Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityAuditory PhysiologyPSY 310Greg FrancisLecture 30Waves, waves, waves.Purdue UniversityOrgan of Corti Tectorial membrane Sits on top Inner hair cells Outer hair cells The “microphone”for the brainProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityHearing Perceptually, we hear sounds that differ in pitch andloudness And several other qualities, as we’ll discuss later What is the physiological response to these perceptualqualities? How are different aspects of sounds represented in thecochlea? At least two possibilities Different neurons code different properties of sound Neural responses vary for different properties of soundPurdue UniversityFrequency We can explore the responses to different frequencies ofsound waves Not the same thing as perceived pitch Georg von Bekesy (Nobel Prize in physiology andmedicine in 1961) Something similar also proposed by Helmholtz (1857) Place theory of hearing The frequency of a sound is coded by the place on the organ ofcorti that responds to the sound bestProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityCochlea Pressurefrom thestapespushes thefluid in thecochlea Causesmembranesto vibratePurdue UniversityBasilar membrane The vibration differs, depending on the frequency of the sound Different places on the basilar membrane have the bestvibration in response to sounds of different frequencies Like a traveling wave on a ropeProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityBasilar membrane The differences are partly due to the mechanical structure ofthe basilar membrane Thickness of material and with of the membranePurdue UniversityTraveling wave Remember that there is constant push and pull by the stapes One can get a variety of interesting wave patternsProf. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTraveling wave Another view of a traveling wavePurdue UniversityTraveling wave As the wave travels along, its amplitude changesDisplacement ofbasilar membranebasilar membraneapexMaximumDisplacementvs. position:envelopeBaseProf. Greg Francis6PSY 310: Sensory and Perceptual ProcessesPurdue UniversityWave envelope The amplitude of the peak of the wave maps out the envelopeof the waveDisplacement ofbasilar membranebasilar membraneapexMaximumDisplacementvs. position:envelopeBase Purdue UniversityFrequency encoding The wave envelopes peak at different places for differentfrequencies of soundsProf. Greg Francis7PSY 310: Sensory and Perceptual ProcessesPurdue UniversityFrequency encoding So different frequencies give rise to peak responses at differentplaces on the cochlea Neurons that respond to the hair cells at different places, representdifferent frequenciesPurdue UniversityWave envelope shape The way waves travel and the properties of the basilarmembrane makes the waves asymmetricalbaseapexProf. Greg Francis8PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTuning curve Pick a placeon the cochlea Measure thefaintest soundthat generates asmallmovement (dB) Vary thefrequency of thesound The threecurves are forthree differentcats (thenumbers aredates)Purdue UniversitySound detection Pure tone sounds produce responses at differentplaces in the cochleabaseapexProf. Greg Francis9PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySound detection A sound that consists of more than one sine wavewill produce separate responses at different places Fourier analysis?baseapexPurdue UniversitySound detection Suppose two sounds are presented together Target Noise The response on the basilar membrane is a mixture of the twosounds We can look at the discriminablity of the tones and relate it to theproperties of the cochlea Masking demonstration Not all noise is created equal Expect significant effect of noise only when its frequency range willinterfere with detection of the tone (e.g, spread out the peak of thetraveling wave)Prof. Greg Francis10PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySound detection There are several ways to do this kind of experiment One way: present a target tone of a given amplitude andfrequency (the number on the graph) Vary the frequencyand amplitude of themask Adjust theamplitude tomake the targettone not heard0.51.02.04.08.0Purdue UniversityA problem The cochlea is about 2.3 cm in length There are about 16,000 - 20,000 hair cells People resolve around 1,500 different pitches E.g., the difference between 1000 Hz and 1003 Hz This would imply that different pitches are coded every 0.002 cm The wave envelope is not that precisebaseapexProf. Greg Francis11PSY 310: Sensory and Perceptual ProcessesPurdue UniversityTuning curves Here the animals (guineapigs) are alive or dead The alive curve showssharp tuning The dead curve showsbroader tuning (lesssensitivity) Something is differentwhen the animal is alivethat changes theproperties of the basilarmembranePurdue UniversityOrgan of corti The outer hair cells apparently move (motile response) tochange the way the basilar membrane vibrates This changes the peak of the wave envelopeProf. Greg Francis12PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySharpening With the motile response, the location of the waveenvelope is more precisePurdue UniversityFrequency tuning Not everything is determined by the place on the cochlea Some neurons respond with bursts of activity at the samefrequency as the stimulus Only for lower frequency stimuli Neurons cannot change fast enough for high frequency soundsProf. Greg Francis13PSY 310: Sensory and Perceptual ProcessesPurdue UniversityConclusions Waves on the basilar membrane are different fordifferent sound frequencies Provides the basis for pitch perception Different neurons respond to different frequencies Very complicated Lots of other neural processing that we havediscussedPurdue UniversityNext time Using sound to understand theenvironment Auditory