Physiology and Biophysics of Physiology and Biophysics of Auditory and Vestibular Auditory and Vestibular

functionfunction

Dr. Charles CapadayDr. Charles CapadayProfessor & DirectorProfessor & Director

Brain and Movement LaboratoryBrain and Movement LaboratoryCRULRG, F-6500CRULRG, F-6500

2601 de la Canardiere, Québec City (QC), 2601 de la Canardiere, Québec City (QC), G1J 2G3, CanadaG1J 2G3, Canada

http://http://www.BrainAndMovementLab.orgwww.BrainAndMovementLab.org//

The sensitivity of the ear is The sensitivity of the ear is remarkable.remarkable.• Some factsSome facts:: At 2-3 kHz the threshold of hearing intensity is about 10At 2-3 kHz the threshold of hearing intensity is about 10-16-16 W/cm W/cm22..

This corresponds to a pressure variation of 2.9 x 10This corresponds to a pressure variation of 2.9 x 10-4-4 dyn/cm dyn/cm22, the , the background atmospheric pressure is 1.013 x10background atmospheric pressure is 1.013 x1066 dyn/cm dyn/cm22

(101.3 kPa).(101.3 kPa). Random pressure variations due to thermal motion of air Random pressure variations due to thermal motion of air

molecules is 0.5 x 10molecules is 0.5 x 10-4-4 dyn/cm dyn/cm2 2 ! The sensitivity of the ear is thus ! The sensitivity of the ear is thus close to the theoretical limit, being just above the thermal noise close to the theoretical limit, being just above the thermal noise level.level.

At the threshold power level (10At the threshold power level (10-16-16 W/cm W/cm22) the displacement of the ) the displacement of the air molecules is less than the size of the molecules themselves!air molecules is less than the size of the molecules themselves!

1 1 2

1 1 2

2

22

4

( )

It p v p v

Ii p v p v

From which it is readily apparent <smile> that only 0.1% of the sound energy enters the water. Water is a very good sound barrier.

The sensitivity of the ear is partly due to its The sensitivity of the ear is partly due to its mechanical construction which amplifies sound mechanical construction which amplifies sound

pressurepressure The area of the eardrum is The area of the eardrum is

30 times larger than that 30 times larger than that of the oval window. So by of the oval window. So by Archimide’s principle…Archimide’s principle…

The ossicles act as a lever The ossicles act as a lever system with a mechanical system with a mechanical advantage of about 2.advantage of about 2.

The ear canal has a The ear canal has a resonant frequency circa 3 resonant frequency circa 3 kHz, amplifying the kHz, amplifying the pressure by a factor of pressure by a factor of about 2.about 2.

Thus, 2 x 30 x 2 =120. Thus, 2 x 30 x 2 =120. However,…However,…

What we will explain…What we will explain…

Sensitivity curve of human earSound Localization

-4

sin

ΔlΔt= , where v is sound velocity

v0.085 m

. . 2.5 x 10 0.25 340m/s

In fact time differences of the order of 30 s can be detected,

corresponding to an angle of about 3 deg.

l d

e g t s ms

Why primary afferent fibre firing rate is Why primary afferent fibre firing rate is nonlinearly related to stimulus intensity (but nonlinearly related to stimulus intensity (but not always)not always)x 0 0.10 1000

y1 1 y2 800

y3 x( ) x0.3

0 500 10000

500

1000

Y-axis in Linear Scale

y1

y2

x

0 500 10000.01

0.1

1

10

100

1 1031 104

Y-axis in Log Scale

y1

y2

x

0 200 400 600 800 10000

5

10

Power Function with Exponent = 0.3

y3 x( )

x

The mammalian central auditory pathways The mammalian central auditory pathways (for dummies version).(for dummies version).

A1 is tonotopically organizedA1 is tonotopically organized

Neural Mechanisms of sound Neural Mechanisms of sound localizationlocalization

• The medial The medial superior olive superior olive transforms a time transforms a time code into a place code into a place code.code.

The Barn Owl’s Story…The Barn Owl’s Story…

A measured behaviour and its A measured behaviour and its underlying neural systemsunderlying neural systems

See, T. Carew. Behavioural Neurobiology. Sinauer Associates, 2000.