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PSYCH 1XX3 (1,043)
Joe Kim (962)
Lecture 15

Lecture 15 Music Perception.docx

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McMaster University
Joe Kim

1 Lecture 15: Music Perception Introduction  Music is more than just a collection of individual notes that are strung together in a certain order  Instead, when you hear music, you perceive it as an organized whole  This organized whole can form an acoustic pattern that is so salient that you can hum the tune after hearing it only once  The acoustic pattern is easily recognizable, even if it is played in a different key or with different instruments  This suggests that what’s important to the perception of this pattern is the relation between the notes and not the individual notes themselves Auditory Scene Analysis  The same Gestalt principles used to organize a visual scene also apply to organizing an auditory scene  Ex. Incoming stream of sounds are separated into figure and ground (figure-ground) o We can consider the “ground” (background) to be whatever sounds you’re not focusing on, such as the random sounds of a subway station itself o The “figure” is the sound of a particular arriving train or a specific voice on the subway platform that you are paying attention to o However, the sounds that make up the figure and ground are not permanent and will change as you focus your attention o Ex. You may be listening to your friend while ignoring the drone of the random sounds around the subway station, then have your attention suddenly drawn to the PA system announcing that your train will be arriving late o You have switched the figure and ground and the friend that you were previously listening to has become background noise to the PA system  Proximity o The principle of proximity organizes sounds that occur close together in time or space o If you played a series of high and low tones both spaced apart in time you would perceive two separate tones o However, if you played the tones closer together in time, you would hear a single tone  Similarity o Allows you to group together auditory input that is similar, such as sounds that are of a similar frequency of timbre o Allows you to pick out and group a series of sounds as all belonging to one particular voice among many voices  Continuity o The principle you would use to follow along with one song, even if you simultaneously heard another song playing with the same instruments 2  Closure o Principle that allows you to understand a conversation, even if every other sound was muffled or missing Pitch Perception  Pitch perception, or how the frequency of a sound wave affects our perception, has been studied extensively  Recall that the lowest frequency we can hear is 20 Hz and the highest is about 20,000 Hz  Recall that sound waves enter the ear canal, vibrating the eardrum, further amplified by the ossicles, which cause a wave in the fluid in the cochlea  This movement of the fluid causes the hair cells along the basilar membrane to move, sending a signal that is sent down the auditory nerve to key regions in the brain  There are two theories required to explain pitch perception along the entire frequency range that we can hear o Frequency theory is so named because it was thought that the entire basilar membrane vibrates at the frequency of the incoming sound wave  This causes impulses of the corresponding frequency to travel up the auditory nerve, effectively allowing the brain to decipher frequency by counting the number of neural impulses  In accordance with the predictions of the theory, physiological evidence indicates that the hair cells on the basilar membrane do indeed vibrate together  Problems  It was learned that axons are incapable of firing more than 1000 impulses per second  This would work fine if all of the sounds that were important to our reproduction and survival were less than 1000 Hz  Although a single axon can’t fire more than 1000 impulses per second, groups of auditory nerve fibres can fire a series of impulses that as a team, can signal to the brain the frequency of sound waves up to 5000 Hz  Called the volley principle and it extended the audible frequency range for the frequency up to 5000 Hz  But still not enough to cover the entire frequency range that reaches 20000 Hz  So theory cannot explain how we perceive pitches between 5000 and 20000 Hz o Although the hair cells along the basilar membrane move together as the frequency theory predicts, they in fact move as a traveling wave that forms a peak at a particular place along the basilar membrane o Place theory of pitch perception states that the brain can decipher the frequency of the sound wave by being tuned to the specific place of the peak of its traveling wave along the basilar membrane 3  Recall that each inner hair cell has roughly 20 direct links with the brain, which would allow the region of each inner hair cell on the basilar membrane to be represented very specifically in the auditory cortex  When a sound causes a wave in the basilar membrane, high frequency sounds maximally displace the hair cells closest to the oval window, where sound initially enters the cochlea  On the other hand, low frequency sounds produce a wave that peaks at the opposite end of the cochlea  This results in tonotopic representation of pitch and this organization is maintained all the way to the primary auditory cortex, with neighbouring regions of the cortex responding maximally to neighbouring frequencies  Tonotopic map contains a map of different locations on the basilar membrane in the primary auditory cortex  Tonotopy in Primary Auditory Cortex  Neurons are arranged in such a way that high frequency sounds activate neurons at one end of the cortical area and low frequency sounds activate neurons at other end and each neuron is maximally sensitive to sounds at a certain frequency  Although each hair cell is maximally responsive to a specific frequency, it will still respond to a range of frequencies  Much like how visual receptors respond maximally to light of a specific wavelength, but will also respond to a range of wavelengths of light  Direct evidence for tonotopic coding of pitch and support for the place theory comes from animal studies that have used drugs that can damage the hair cells  In one experiment, Stebbins and colleagues administered the drug and then tested the monkeys’ ability to perceive different frequencies of sound  When the cochleae was later observed, they found that even brief exposure to the drug damaged hair cells near the entrance to the cochlea at the oval window  With longer exposure to the drug, damage to the hair cells extended toward the other end of the basilar membrane  The behavioural tests showed that monkeys with damage to the hair cells near the oval window were unable to perceive high frequency sounds  More damage along the basilar membrane translated into a growing inability to hear progressively lower frequency sounds  Taken together, these results demonstrate that different frequencies are represented at specific places along the basilar membrane, with high frequencies at the entrance of the cochlea and lower frequencies at the opposite end of the cochlea 4  Problems with place theory  As the frequency of sound gets lower, the location of the peak of the wave along the basilar membrane gets more and more variable  For very low frequencies under 50 Hz, the peak actually disappears completely  So the place theory alone cannot account for the full audible range of hearing, much like the frequency theory  It turns out that both theories are needed to explain the full range of hearing  Frequency theory is useful to explain how we hear low frequencies that are below 1000 Hz and place theory explains how we perceive high frequencies above 5000 Hz  Both mechanisms are theoretically used for frequencies between 1000 to 5000 Hz, which is coincidentally the range of frequencies that we discriminate most effectively Bird Song  Bird song is defined as the music-like vocalizations that are made primarily by the male of a species during the breeding season in order to attract a female or defend his territory from other males  Songbirds have evolved two key brain regions to deal with the complexities of producing song o The high vocal centre (HVC) and the robust nucleus of the archistriatum (RA) o As males are the primary producers of song, both of these regions are larger in males than females o The sex differences are controlled in part by hormones  A female given testosterone will show an increase in the size of these brain regions and will begin to sing like a male o These key brain regions are further modified by experience  Males that are particularly at song have enlarged HVC and RA brain regions  Birdsong is an excellent collaboration between inherited and learned components during development to produce a species-typical behaviour  Birds may inherit a genetic predisposition to sing, but they must learn and practice to produce correct songs  Evidence for Learning o Marler and Tamura analyzed the songs of white-crowned sparrows that lived in three different local regions o Their findings confirmed that songs were at least partially learned because the same species birds living in these different regions had different dialects o Although their songs were similar and these birds would understand each other, they had different accents and would be recognized as strangers  Evidence for Inheritance o Marler and Peters conducted a simple experiment to make the point 5 o They took two closely related species of sparrow, the swamp sparrow and the song sparrow, and raised them in isolation, while exposed to tape recorded birdsongs from both species o Results showed that each species learned to produce the song of its own species, even though each group was equally exposed to the auditory input from both species o This suggests that there is some genetic hardwiring that guides learning toward the birds’ own species-specific songs  Birds usually hatch in the spring or early summer and are only exposed to the specific bird song from the father until the end of summer  The young bird then gets no more exposure to the song and has to remember it for
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