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Psy280: Perception Prof. Anderson Department of Psychology Audition 1 & 2 1 Hearing: What’s it good for? Remote sensing Not restricted like visual field Can sense object not visible 2 Hearing: The sound of silence A tree in the forest 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 Physical signal but no perception Or water pressure when under water John Cage’s 4:33 No. 2, 1962 3 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 4 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 5 The human ear Outer ear Focusing of sound Resonance amplifies 20005000 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 6 Auditory sensory transduction: The inner ear Cochlea 3 layers Cochlear partition Contains organ of corti Organ of corti Coiled and liquid filled Cilia (hair) cells Between basilar and tectorial membranes Transduction Movement of cilia between membranes 7 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 8 Auditory pathways QuickTime™ and a GIF decompressor are needed to see this picture. 9 Audition: What and where What is it? *Pitch Identification Surprisingly, little is known beyond speech Where is it? *location 10 What: Pitch How does neural firing signal different pitches? 1) Timing codes 2) Place codes 11 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 12 Problems with temporal coding Problem: A single neuron can’t fire at the rate necessary to represent higher f tones Solution: volley principle E.g., 1000-20,000 Hz (i.e., 1000-20000 per second) Max neuron firing rate: 500-800 per second 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 13 Pitch: Place coding Related to doctrine of specific nerve energies What is pitch? Activation of different places in auditory system Frequency specific Tonotopy Owl brainstem Cochlear Brainstem Cortical Stimulate these regions Should result in pitch perception Human auditory cortex 14 Place coding starts in cochlea Von Bekesy studied basilar membrane in cadavers Observed traveling waves Diff frequencies (f) result in waves w/ diff envelopes Base more narrow and stiffer Apex wider and more flexible Higher f: Peak closer to base Lower f: Peak closer to apex Thus, f related to “place” where peak fluctuation 15 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 16 Complex tones: Fourier decomposition Basilar membrane acts as f analyzer Breaks down complex f inputs into constituent pure tone components 17 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 400 Hz mask Increases threshold for 800 more than 200 Hz Consistent with basilar vibrational overlap E.g. 400 Hz mask overlaps more with 800 than 200 Hz 18 Mystery of the missing fundamental 400 Hz fundamental plus harmonics (800, 1200, 1600, 2000) What if remove fundamental f (400Hz)? Perceived pitch doesn’t change! Hence: The missing fundamental Problem for place coding Sounds like 400 Hz pitch with complex timbre 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 19 our brain does f What does Barry White sound like on the telephone? Telephone carries 3003400Hz Typical male voice Barry white Fundamental f = 120 Hz 30 Hz? Can’t speak to Barry on the telephone? Missing fundamental allows us to hear “virtual” pitch of voice 20 If its too loud your too old Db (SPL) scale Attenuated low and high f relative to midrange High volume Loudness varies with f Low volume Pain and pleasure Audibility curves Loudness doubles about every 10 db at 1000 Hz Less frequency attenuation Low volume sounds muddy Mostly mid range I like my music loud Each curve represents equal loudness 21 Otoacoustic emissions: Talking ears Ears don’t only receive sounds, they make them! Occur spontaneously and also in response to sound It like your ears are talking back! Created by movement of outer hair cells (ohc) Discovered in 1978 Tiny microphones 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 22 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 23 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 24 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 25 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 26 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 Above Level Below 27 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 Use sound level for familiar sources Frequency: Auditory atmospheric haze Suggests that sound serves as signal for visual search Absorption of high f Sound muffled Auditory parallax Sounds move faster across ears at near relative to far distances 28 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 29 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? 30 Principles of auditory grouping Like gestalt visual principles Auditory stream segregation Similarity Timbre Location Pitch Time 1 stream 2 streams 31 Auditory-visual interactions: Location and pitch Visual capture of sound Location: Ventriloquism effect Pitch: McGurk effect QuickTime™ and a Cinepak decompressor are needed to see this picture. “Ba” “Va” “Tha” “Da” Visual information is integrated with audition Creates fused auditory visual perception 32 Auditory-visual interactions: Location and pitch Auditory experience is much more than pressure level changes 33