Frequency Coding and Pitch Perception

1. Frequency Coding and Pitch Perception Your host: Ben Friedman

2. TERMS TIMBRE - The quality of the sound being produced. Timbre is caused by the spectrum of a complex tone Consisting of a fundamental and its overtones or harmonics PITCH - Relative perceived height of a tone FREQUENCY - Number of cycles per second of a tone measured in Hz HARMONICS - Also called overtones, these are individual tones of frequencies which are multiples of that of the fundamental that comprise the spectrum of the tone. FUNDAMENTAL - The perceived tone of a spectrum of harmonics.

3. Basilar Membrane • Involved with pitch perception. Sheet-like part of cochlea. • The sheet moves creating an envelope which is sensitive to frequency and timbre.

4. Frequency Analysis in the Auditory System • Place Theory • Temporal Theory

5. Place Theory • Information about the particular frequency of an incoming sound wave is coded by which segment of the basilar membrane vibrates in response to that frequency.

6. Place Theory • This takes place because different sections of the basilar membrane vibrate at their maximum when subjected to different frequencies due to the stiffer nature of the membranes base as opposed to its floppy nature at the apex. Therefore low-frequency sounds will be maximized toward the apex and vice versa.

7. Notice the frequency range along the membrane

8. Lateral Inhibition • It is not easily explained under this theory how a people can distinguish such small differences in frequency. One theory is that much like the visual system, the auditory system makes use of lateral inhibition and inhibitory/excitatory cells to enhance the neural responses to pure tones. • This is however dependent upon the intensity of the stimulus

9. Coding on the Auditory Cortex • Under Place Theory corresponding frequency specific cells of the cochlea are mapped tonotopically on the auditory cortex • This is also analagous to the the way visual cortex is mapped in relation to the retina

10. Temporal (telephone) Theory • Auditory receptors respond to the vibration of the basilar membrane by firing the same temporal pattern in the auditory nerve. • Volley Principle - firings of individual nerve fibers follow alternating patterns of phase which collect to produce the original pattern of the stimulus. This is necessary because we can perceive sounds that have higher frequency than a nerve fiber can fire.

11. Evidence for Temporal theory • Monkey auditory nerve fibers • Stimulated with pure tones • The fibers respond to frequency tones with varied patterns that reflected the original frequency. • Pitches lose distinction at higher levels of frequency because the patterns become to fine to resolve.

12. Pattern Recognition Theory • Upon hearing a complex tone with missing harmonics we imagine a fundamental which fits in it.

13. Fourier Analysis • This is a Fourier Analysis of a tone comprised of 7 harmonics of the fundamental.

14. Complex Tones • Two tones of similar Frequency • Harmonic Series and Missing Fundamentals • Intervals and Chords?

15. How do we hear two tones of similar frequency? • Beating - Two waves of relatively equal amplitude traveling in the same direction causing a wave that oscillates with average frequency.

16. Beating • Note to self - learn power point.

17. Harmonic Series • Like we said earlier, tones found in nature are usually not a pure sine wave. They are a complex of overtones that create the perception of a single tone, the fundamental. • These harmonics have frequencies that ascend as multiples of the fundamental. • You can remove a number of tones from the harmonic series including the fundamental and still perceive the fundamental alone. • Spectral Music in the 1960’s made use of this phenomenon

18. Harmonic Series of a Trumpet • There are 20 differentiated harmonics composing this tone which help to give it a unique timbre.

19. Intervals and Chords • There is still much to be learned about how the brain processes pitches of different frequency played simultaneously(chords) or in succession(intervals). • Deutsch proposes a parallel channel theory in which the brain not only has separate channels for pitches separated by octaves but also for those separated by other intervals. • But we could talk all day about this alone…..so let’s wrap it up.

20. Summary • There are competing theories as to how the auditory system codes for pitch. The reality is it probably uses aspects from both theories, place and temporal to extract information about the pitch. • Pitch recognition as well as interval and chord recognition is not understood fully.

21. References • http://www.vimm.it/cochlea/cochleapages/theory/bm/bm.htm • http://www.kettering.edu/~drussell/demos/superposition/superposition.html • Duetsch, Diana. The Processing of Pitch Combinations. Ch 10. University of San Diego. La Jolla, CA. Academic Press. 1999 • Handbook for Acoustic Etiology. Ed: Barry Truax. Second Edition Cambridge Publishing 1999. http://sfa.ca/sonic-studio/handbook • Levine, Michael W. Fundamentals of Sensation and Perception. Oxford University Press, 2000