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PSYCH 1XX3 (1,109)
Joe Kim (1,028)
Lecture 14

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Joe Kim

1 Lecture 14: Audition (which ear structure cause the most amplification?) (What ear structure protect our ear from being damaged by loud noise?) > Ear canal : first amplification occurs The Auditory Mechanisms of Different Species  Auditory mechanisms vary across different species according to specific needs  Sound Frequency o Different species can hear different ranges of frequencies o Ex. Dog whistle o Humans can perceive sounds that lie anywhere between 20 and 20,000 Hz o Whales, dolphins and dogs have a wider hearing range, while frogs and birds have a narrower range o At the lower frequency detection extreme are fish, while at the higher frequency detection extreme are bats and rodents o Audible frequency range is determined in part by evolution of structures of auditory system o One key structure is the basilar membrane, which contains the hearing receptors o Sounds of different frequencies are processed along different areas of basilar membrane  The Basilar Membrane o Varies in length across species o It is shortest in amphibians and reptiles, longer in birds and longest in mammals o A longer basilar membrane allows processing of a wider range of frequencies The Stimulus: Sound Waves  Sound travels in waves, although sound waves travel much slower and require some medium to travel through  Sound waves are initiated by either a vibrating objet, like vocal cords or a guitar string, or by forcing air past a small cavity, like a pipe organ  This causes air molecules surrounding the source of the sound to move, causing a chain reaction of moving air particles  These alternating bands of more and less compressed air molecules interact with eardrum to begin auditory processing  A band of compressed air molecules causes your eardrum to get pushed slightly inwards whereas a band of less dense air particles causes the eardrum to move outwards  Changes in air pressure over time that make up a sound wave can be graphed as a sine wave 2  The three physical characteristics of the wave, amplitude, wavelength and purity, when applied to sound waves, translate into the three psychological properties of loudness, pitch and timbre  Amplitude: Measure of Loudness o Variations in amplitude (can be measured) or height of sound wave affect the perception of loudness o Since waves of greater amplitude correspond to vibrations of greater intensity, higher waves correspond to louder sounds o Loudness is measured using a logarithmic scale of decibels (db) o In this scale, the perceived loudness of a sound doubles for every 10 dB increase o A whisper is at around 27 dB, a normal conversation at 60 dB and the front row of a rock concert at around 120 dB  Frequency: Measure of Pitch o Sound waves also vary in the distance between successive peaks, called wavelength or frequency of sound and this property affects the perception of pitch o Pitch is the internal representation on how we present frequency. o Pitch is measured in hertz (Hz), which represents the number of cycles per second or the number of times in a second that a sound wave makes one full cycle from one peak to the next o So if many wave peaks are condensed into one second, then this sound will be of a high frequency and result in a perception of a high pitched sound o Similar to light, the audible zone of frequencies that humans can detect represents only a portion of the possible frequencies that can be produced  Timbre: Measure of Complexity/Purity o Most sounds we hear everyday are complex sounds that are composed of multiple sound waves that vary in frequency o Timbre refers to the complexity of a sound o Ex. When you pluck a guitar, it vibrates as a whole (fundamental), but also vibrates at shorter segments along the string (overtones)  The final sound you hear is a mixture of the fundamental tones and all the overtones and this combination is timbre o So a piccolo and a bassoon may both play the same note, but because each instrument produces a unique combination of fundamental frequency and overtones, they still sound different to us even though each instrument is producing same frequency and amplitude The Ear  Structure of Ear o Can be divided into the external, middle and inner ear and each area conducts sound in a different way o Incoming changes in air pressure are channelled through the external ear, onto the middle ear and amplified so that it can be detected as changes in fluid pressure by inner ear 3 o These changes in fluid pressure are then finally converted to auditory neural impulses  The External Ear o Made up of the pinna, ear canal and eardrum o The pinna is the folded cone that collects sound waves in the environment and directs them along the ear canal o Since the ear canal narrows as it moves towards the eardrum, it functions to amplify the incoming sound waves, much like a horn o The eardrum is a thin membrane vibrating at a frequency of the incoming sound wave and forms the back wall of the ear canal  The Middle Ear o Begins on the other side of the eardrum, which connects the ossicles, the three smallest bones in the body o The ossicles consist of the hammer, the anvil and the stirrup o The amplification of vibrating waves continues here in the middle ear o Vibrating ossicles are about 20 times larger than the area of the oval window to which they connect to create a lever system that amplifies the vibrations even more o The additional amplification is necessary because the changes in air pressure originally detected by the external ear are about to be converted to waves in the fluid-filled inner ear  The Inner Ear o Vibrating oval window connects to the cochlea of inner ear o Cochlea is a fluid-filled tube about 35 mm long, coiled like a snail shell o Cochlea contains the neural tissue that is necessary to transfer the changes in fluid to neural impulses of audition  The Cochlea o The oval window is actually a small opening in the side of the cochlea and when it is made to vibrate, it causes the fluid inside the cochlea to become displaced o The round window, located at the other end of the cochlea, accommodates for the movement of the fluid by bulging in and out accordingly  The Basilar Membrane o Inside the cochlea is a flexible membrane, called the basilar membrane that runs the length of the cochlea like a carpet o So when the basilar membrane is pushed downwards, the fluid inside the cochlea causes the round window to bulge out and when the basilar membrane is forced upwards, the round window bulges inwards o Although the cochlea itself gets narrower toward the end, the basilar membrane gets wider o Because the length of the membrane varies in both flexibility and width, sounds of different frequencies cause different regions of the membrane to vibrate o Higher frequency sounds cause the end nearest the oval window to vibrate whereas lower frequency sounds cause the end nearest the round window to vibrate  Hair Cells 4 o Basilar membrane houses auditory receptors, which are called hair cells o As membrane moves in response to waves in the fluid, the hair cells also move and this movement is finally converted to neural impulses that the brain can understand o Hair cells only respond maximally to one particular frequency. High frequencies displace the membrane closest to the oval window. Low frequencies displace the membrane closest to the round window. Inner hair cells have multiple afferents that run along the cochlear nerve, leading to signal amplification. Having fewer afferents reduces the amplification. Auditory Pathway: From Receptors to Auditory Cortex  When activated, hair cells along basilar membrane release a neurotransmitter  Hair cells form synapses with bipolar cells, whose axons make up the cochlear nerve, a branch of the main auditory nerve  Although the outer hair cells outnumber the inner hair cells by about 4 to 1, it is the inner hair cells that mainly contribute to signal in the cochlear nerve  Some important differences between inner and outer hair cells o Each inner hair cell communicates with roughly 20 afferent fibres, which means that signal from each inner hair cell has exclusive rights to 20 direct links to the brain o The outer hair cells have to share one direct link to the brain with about 30 other outer hair cells (slower) o The axons that synapse with outer hair cells are thin and unmyelinated, whereas axons that carry information from inner hair cells are thick and myelinated o The arrangement of these connections suggest that even though there are far fewer inner hair cells than outer hair cells, inner hair cells are primarily responsible for transmitting auditory signal to brain  Cochlear Nucleus o Neurotransmitter released by hair cell is cap
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