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Lecture 9

Lecture 9 PSYB51.docx

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University of Toronto Scarborough
Mathias Niemier

Lecture 9 Hearing: Physiology and Psychoacoustics Explain the premotor theory of attention:  Stages before the actual execution of the motor command, that the muscles are twitching somehow and moving the eyeballs  Attention is ready-to-put-in-place occumotor command  You can actually suppress these eye movements The Basics: – Nature of sound – Anatomy and physiology of the auditory system – How we perceive loudness and pitch – Impairments of hearing 
 • Hearing in the Environment
 • Sound Localization (where?)  Sounds are created when objects vibrate – Vibrations of object cause molecules in object‟s surrounding medium to vibrate as well, which causes pressure changes in medium – Longitudinal direction
 – Speed of sound depends on density of medium – Air: ~340 m/s
 – Water: ~1500 m/s  Basic qualities of sound waves
 – Amplitude: Magnitude of displacement of a sound pressure wave-> molecules in the air are going to be pushed away and towards the original vibrating object. Molecules themselves don‟t travel very far. – Intensity: Amount of sound energy falling on a unit area  Measured in decibels sound pressure level (dB SPL) ex. from 6-> 12-> 18-> 24  Loudness is indirectly related to intensity. Ex. a gun shot is very loud but in reality it‟s intensity is much higher. If you had a constant tone that was louder that the gun shot, you would have the constant tone being lower than the gun shot, because brief loud sounds are not perceived as loud – Decibels: Ratio between the pressure of some sound and the pressure of a reference sound p (~minimum).– dB = 20*log(p/p ); e.g. 20*log(2) = 6 0 0 – Loudness: The psychological aspect of sound related to perceived intensity or magnitude  Humans can hear across a wide range of sound intensities – Ratio between faintest and loudest sounds is more than one to one million, 6dB = double amount of pressure Basic qualities of sound waves(cont‟d)
 – Frequency: For sound, the number of times per second that a pattern of pressure change repeats – Measured in 1/s = Hz – Frequency is associated with pitch – Pitch: Psychological aspect of sound related to the fundamental frequency (low: tuba, high: piccolo). – (Phase): high level of molecules (high pressure) or low pressure, which tells you in which phase the sound is > In industries, loud machines or plants that creates a lot of noise, you want to avoid too much noise spreading out of that plant. You can record the noise created by the machine, put in in the computer, and play it with the speakers (the speakers will duplicate the noise with the opposite phase- like an anti-noise)  Interactions between amplitude and frequency • Human hearing uses a limited range of frequencies: From about 20 to 20,000 Hz  at certain frequencies,, around 5000 Hz, this is where your threshold is very low.  Even small amounts of energy are perceived very well  Red line: In terms of psysics, it doesn‟t matter how loud the frequency is , it‟s just extremely painful  Blue line: indicates that it‟s going to have a high chance of damaging your ear (very dangerous)  One of simplest kinds of sounds: Sinewave, or pure tone – Sine wave: Waveform for which variation as a function of time is a sine function
 – Most sounds in world: Complex sounds, (e.g., human voices, birds, cars, etc.) – All sound waves can be described as some combination of sine waves  Complex sounds can be described by Fourier analysis – Spectrum: A representation of the relative energy present at each frequency  Harmonic spectrum: Typically caused by simple vibrating source, (e.g., string of guitar). The frequencies of its components are integer multiples of lowest frequency. st – Fundamental frequency: Lowest frequency component of a sound (= 1 harmonic) – 262, 524, 786, 1048 Hz – You only hear the first harmonic, not the second or third, or fourth. ; all joint together, you hear it as the fundamental frequency because they play together – Sounds w/ same pitch and loudness may still sound different (piano vs. guitar) – timbre  Anatomy and physiology of the auditory system  Outer, middle, inner ear
  Two strategies to code frequency  Auditory pathway
  Cortical structures Outer ear: – Sounds are first collected from environment by the pinna (ear lobe) – Sound waves are funneled by the pinna into ear canal (you want to protext them from being damaged because there are tiny bones) – Length and shape of ear canal enhance sound frequencies – Main purpose of canal is to insulate structure at its end: Tympanic membrane, vibrates in response to sound  Teach yourself about the middleear!
 • Step1:What‟s the problem with audition? > the higher frequencies get deflected Middle ear: 3 ossicles: malleus, incus, stapes; smallest bones in body Step 2: physics refresher -man trying to lift his rock  when the prof. „hit‟ the student‟s hand, it did not hurt him, but if her applied the same level of pressure with a pencil, it would definitely hurt his hand because the surface area is much smaller Step 3: Solution – Enhance sound (lever mechanism + focusing pressure on smaller area) – Stapes transmits vibrations of sound waves to oval window – Loud sounds: muscles Inner ear: Fine changes in sound pressure are translated into neural signals – Cochlea: oval/round window, three canals -- snail looking thing: auditory nerve  Cochlear canals and membranes
 – Cochlea: Spiral structure of the inner ear containing the organ of Corti – Cochlea is filled with watery fluids in three parallel canals • Middle canal surrounded by ...
 • Vestibular canal + tympanic canal – Separated by membranes » Organ of Corti sits on top of basilar membrane, covered by tectorial membrane  Vibrations transmitted through tympanic membranes and middle-ear bones cause stapes to push and pull flexible oval window in and out of vestibular canal at base of cochlea – If sounds are extremely intense, any remaining pressure is transmitted through helicotrema and back to cochlear base through tympanic canal, where it is absorbed by another membrane: Round window  Organ of Corti – Sound waves transformed into movements of the 
 ossicles ... – Movements of cochlear partition which then are translated into neural signals by structures in the organ of Corti; extends along top of basilar membrane – Made up of specialized neurons called hair cells, dendrites of auditory nerve fibers that terminate at base of hair cells, and scaffold of supporting cells  Tectorial membrane: Extends a top organ of Corti; gelatinous structure • Hair cells in each human ear: Arranged in fourrows that run down length of basilar membrane – Inner hair cells: afferent auditory information. Inner hair cells send sound information to the brain stem, and it sends the information back through the outer hair cells, so inner hair cells become more sensitive -- Outer hair cells: 
 efferences (flow of signals out of the brain; exclusively muscles or like it), feedback system ; crucial for hearing. They expand in size. The hair itself creates very faint sounds  Coding of amplitude and frequency in the cochlea – Place code: Different parts of cochlea tuned to different frequencies; information about the frequency of an incoming sound is coded by place along cochlear partition, i.e. at the location with greatest mechanical displacement • Thickness & width of basilar membrane The auditory nerve(AN) – AN fibers sensitive to certain frequencies depen
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