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Chapter 11

Chapter 11. Sound, Audition, and Pitch. Sound Perception. Areas outside of the auditory cortex also process some sound information Parietal lobe Frontal lobe These areas are activated by both visual and auditory info This overlap occurs in areas associated with what and where processing.

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Chapter 11

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  1. Chapter 11 Sound, Audition, and Pitch

  2. Sound Perception • Areas outside of the auditory cortex also process some sound information • Parietal lobe • Frontal lobe • These areas are activated by both visual and auditory info • This overlap occurs in areas associated with what and where processing

  3. Sound Perception • Dorsal stream of processing (where) • Involves the parietal lobe • Concerned with the spatial location of sounds • Ventral stream of processing (what) • Involves the temporal lobe • Analyzes various components of sound

  4. Can the ear produce sound? • Yes. • EOAE (evoked otoacoustic emissions) • Sounds that the cochlea produces in response to acoustic stimulation • SOAE (Spontaneous otoacoustic emissions) • Less than 20dB above threshold • Sounds not heard • People who have these have more sensitive hearing

  5. Pitch Discrimination • Range: 20Hz to 15,000Hz or 20,000Hz • Minimal discriminable frequency difference • Between two tones • About 2Hz for sounds up to 2000Hz; above these frequencies is a larger difference

  6. Pitch Discrimination(Place theory) • Bekesy (1960) • States that the frequency of sound is indicated by the place along the organ of corti where nerve firing is the highest. • Remember: perception of pitch is based on frequency • Vibrating motion of the basilar membrane is similar to “snapping” a rope (called a traveling wave)

  7. Pitch Discrimination(Place theory) • The base of the basilar membrane is 3-4 times narrower than the apex • The base of the basilar membrane is also about 100 times stiffer than the apex • These physiological characteristics create the traveling wave.

  8. Pitch Discrimination(Place theory) • Envelope of the traveling wave • Maximum displacement that the wave causes at each point along the membrane. • This indicates which hair cells are moving along the basilar membrane • The hair cells that vibrate the most will be stimulated the most strongly • The peak is the frequency of the sound

  9. Pitch Discrimination(Place theory) • So, the pitch perceived depends on where sound causes max displacement on basilar membrane. • Pitch is encoded in the physical location of activated receptors along the basilar membrane • Remember: Base = High freq; Apex = Low freq.

  10. Pitch Discrimination(Place theory) • Evidence for Place theory • Measuring electrical responses along the cochlea results in a tonotopic map

  11. Pitch Discrimination(Place theory) • Evidence for Place theory • Auditory masking • One tone can mask or decrease our perception of another tone that is occurring at the same time • When the masking tone is playing, the thresholds for all frequencies are redeterimined. • There is a large increase in the threshold for the test tones

  12. Pitch Discrimination(Place theory) • Evidence for Place theory • Auditory masking • The masking tone effects the other tones because the vibration patterns overlap • More of an effect for 800 Hz then 200 Hz. • Place theory predicts masking functions

  13. Pitch Discrimination(Place theory) • Evidence for Place theory • As it turns out, we are actually even more sensitive to the vibration patters of the basilar membrane than Bekesy originally thought • We can distinguish between 400 and 405 Hz with no problem even though the patterns are almost identical • This sharp localization is the result of the outer hair cells

  14. Pitch Discrimination(Place theory) • Evidence for Place theory • Motile Response • It is not just the cilia on the outer hair cells that moves in response to sound • The entire hair cell moves • It tilts slightly and changes length in response to sounds

  15. Pitch Discrimination(Place theory) • Evidence for Place theory • Cooperation between inner and outer hair cells

  16. Pitch Discrimination(Volley theory) • Early frequency theory - neurons in the cochlea fire at the frequency of the sound waves heard. • Overly simplistic view • a single neuron is capable of firing at a maximum of only about 1,000 cycles per second, but yet we can hear sounds up to 20,000 cps. • So, the volley theory was proposed.

  17. Pitch Discrimination(Volley theory) • The volley theory says that specific frequencies are represented by groups of neurons firing in succession. • One after the other • So, a 500-cps tone would cause one group of neurons to fire at 500 cps. • A 1,000-cps tone would cause two groups of neurons to fire alternately at 500 cps. • A 2,000-cps tone would cause four groups of neurons to fire alternately at 500 cps (or, perhaps, 20 groups of neurons to fire alternately at 100 cps).

  18. Pitch Discrimination • Most recent thinking: Update of place theory explains all of our sound perception. • Pitch discrimination is accomplished by two functions • A traveling-wave mechanism for high frequencies and a frequency mechanism for low frequencies. • So, volley theory is no longer considered

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