this effect to be modulated by CNS Auditory nerve: conveys signals from the hair cells in the organ of Corti to the brain Made up of type 1 and type 2 auditory nerve fibers bundled together Neural Representations Frequency Represented by a place code and a temporal code in the auditory system 1. Place code: based on the displacement of the basilar membrane at different locations ◦ Different locations on the basilar membrane respond selectively to different frequencies The location of an inner hair cell will determine the frequency that it will respond the strongest to ◦ Frequency tuning of type 1 auditory neurons can be almost entirely accounted for by the frequency response of the basilar membrane 2. Temporal code: based on a match between the frequencies in incoming sound waves and the firing rates of auditory nerve fibers ◦ Inner hair cells will also fire with specific timing relative to the stimulus wave ◦ Time-locked mechanism works as long as each nerve fiber in a population of fibers produces APs in phase with the incoming sound stimulus APs are produced at the same time as the peaks in the incoming sound wave At higher frequencies, neurons are not phase-locked to the stimulus fire with specific timing relative to the stimulus wave, creating a temporal representation of frequency ◦ Not every fiber produces an AP, but when it does its at the peak Volley principle: Phase-locked response doesn’t mean that it fires every cycle of the sine wave Activity of the population of neurons will have a response at that frequency (and integer multiples of it) Tonotopic organization based on frequency is maintained throughout the auditory system Auditory neurons with higher characteristic frequencies have sharper tuning curves ◦ Cat neurons: response to higher frequencies than humans can detect Psychophysics experiments show similar responses in humans Temporal code for frequency arises from the phase-locked responses of neurons to a sound stimulus ◦ Neuron fires at a specific phase of the sine wave Phase-locking occurs in auditory neurons up to ~5 kHz Sound Intensity Represented as both a rate code and number of active hair cells As the sound intensity increases, a larger area of the basilar membrane will move ◦ The more hair cells will be active Firing rate of a type 1 auditory neuron will increase as the intensity/amplitude of the vibrations of the basilar membrane increase ◦ Saturation level: When reaching the max firing rate, signal saturates Different sensitivities of different hair cells allow the encoding of a wide dynamic range