motor theory remnants april 3, 2012 dirty work project reports #5 to turn in. on thursday, we’ll...
TRANSCRIPT
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Motor Theory Remnants
April 3, 2012
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Dirty Work• Project Reports #5 to turn in.
• On Thursday, we’ll talk about the muscles that control articulation…
• And do a slightly messy static palatography demo
• At the end of today, we’ll do the USRI evaluations.
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Another Piece of the Puzzle• Another interesting finding which has been used to argue for the “speech is special” theory is duplex perception.
• Take an isolated F3 transition:
and present it to one ear…
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Do the Edges First!• While presenting this spectral frame to the other ear:
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Two Birds with One Spectrogram
• The resulting combo is perceived in duplex fashion:
• One ear hears the F3 “chirp”;
• The other ear hears the combined stimulus as “da”.
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Duplex Interpretation• Check out the spectrograms in Praat.
• Mann and Liberman (1983) found:
• Discrimination of the F3 chirps is gradient when they’re in isolation…
• but categorical when combined with the spectral frame.
• (Compare with the F3 discrimination experiment with Japanese and American listeners)
• Interpretation: the “special” speech processor puts the two pieces of the spectrogram together.
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fMRI data• Benson et al. (2001)
• Non-Speech stimuli = notes, chords, and chord progressions on a piano
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fMRI data• Benson et al. (2001)
• Difference in activation for natural speech stimuli versus activation for sinewave speech stimuli
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Mirror Neurons• In the 1990s, researchers in Italy discovered what they called mirror neurons in the brains of macaques.
• Macaques had been trained to make grasping motions with their hands.
• Researchers recorded the activity of single neurons while the monkeys were making these motions.
• Serendipity:
• the same neurons fired when the monkeys saw the researchers making grasping motions.
• a neurological link between perception and action.
• Motor theory claim: same links exist in the human brain, for the perception of speech gestures
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Moving On…• One important lesson to take from the motor theory perspective is:
• The dynamics of speech are generally more important to perception than static acoustic cues.
• Note: visual chimerism and March Madness.
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Auditory Chimeras• Speech waveform + music spectrum:
• Music waveform + speech spectrum:
frequency bands
1 2 4 8 16 32
frequency bands
1 2 4 8 16 32
Source: http://research.meei.harvard.edu/chimera/chimera_demos.html
Originals:
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Auditory Chimeras• Speech1 waveform + speech2 spectrum:
• Speech2 waveform + speech1 spectrum:
frequency bands
1 2 4 6 8 16
frequency bands
1 2 4 6 8 16
Originals:
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Motor Theory, in a nutshell• The big idea:
• We perceive speech as abstract “gestures”, not sounds.
• Evidence:
1. The perceptual interpretation of speech differs radically from the acoustic organization of speech sounds
2. Speech perception is multi-modal
3. Direct (visual, tactile) information about gestures can influence/override indirect (acoustic) speech cues
4. Limited top-down access to the primary, acoustic elements of speech
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Audition (or, how we hear things)
April 3, 2012
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How Do We Hear?• The ear is the organ of hearing. It converts sound waves into electrical signals in the brain.
• the process of “audition”
• The ear has three parts:
• The Outer Ear
• sound is represented acoustically (in the air)
• The Middle Ear
• sound is represented mechanically (in solid bone)
• The Inner Ear
• sound is represented in a liquid
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The Ear
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Outer Ear Fun Facts• The pinna, or auricle, is a bit more receptive to sounds from the front than sounds from the back.
• It functions primarily as “an earring holder”.
• Sound travels down the ear canal, or auditory meatus.
• Length 2 - 2.5 cm
• Sounds between 3500-4000 Hz resonate in the ear canal
• The tragus protects the opening to the ear canal.
• Optionally provides loudness protection.
• The outer ear dead ends at the eardrum, or tympanic membrane.
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The Middle Ear
eardrum
the hammer (malleus)
the anvil (incus)
the stirrup (stapes)
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The Middle Ear• The bones of the middle ear are known as the ossicles.
• They function primarily as an amplifier.
• = increase sound pressure by about 20-25 dB
• Works by focusing sound vibrations into a smaller area
• area of eardrum = .55 cm2
• area of footplate of stapes = .032 cm2
• Think of a thumbtack...
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Concentration• Pressure (on any given area) = Force / Area
• Pushing on a cylinder provides no gain in force at the other end...
• Areas are equal on both sides.
• Pushing on a thumb tack provides a gain in force equal to A1 / A2.
• For the middle ear , force gain
• .55 / .032 17
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Leverage• The middle ear also exerts a lever action on the inner ear.
• Think of a crowbar...
• Force difference is proportional to ratio of handle length to end length.
• For the middle ear:
• malleus length / stapes length
• ratio 1.3
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Conversions• Total amplification of middle ear 17 * 1.3 22
• increases sound pressure by 20 - 25 dB
• Note: people who have lost their middle ear bones can still hear...
• With a 20-25 dB loss in sensitivity.
• (Fluid in inner ear absorbs 99.9% of acoustic energy)
• For loud sounds (> 85-90 dB), a reflex kicks in to attenuate the vibrations of the middle ear.
• this helps prevent damage to the inner ear.
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The Attenuation Reflex• Requires 50-100 msec of reaction time.
• Poorly attenuates sudden loud noises
• Muscles fatigue after 15 minutes or so
• Also triggered by speaking
tensor tympani
stapedius
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The Inner Ear• In the inner ear there is a snail-shaped structure called the cochlea.
• The cochlea:
• is filled with fluid
• consists of several different membranes
• terminates in membranes called the oval window and the round window.
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Cochlea Cross-Section
• The inside of the cochlea is divided into three sections.
• In the middle of them all is the basilar membrane.
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Contact
• On top of the basilar membrane are rows of hair cells.
• We have about 3,500 “inner” hair cells...
• and 15,000-20,000 “outer” hair cells.
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How does it work?• On top of each hair cell is a set of about 100 tiny hairs (stereocilia).
• Upward motion of the basilar membrane pushes these hairs into the tectorial membrane.
• The deflection of the hairs opens up channels in the hair cells.
• ...allowing the electrically charged endolymph to flow into them.
• This sends a neurochemical signal to the brain.
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An Auditory Fourier Analysis• Individual hair cells in the cochlea respond best to particular frequencies.
• General limits:
20 Hz - 20,000 Hz
• Cells at the base respond to high frequencies;
• Cells at the apex respond to low.tonotopic organization of the
cochlea
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How does this work?• Hermann von Helmholtz (again!) first proposed the place theory of cochlear organization.
• Original idea: one hair cell for each frequency.
• a.k.a. the “resonance theory”
• But...we can perceive more frequencies than we have hair cells for.
• The rate theory emerged as an alternative:
• Frequency of cell firing encodes frequencies in the acoustic signal.
• a.k.a. the “frequency theory”
• Problem: cell firing rate is limited to 1000 Hz...
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Synthesis• The volley theory attempted to salvage the frequency rate proposal.
• Idea: frequency rates higher than 1000 Hz are “volleyed” back and forth between individual hair cells.
• There is evidently considerable evidence for this proposal.
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Traveling Waves (in the ear!)• Last but not least, there is the traveling wave theory.
• Idea: waves of different frequencies travel to a different extent along the cochlea.
• Like wavelength:
• Higher frequency waves are shorter
• Lower frequency waves are longer
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The Traveling Upshot• Lower frequency waves travel the length of the cochlea...
• but higher frequencies cut off after a short distance.
• All cells respond to lower frequencies (to some extent),
• but fewer cells respond to high frequency waves.
• Individual hair cells thus function like low-pass filters.
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Hair Cell Bandwidth
• Each hair cell responds to a range of frequencies, centered around an optimal characteristic frequency.
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Frequency Perception• In reality, there is (unfortunately?) more than one truth--
• Place-encoding (traveling wave theory) is probably more important for frequencies above 1000 Hz;
• Rate-encoding (volley theory) is probably more important for frequencies below 1000 Hz.
• Interestingly, perception of frequencies above 1000 Hz is much less precise than perception of frequencies below 1000 Hz.
• Match this tone:
• To the tone that is twice the frequency:
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Higher Up• Now try it with this tone:
• Compared to these tones:
• Idea: listeners interpret pitch differences as (absolute) distances between hair cells in the cochlea.
• Perceived pitch is expressed in units called mels.
• Twice the number of mels = twice as high of a perceived pitch.
• Mels = 1127.01048 * ln (1 + F/700)
• where acoustic frequency (F) is expressed in Hertz.
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The Mel Scale
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Equal Loudness Curves• Perceived loudness also depends on frequency.