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Psych 1XX3 Music Perception Lecture Notes.pdf

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Joe Kim

Psych 1XX3 – Music Perception Notes – Mar 18, 2010 Introduction:  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's 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: Gestalt Principles and Auditory Analysis:  How are we able to organize our auditory world so easily? The same Gestalt principles used to organize a visual scene also apply to organizing an auditory scene. Figure Ground Principle:  For example, incoming stream of sounds are separated into figure and ground.  We can consider the “ground” (or background) to be whatever sounds you’re not focusing on, such as the random sounds of the subway station itself and the “figure” as the sound of a particular arriving train, or a specific voice on the subway platform that you are paying attention to.  Keep in mind, however, that the sounds that make up the figure and ground are not permanent, and will change as you focus your attention. Proximity:  The principle of proximity organizes sounds that occur close together in time or space.  If you played a series of high (A) and low (B) tones both spaced apart in time you would perceive two separate tones.  However if you played the tones closer together in time, you would hear a single tone. Similarity:  The principle of similarity allows you to group together auditory input that is similar, such as sounds that are of a similar frequency or timbre.  This would allow you to pick out and group a series of sounds as all belonging to one particular voice among many voices. Continuity:  Continuity is the principle that you would use to follow along with one song, even if you simultaneously heard another song playing with the same instruments. Closure  Closure is the principle that would allow you to understand a conversation, even if every other sound was muffled or missing. Pitch Perception:  Recall from our coverage of audition that frequency is measured in Hz, and that the lowest frequency we can hear is 20 Hz, and the highest is about 20000 Hz.  Also, 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. (See image below.)  There are two theories required to explain pitch perception along the entire frequency range that we can hear. Frequency Theory:  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. Problems  In accordance with the predictions of the frequency theory, physiological evidence indicates that the hair cells on the basilar membrane do indeed vibrate together.  The frequency theory made perfect sense, until 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. However we learned that humans can hear sounds with frequencies as high as 20000 Hz. Volley Principle:  Although a single axon cannot 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.  This is called the volley principle, and it extended the audible frequency range for the frequency theory up to 5000 Hz. BUT, it’s still not enough to cover our entire frequency range that reaches 20,000 Hz.  So the frequency theory of pitch perception cannot explain how we perceive pitches between 5000 and 20,000 Hz. (See image below.) Place Theory:  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.  And so, the 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 travelling wave along the basilar membrane. (See image below.)  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 with that peaks at the opposite end of the cochlea. Tonotopic Representation of Pitch in A1:  This results in a 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. Hair Cells Respond Maximally to One Frequency:  Although each hair cell is maximally responsive to a specific frequency, it will still respond to a range of frequencies.  Direct evidence for tonotopic coding of pitch, and support for the place theory, comes from animal studies which 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. (See image below.)  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. Problems w/ Place Theory:  These trends are generally true, but a problem with the place theory is that as the frequency of the 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 also cannot account for the full audible range of hearing; it turns out that both the frequency and place 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:  Def’n of Bird Song: 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. High Vocal Centre and Robust Nucleus:  Songbirds have evolved two key brain regions to deal with the complexities of producing song: the high vocal center (HVC) and the robust nucl
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