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Psychology 135C.F.Tiangco

UPDEPP

Do you hear what I hear? :A lecture on the auditory sense

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The characteristics of sounds enable us tolocate, identify, and react to the objects orpeople from which they arise.

For this reason, hearing plays a veryimportant role in defining your perceptualworld throughout your lifetime.

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An old puzzleIf a tree falls in the forest but no one isaround to hear it, does the tree make asound?

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Auditory StimulusSound refers to a perceptualexperience, which requiresthe presence of a perceiverto perceive the disturbancein air pressure.

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Pressure waves are transmitted through media whoseconstituent molecules are sufficiently close together tocollide with one another when they are set in motion,and at the same time, are sufficiently elastic to returnto their original position following disturbance.

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Regardless of the medium through which it is propagatedand regardless of the source, sound energy becomes weakeras it travels farther from its source.

However, sound continues to travel at a constant speed, so

long as it travels in the same medium.

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Sound can travel aroundand sometimesthrough---solid objects.

When sound waves strike a surface, some portion

of the sound waves bounce off the surface(echoes).

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Sound waves

Wavelength: The distance between any pointon a wave and the equivalent point on the nextphase.

Amplitude: The strength or power of a wavesignal.

Frequency: The number of times thewavelength occurs in one second. Measured inkilohertz (Khz), or cycles per second.

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The Auditory System: TheEAR

Outer Ear : functions like a directionalmicrophone that picks up sound and modifies it

- Pinna : corrugations act like small reflectingsurfaces that modify or color the complexity of sound actually entering the ear

- Auditory canal : a slightly bent tube approx. 2.5cm long and 7 mm in diameter; behavessomewhat as a resonant pipe

- Eardrum/tympanic membrane : thin, oval-shaped

membrane vibrates when sound pressure wavesstrike it

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The Middle Ear :

- Ossicles : three smallest bones in the human body, bridgingthe gap between the eardrum and oval window

- Eustachian tube: connects middle ear and the throat

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More on the role of the ossicles:- Sound waves travel through the air until

they reach the inner ear- Liquid offer opposition to the flow of

soundwaves (impedance)- When the impedances for two media differ,

impedance mismatch results, and sound

waves cannot be readily transmitted fromone medium to another (loss of 30 dB)

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More on the role of the ossicles:- Three important processes help solve the

impedance mismatch prob by increasing theefficiency with which sound is transmitted to the

inner ear:(1) The eardrum is somewhat curved, which makes it

respond more efficiently(2) The three ossicles act like a lever

(3) The force of the relatively large tympanicmembrane is transmitted to the much smallerregion where the stapes meets the oval window of the cochlea

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tensor tympani

stapedius

In the presence of loud sound,

these two muscles contractthereby stiffening the eardrumand restricting the movement of the ossicles ( Acoustic Reflex ).

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The Inner Ear: consists of a series of hollowcavities carved into the temporal bone of the skull.

- Semicircular canals : concerned with themaintenance of body posture and balance

- Cochlea : a coiled, fluid-filled cavitycontaining the specialized receptors forhearing

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1. Auditory canal2. Ear drum3. hammer4. Anvil5. Stirrup6. Round window7. Oval window8. Semicircular canals9. Cochlea10. Eustachian tube

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3 chambers of cochlea:- Vestibular canal- Cochlear duct- Tympanic canal

The chambers run parallelto one another the entirelength of the cochlea,except at the apex where

VC and TC merge.

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The VC and TC contain the same fluid,similar in composition to spinal fluid.

The fluid of the CD is chemicallydifferent.

This chemical difference between the

two fluids plays a crucial role in the initialstage of hearing. Fluids also supply allthe nourishment for cells in the cochlea.

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Each IHC is shaped like a flask and is surrounded bysupporting cells.Each OHC is more cylindrical in shape and each issurrounded by fluid.Both IHC and OHC terminate in tiny bristles (cilia).Only cilia from OHCs actually touch the tectorialmembrane; IHC attached only to the BMBending of the cilia constitutes the crucial early event inthe process that leads to hearing, for it is this event thattriggers the electrical signals that travel from the ear tothe brain.

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Stimulation produces electrical charges that releaseneurotransmitters, chemicals picked up by the nerveendings leading to the auditory nerve.

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Registration of Frequency(1) Place Theory different frequencies of

vibration of the cochlear fluid disturbdifferent regions of the BM. Thesedifferent regions of disturbance activatedifferent hair cells and hence, differentauditory nerve fibers.

- The theory says that very high soundsmight vibrate one area of the basilarmembrane and that very low soundsvibrate another area. (Helmholtz version)

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Georg von Bekesy:- Proposed that the vibration pattern of the BM leads to the

stimulation of different places along the membrane.- There is systematic variation in the stiffness of the membrane.

Because of this, vibration in the cochlea produces a pressure wavethat travels from the stapes to the other end (traveling wave).

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(2) Temporal /FrequencyTheory temporalstructure of some sound is represented bytemporal fluctuations in the firing rates of auditory nerve fibers

- the stapes taps out a series of beats on theoval window, and the entire BM dances to thatbeat- assumes that the auditory nerve can fire at

rates of 20 to 20,000 times per second(Rutherford version). BUT, maximum numberof responses of neuron is 1000 each second.

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Wever:- Proposed a volley principle whereby

clusters of neurons share in producing therequired firing rate.

- However, even with the addition of thevolley principle, the system cannot handlefrequencies above 4000 Hz.

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Registration of AmplitudeAs stimulus amplitude increases, a greaternumber of neural fibers are active and eachneural fiber fires more often.

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Auditory PathwaysNeural info is carried out of the inner ear by the

auditory nerve, which branches out into severaldifferent pathways that eventually reconverge atthe auditory cortex

After leaving the inner ear, the auditory nerve

travels to the cochlear nucleus. Here, theauditory nerve cells transmit their information tonew cells.

The auditory system contains specialized neuralanalyzers for locating and identifying sound

sources within the auditory environment

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Auditory PathwaysLeft auditory nerve projects to the

cochlear nucleus on the L side of the brain (same for the right)

At processing stages beyond thecochlear nucleus, auditory systembecomes binaural

Some binaural neurons areexcitatory (prefer low-freq); someexcitatory/inhibitory (prefer high-freq)

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Auditory PathwaysBeyond the cochlear nucleus is

the superior olivary nucleus,which can compare theinformation it receives from

the two ears.

Each superior olivary nucleussends its information to aninferior colliculus, then to themedial geniculate nucleus of the thalamus.

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Auditory PathwaysCells in the MGN send

projections to the primaryauditory projection area(A1/Brodmanns area 41) of the temporal cortex of thebrain, where multipleelementary auditory featuresare organized into meaningful

auditory objects.

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Hearing LossConduction lossstems from somedisorder within theouter or middle ear.

Sensory/neural lossoriginates within theinner ear or in theauditory portion of the brain.

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Signs and symptoms of hearing loss may include:Muffled quality of speech and other soundsDifficulty understanding words, especially againstbackground noise or in a crowd of peopleFrequently asking others to speak more slowly, clearlyand loudlyNeeding to turn up the volume of the television or radioWithdrawal from conversationsAvoidance of some social settings

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Factors that may damage or lead to loss of the hairs and nerve cells in your inner earinclude:Aging.Heredity.Occupational noises.Recreational noises.

Some medications.Some illnesses.

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On to Homework!

The end (for now).

S d fi

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Responses to Sound. Definethe terms below.1) Case of the missing fundamental2) Harmonics3) Equal loudness contour4) Dissonance5) Interaural time difference6) Interaural intensity difference7) Sound shadow8) Cone of confusion9) Auditory adaptation10) Auditory fatigue