The Auditory System

The AUDITORY STIMULUS
Pressure waves created by vibrating objects, transmitted through air medium
Condensed and rarified air molecules (no sound in space)
Travels at 340 meters/sec in air (1,500 meters/sec in water)
Sound wave properties
Pure tone is represented by mathematical function - sine wave

Timbre: 2 tones w/same loudness, pitch, and duration but can sound different

dB scale makes wide range more manageable

Most natural sounds are made up of many frequencies
periodocity: regular repetition of pressure changes
Human frequency range about 20-20K Hz
Fourier analysis and sound
- sine wave with the lowest frequency is the fundamental frequency (first harmonic)
- the other sine waves are the tone's harmonics
- acomplex waveform can be broken down by Fourier analysis & represented by a Fourier frequency spectrum
- frequency & amplitude are represented
440 880 1320
- harmonics are all multiples of the fundamental frequency

AUDITORY ANATOMY & PHYSIOLOGY

PARTS of the EAR
pinnae help to funnel sound and play role in sound localization

Auditory canal enhances intensities of sound waves at 2-5000 Hz by its property of resonance: sound waves bouncing back from end of canal at (resonance frequency of 3,400Hz) reinforce the incoming waves

Ossicles - malleus is set into vibration by tympanic membrane, to incus, to stapes which hits the membrane covering the oval window of the cochlea
Ossicles transmit sound vibrations from air to liquid (w/o them, 97% of sound would be reflected away)

<<< FIGURE: Parts of the Ear >>>

Cochlea has cochlear partition separating the two primary chambers, scala vestibuli and scala tympani
- cochlear partition contains the organ of Corti,which sits on the basilar membrane and consists of hair cells, tectorial membrane

<<< FIGURE: The Cochlea >>>

2 types of hair cells - inner and outer hair cells, synapse w/dendrites of auditory nerve
Sound cause pushing and pulling of cochlear partition, vibrates at same freq as stapes
- basilar membrane moves up and down, tectorial membrane moves back and forth, causes cilia to bends
- Outer cells physically touch overlying tectorial membrane
- Inner cells free to "go with the flow"

<<< FIGURE: Cochlea & Hair Cells >>>

- both have cilia connected by tip links, at point of attachment there is a membrane channel
- auditory hair cell resting potential at -160mV (lots of K+ outside)
- slight resting tension on tip link allows small amount of K+ and Ca2+ to diffuse into cilium
- stretch of tip link allows all channels to open, membrane depolarizes, NT release increases
- in opposite direction tip link slack, no channels open, NT release decreases

<<< FIGURE: Transduction in the Hair Cells >>>

CENTRAL AUDITORY PATHWAYS
From auditory nerve mostly contralateral projections, synapse on:

Cochlear nuclei --> Superior Olive --> Inferior Colliculus --> MGN --> Auditory cortex
                                   (medulla)                    (tectum)          (thalamus)         (temporal lobe; A1, A2)
Input to cochlear nuclei: 95% from inner hair cells, 5% from outer hair cells
- But inners are only 29% of hair cell population

Mutant mice w/out inner hair cells appear deaf
- Outer hair cells demonstrate a motile response, mechanically amplifies basilar membrane vibration
- destruction of outer hair cells causes decrease in inner hair cell response

NEURAL CODING IN AUDITION

CODING for FREQUENCY
PLACE CODE and TIMING CODE
Place Code: diff. freq are signaled by neurons in diff. places in the auditory system; diff. placement of receptors in cochlea
Georg von Bekesy (work begun 1928, Nobel prize in 1961)
How does basilar membrane vibrate in response to diff frequencies?
Studied two primary ways:
Human Cadavers & Physical Models of Membrane
- generated signals and watched the movement of membrane with a microscope
2 facts built into model:

1. base is 3-4X narrower than apex
2. base is 100X stiffer than apex

- Found traveling wave motion and the envelope of the traveling wave (the maximum displacement)
- concluded that the peak of the wave is a function of sound wave
- greater displacement of the membrane = greater firing rates, bending of hair cells there
Low freq cause max. vibration at apex, higher freq cause max. vibration nearer the base

Physiological Evidence for Place Theory
Tonotopic maps: an orderly map of frequencies along the length of the cochlea
electrophysiological recording confirms- high freq=base, low freq=apex
Frequency tuning curve: plot of senstivity (dB required for small response) X freq (kHz)
characteristic freq: freq. a hair cell or auditory nerve fiber is most sensitive to

* * * THE FOLLOWING PARAGRAPHS INDICATED LIKE THIS WILL NOT BE ON THE EXAM!!! * * *
Psychophysical Evidence for Place Theory
Masking: presence of one sound reduces our ability to hear another sound
Masking stimulus (noise, or white noise), contains large # of frequencies
e.g. 365-455Hz, bandwidth =90Hz (range), center freq = 410Hz (mean)
Masking Experiment:
1) Measure threshold of diff frequencies
2) Re-measure in presence of masking white noise
- What effect does the mask have on test tones of diff freq.?
- Results: Masking tone affects freqs above it (higher freqs) more than below it
*Reflects asymmetrical vibration pattern of basilar membrane*
*High intensity masking noise = greater vibration at base (high freq end) of basilar membrane*
Psychophysical Tuning Curves: plot of mask intensity X mask frequency
Experiment:
1) Low intensity test tone at constant frequency
2) Present series of masking pure tones; measure mask intensities (dB SPL) that make test tone barely audible
- How loud do diff mask freq have to be to make the test tone barely detectable (at its threshold)?
- Results: Low dB masks will only affect test tone if freq is at or near test tone's freq
if mask at freqs above or below test tone, mask intensity must be high
*Mask only affects small range of frequencies*
*Shows that sound perception depends on multiple, narrowly tuned fibers operating across freq range*

Freq. on Auditory Cortex represented by
- Tonotopic surface maps, columnar organization (perpendicular electrode track)

Timing Code: freq of stimulus is directly represented by freq of nerve firing
Rutherford (1886): 3,000Hz = 3,000 impulses/sec by nerve fiber
Volley principle: high freq stimuli could be signaled in the firing rates of several nerve fibers which are phase locked to different peaks of the sound wave
Does occur early in the auditory pathways, weak at auditory cortex

Neural Response to Complex Stimuli
- Cells have been found in auditory cortex that do not respond to pure tones or tone combinations, but will respond to complex sounds (tearing paper, keys jingling)
Some cells respond only when tone shifts freq (low to high or vice versa)
frequency sweep detectors: respond well to freq changes but poorly to constant tones
monkey "call" neurons in A1 and A2 (implications for special human speech neurons?)
Efferent feedback (e.g., superior olive to hair cells) could function to:
- decreases sensitivity of inner hair cells (dampens response)
- reducing effect of background noise
- focus attention on one stimulus (auditory attention)