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Chapter 4d - Audition and Music Perception Video Lecture Psych 1XX3.docx

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McMaster University
Joe Kim

Psych 1XX3 Chapter 4: Sensory Systems Audition  “If a tree falls in the forest and no one is around to hear it, does it make a sound?” depends what you define as sound o Falling tree would produce sound waves o Sound waves themselves do not make sound unless an auditory system is present to translate those sound waves into the perceptual experience of audition The Auditory Mechanism of Different Species  Sound Frequency Perception in Vertebrates o Range of frequencies that can be detected vary between species  Humans can perceive sounds that lie between 20 and 20,000 Hz  Whales, dolphins and dogs have a higher range  Frogs and birds have a narrower range  Fish have lower frequency detection  Bats and rodents have higher frequency detection  Environmental Impacts on Auditory Structure o Audible frequency range is determined in part by the evolution of the structures of the auditory system  Basilar Membrane – contains hearing receptors; sounds of different frequencies are processed along different areas of the basilar membrane o Varies in length across species  Shortest in amphibians  Longer in birds  Longest in humans – allows processing of a wider range of frequencies The Stimulus – Sound Waves  Sound waves are initiated by an action that causes air molecules surrounding the source of the sound to move, causing a chain reaction of moving particles o A vibrating object (like vocal cords or guitar string) o A sudden burst of air (clap) o Forcing air past a small cavity (pipe organ)  Waves travel in all directions  Alternating bands of more and less compressed air molecules interact with the eardrum to begin auditory processing o Compressed air molecules – causes eardrum to get pushed slightly inwards Psych 1XX3 o Less dense air molecules – causes eardrum to get moved outwards  Sine Waves – changes in air pressure over time that make up a sound wave can be graphed as a sine wave o Three physical characteristics of a wave translate into three psychological properties  Amplitude – loudness  Wavelength – pitch  Purity – timbre  Amplitude: Measure of Loudness o Waves of greater amplitude correspond to vibrations of greater intensity o Loudness is measured using a logarithmic scale of decibels (dB)  Perceived loudness of a sound doubles for every 10 dB increase o Normal Conversation – 60 dB o Whisper – 27 dB o Concert (sometimes every iPod) – 120 dB  Can cause physical pain and permanent damage  Frequency: Measure of Pitch o Wavelength – distance between successive peaks o Frequency – cycles per second o Pitch is measured in Hertz (Hz) – represents the number of cycles per second  Number of times in a second that a sound wave makes one full cycle from one peak to the next o Many peaks condensed into one second – high frequency  high pitch o Audible zone of frequencies that humans can detect represent only a portion of the possible frequencies that can be produced Psych 1XX3  Timbre: Measure of Complexity/Purity o When you pluck a guitar string  It vibrates as a whole – fundamental tone  Also vibrates at shorter segments along the string – over tones  Final sound you hear is a mixture of fundamental tone and all the overtones  combination is timbre o Different instruments produce a unique combination of the fundamental frequency and overtones – same amplitude and frequency – yet sound different The Ear  Detects the sound waves and converts them into something the brain can interpret  Can be divided into the external, middle and inner ear o Each area conducts sound in a different way o Incoming changes in air pressure are channeled through the external ear, onto the middle ear and amplified so that it can be detected as changes in fluid pressure by the inner ear o These changes in fluid pressure are then finally converted to auditory neural impulses  External Ear o Made up of the pinna, the ear canal and the eardrum o Pinna – folded cone that collects sound waves in the environment and directs them along the ear canal o Ear Canal – narrows as it moves toward the eardrum; functions to amplify the incoming sound waves o Eardrum – thin membrane vibrating at the frequency of the incoming sound wave and forms the back wall of the ear canal  Middle Ear o Connects to the ossicles, the three smallest bones in the body o Ossicles  Hammer  Anvil  Stirrup o Amplification of the vibrating waves continues here in the middle ear o The vibrating ossicles are about 20x larger than the area of the oval window to which they connect to create a lever system that amplifies the vibrations even more  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  Vibrating oval window connects to the cochlea of the inner ear  Inner Ear o Cochlea  Fluid filled tube; about 35 mm long, coiled like a snail shell  Contains neural tissue that is necessary to transfer the changes in fluid to neural impulses of audition o Oval Window – small opening in the side of the cochlea  When made to vibrate – causes the fluid inside the cochlea to become displaced o Round Window Psych 1XX3  Located at the other end of the cochlea  Accommodates for the movement of the fluid by bulging in and out accordingly o Basilar Membrane  Runs the length of the cochlea (inside the cochlea)  When pushed backward – fluid inside the cochlea causes the round window to bulge out  When forced upwards – the round window bulges inward  Gets wider toward the end (even though the cochlea gets narrower toward the end)  Length varies in flexibility and width – sounds of different frequencies cause different regions of the membrane to vibrate  Higher frequency sounds cause the end nearest the oval window to vibrate  Lower frequency sounds cause the end nearest the round window to vibrate  Hair Cells – auditory receptors in the basilar membrane  As the membrane moves in response to the waves in the fluid, the hair cells also move  This movement is finally converted to neural impulses that the brain can understand Psych 1XX3 Auditory Pathway – From Receptors to Auditory Cortex  When the hair cells along the basilar membrane are activated o Neurotransmitter is released o The hair cells synapse with bipolar cells, whose axons make up the cochlear nerve, a branch of the main auditory nerve  Inner hair cells mainly contribute to the signal in the cochlear nerve  Outer hairs more numerous but slower o Outer to Inner Hairs – 4:1 ratio o Outer hair cells have to share one direct link to the brain with ~30 other outer hair cells  Inner hair cells are less numerous but are faster and have more connections with the brain o Inner hair cell communicates with 20 afferent fibers – signal from each inner hair cell has exclusive rights to 20 direct links to the brain  The axons that synapse with the outer hair cells – thin and unmyelinated  The axons that carry information from the inner hair cells – thick and myelinated  Primarily responsible for transmitting the auditory signal to the brain  Cochlear Nucleus o The neurotransmitter released by the hair cells is capable of triggering EPSPs in the cochlear nerve fibers, which then sends this signal to the cochlear nuclease in the hindbrain o Separate dorsal and ventral streams  Vision o Ventral Stream – processes object recognition o Dorsal Stream – location of an object o Topographical Organization  Mapping to the neural pathways  Spatial organization of our visual world is maintained at all levels along the visual pathway  Neighboring locations in space fall on neighboring regions of our retinas  Tonotopic Organization o Frequency is coded along different regions of the basilar membrane because sounds of different frequencies displace the hair cells in these different regions o The hair cells connect to the cochlear nerve such that neighboring regions of hair cells remain together o This organization is maintained all the way through the auditory pathway to the primary auditory cortex  Frequency and the Basilar Membrane o The region of the basilar membrane that is closest to the oval window and responds to low frequency sounds is represented at one end of area A1 Psych 1XX3 o The region of the basilar membrane that is closest to the round window and responds to high frequency sounds is represented at the other end of area A1 o Information about similar frequencies is processed together Auditory Localization  Able to localize where a sound is coming from in space through auditory localization o Rely on the fact that our sense organs are separated in space  Auditory Localization is calculated from the neural representation of incoming sound  The fact that ears are located on opposite sides of our head results in interaural differences in sound that give us cues for auditory localization  Interaural Cues o The difference in time it takes for the sound to reach each ear  Can be measured in the sub-milliseconds – dependent on the direction of the incoming sound  Specific neurons in the superior olivary complex respond to these slight differences in the timing of arrival of the action potentials from each ear in response to the same sound o Intensity difference at each ear  For very close sounds, there is a detectable loss of intensity because the sound wave has to travel farther to reach one ear than the other  For sounds that are further away – difference in timing is less detectable; ears rely on the differences in intensity caused by the head which casts a “sound shadow” which diminishes the intensity at the distal ear  Input from each ear travels to both sides of the brain; these differences in intensity can be directly compared to calculate the location of the sound  Some neurons in the superior olivary complex respond specifically to these intensity differences from each ear, while others respond specifically to the interaural difference in arrival times for the sounds  Harder to localize a sound directly in front or behind you – reaches both ears at same time – must rotate head to cause slight change in sound intensity reaching each ear to localize sound o Pinna Cues  The sound direction produced by the characteristic folds and ridges of the pinnae  The pinna diffracts incoming sound waves to make significant changes in the frequency content of the sound that reaches the inner ear; some frequencies become amplified, while others are attenuated  These changes are collectively called pinna cues and are required for accurately localizing the elevation of a sound source  Are particular to an individual – each person has a different shaped pinna; “ear prints”  When pinna cues are altered (by placing plastic molds into the pinna cavities) – dramatic disorienting effect on localization ability  Subjects are able to adapt over a few weeks Echolocation in Bats  Bats are visually adept  Also able to use an entirely different system that is based on hearing – allows them to identify prey, navigate their way through thick forests and catch up to two mosquitoes per second in total darkness  One study found that a bat with a 40cm wingspan was able to fly through a 14x14 cm opening in a grid in total darkness  Echolocation – process by which a receiver emits sound pulses and analyses the returning echo to form a perceptual image of objects in the environment Psych 1XX3  Bats o Emit a burst of sound waves of very high frequency which bounces off the object and returns to the bats ears  1/3 to 1/5 msec  12 to 200 kHz o The bats brain analyzes the slight differences in the frequency content and timing if the returning sound waves to determine the characteristics of objects in its environment o Close object – return echoes sooner in time than objects further away o Moving objects – echoes will be Doppler-shifted compared to stationary objects o Textured objects – echoes will vary slightly in their return times relative to echoes from objects that are smooth  Co-evolution – the process by which the evolution and adaptation of traits of one species can directly affect the evolution of traits in another species o Bats have evolved an efficient system for navigation and prey detection o Prey have evolved a sense of hearing designed for detection of bat calls  Moths have evolved the ability to hear sounds that match the frequency range used by most bats when they’re hunting insects using echolocation  Chance of survival has increased by 40% o Selection pressures of a predator can drive
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