Psy280: Perception

1. Psy280: Perception Prof. Anderson Department of Psychology Audition 1 & 2

2. Hearing: What’s it good for? • Remote sensing • Not restricted like visual field • Can sense object not visible

3. Hearing: The sound of silence • A tree in the forest • Physical signal but no perception • One hand clapping • No physical signal, no perception • Separate physical quantity from perceptual quality • Sound is the perceptual correlate of the physical changes in air pressure • Or water pressure when under water • John Cage’s 4:33 No. 2, 1962

4. What are the physical attributes associated with sound? • Loudness • Amplitude or height of pressure wave • Pitch • Frequency of times per second (Hz) a pressure wave repeats itself

5. What is sound quality? • Pure tones • Single frequency (f) • Rarely exist in real world • Complex tones • More than one f • Due to resonance • Air pressure causes reverberations • E.g., tuning forks • E.g., Plucking the A string on a guitar • Fundamental frequency 440 Hz (cycles/s) • Harmonics • Reverberations at multiples of the fundamental • E.g., 880, 1320 • Creates fullness of complex sounds • Timbre is the relative amplification of harmonics

6. The human ear • Outer ear • Focusing of sound • Resonance amplifies 2000-5000 Hz range • Converts from air to mechanical vibration • Middle ear • Amplification • Fluid denser than air • Focus vibrations onto stapes/oval window • Increased leverage from ossicles • Inner ear • Sensory transduction • Physical to neural energy • Fluid pressure changes • Bending of hair cells

7. Auditory sensory transduction: The inner ear • Cochlea • Coiled and liquid filled • 3 layers • Cochlear partition • Contains organ of corti • Organ of corti • Cilia (hair) cells • Between basilar and tectorial membranes • Transduction • Movement of cilia between membranes

8. Auditory transduction • Bending—>physical energy • Converted to neural signals • Bend one direction —> depolarization • More likely to fire AP • Other direction —> hyperpolarization • Less likely to fire AP

9. Auditory pathways

10. Audition: What and where • What is it? • *Pitch • Identification • Surprisingly, little is known beyond speech • Where is it? • *location

11. What: Pitch • How does neural firing signal different pitches? • 1) Timing codes • 2) Place codes

12. Pitch: Temporal coding • Idea: Diff f’s signaled by rate of neuronal firing • Hair cell response • Bend one direction —> depolarization • Other direction —> hyperpolarization • Result? • Bursting pattern of neural response related to frequency of oscillation

13. Problems with temporal coding • Problem: A single neuron can’t fire at the rate necessary to represent higher f tones • E.g., 1000-20,000 Hz (i.e., 1000-20000 per second) • Max neuron firing rate: 500-800 per second • Solution: volley principle • No single neuron represents f • Coding across many neurons with staggered firing rates • Evidence: Phase locking • Diff neurons respond to diff peaks • Not every peak • Pool across multiple neurons to represent high f’s

14. Pitch: Place coding Owl brainstem • Related to doctrine of specific nerve energies • What is pitch? • Activation of different places in auditory system • Frequency specific • Tonotopy • Cochlear • Brainstem • Cortical • Stimulate these regions • Should result in pitch perception Human auditory cortex

15. Place coding starts in cochlea • Von Bekesy studied basilar membrane in cadavers • Base more narrow and stiffer • Apex wider and more flexible • Observed traveling waves • Diff frequencies (f) result in waves w/ diff envelopes • Higher f: Peak closer to base • Lower f: Peak closer to apex • Thus, f related to “place” where peak fluctuation occurs

16. Frequency tuning: Neural place coding • Tonotopic arrangement of hair cell nerves • Diff nerves innervate diff parts of basilar membrane • Allows for “place” code for frequency Frequency tuning curves of single hair cells

17. Complex tones: Fourier decomposition • Basilar membrane acts as f analyzer • Breaks down complex f inputs into constituent pure tone components

18. Auditory masking: Evidence for cochlear place coding • Auditory masking • Presence of certain tones decreases perception of nearby tones • Similar f result in greater masking • Asymmetry in spread of masking • Consistent with basilar vibrational overlap • E.g. 400 Hz mask overlaps more with 800 than 200 Hz 400 Hz mask Increases threshold for 800 more than 200 Hz

19. f Mystery of the missing fundamental • 400 Hz fundamental plus harmonics (800, 1200, 1600, 2000) • Sounds like 400 Hz pitch with complex timbre • What if remove fundamental f (400Hz)? • Perceived pitch doesn’t change! • Hence: The missing fundamental • Problem for place coding • No direct stimulation of 400 Hz on basilar membrane • Harmonic structure determines perceived pitch • Not what is present on basilar membrane • What we hear is not what the basilar membrane tell us, but what our brain does

20. What does Barry White sound like on the telephone? • Telephone carries 300-3400Hz • Typical male voice • Fundamental f = 120 Hz • Barry white • 30 Hz? • Can’t speak to Barry on the telephone? • Missing fundamental allows us to hear “virtual” pitch of voice

21. If its too loud your too old Pain and pleasure • Db (SPL) scale • Loudness doubles about every 10 db at 1000 Hz • Audibility curves • Loudness varies with f • Low volume • Attenuated low and high f relative to midrange • High volume • Less frequency attenuation • Low volume sounds muddy • Mostly mid range • I like my music loud Each curve represents equal loudness

22. Otoacoustic emissions: Talking ears • Ears don’t only receive sounds, they make them! • Discovered in 1978 • Tiny microphones • Occur spontaneously and also in response to sound • It like your ears are talking back! • Created by movement of outer hair cells (ohc) • Part of auditory sensitivity is movement of ohc to change region specific flexibility of basilar membrane • Allows tuning curves to be so narrow • Hearing impairments often start with loss of ohc function

23. Auditory localization • Where is the sound coming from? • Distance • Elevation (vertical) • Azimuth (horizontal) • Localization not nearly as precise as vision • Localization within 2-3.5 degrees in front of head • 20 degrees behind head • Suggests important role of vision • Tunes auditory localization

24. Why is is auditory localization not obvious? • Vision • Stimulate different photoreceptors in eye • Audition • No such separation of sounds sources on sensory surface • Sources combine to equally stimulate ear receptors

25. Why have two ears? • Two aural perspectives on the world • Like vision, can be used to get different sound pictures of environment • Binaural cues • The disparities between ears is used for localization

26. Azimuth • Interaural (between ears) Time Difference (ITD) • Air pressure changes are very slow relative to speed of light • ITD at side = max 600 µS • ITD at front = 0 • Can induce perception of location by varying ITD using headphones • Interaural Level (intensity) Difference (ILD) • Amplitude decreases w/ distance • Head casts sound/acoustic shadow • Reduced amplitude due to reflection • Measure w/ tiny microphones • f dependent • Greater shadow for higher f

27. Above Level Below Elevation • ITD/ILD not very useful • Use spectral cues • Frequency information can result in different perceptual qualia • Monaural: f serves as signal for pitch • Binaural: f serves as signal for location • Pinna differentially absorb f • Result: Notches in frequency spectra

28. Distance • At close distances (< 1 meter) • ILD can discriminate near and far • At very close distances ILD is very large (e.g. 20 Db) • But what’s that going to do for us? • At far distances • We are very poor judges for unfamiliar sounds • Suggests that sound serves as signal for visual search • Use sound level for familiar sources • Frequency: Auditory atmospheric haze • Absorption of high f • Sound muffled • Auditory parallax • Sounds move faster across ears at near relative to far distances

29. Brain basis for localization Sound to right • ITD detectors • Brainstem: Superior olivary nucleus • Primary auditory cortex • Coincidence detection • Neurons fire maximally when signals arrive at same time • Thus: “coincidence” • Axonal distance create input delays Sound to left

30. Auditory scene analysis • How do we segregate different sounds being produced by many sources simultaneously? • How do we tell what frequencies belong to what source? • E.g., Cocktail party • Don’t perceive an unorganized jumble of frequencies • Not simply high vs low f • Most f ranges overlap • How do we segregate information as belonging to distinct auditory objects?

31. Principles of auditory grouping • Like gestalt visual principles • Auditory stream segregation • Similarity • Timbre • Location • Pitch • Time 1 stream 2 streams

32. Auditory-visual interactions: Location and pitch • Visual capture of sound • Location: Ventriloquism effect • Pitch: McGurk effect • “Ba” • “Va” • “Tha” • “Da” • Visual information is integrated with audition • Creates fused auditory visual perception

33. Auditory-visual interactions: Location and pitch • Auditory experience is much more than pressure level changes