P105 Lecture #20 visuals 25 Feburary 2013

2 Acoustic Pressure is measured in decibels (dB) 1 atm = 100,000 pascals = micropascals Threshold: the softest sound detectable is 20 micropascals (at 1000 Hz). 2 parts in 10 billion of an atmosphere We hear sounds 1-10 million times more intense than threshold dB are logarithmic units with 0 dB at threshold adding 20 dB = factor of 10 increase in pressure – 6 dB approximately doubles the pressure 40 dB SPL = 20 x 100 = 2,000 micropascals Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

3 Hearing threshold of a profoundly deaf person (ex: Shipsey) Hearing threshold of a severely deaf person soft loud

4 The Ear Has Three Distinct Regions ca. 550 B.C. Pythagoras & successors ca. 175 A.D. Galen Nerve transmits sound to the brain It has taken until the present to unravel the rest Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

Auditory System Physiology Illustration from E.J. Heller, “Why you hear what you hear”

3D Rendering of Auditory Transduction System Show video “Auditory Transduction”, by Brandon Pletsch. (This video was awarded 1 st prize in the 2003 NSF/AAAS Science & Engineering Visualization Challenge)

Anatomist Andreas Vesalius describes the structure of the middle ear. The tympanic membrane & ossicles Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

8 Why is our “ sound sensor ” not on the outside of our head? Impedance mismatch overcome by ratio of areas and lever action Hermann Ludwig von Helmholtz first to understand the role of the ossicles ( 1860’s) Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

Pressure Amplification in middle ear Lever action of ossicles (gives 1.5x amplification of force) Ratio of areas of oval window to tympanum (20x amplf’n of pressure Illustration from E.J. Heller, “Why you hear what you hear”

Inner Ear Illustrations from E.J. Heller, “Why you hear what you hear”

11 The cochlea and its chambers 1561 Gabriello Fallopio discovers the snail-shaped cochlea of the inner ear. The cochlea is about the size of a pea Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

12 The Cochlea houses the Organ of Corti Auditory Nerve Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

13 Organ of Corti 1 st detailed study of Organ of Corti by Alfonso Corti Original figures (scanned) from: Zeitschrift für wissenschaftliche Zoologie (1851) Hair Cells are mechano-electric transduction devices Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

14 Georg von Békésy (Nobel 1961) The Middle Ages Experimentally measured traveling wave profiles published by von Békésy in Experiment in Hearing, McGraw-Hill Inc., End of Early History Hermann Ludwig von Helmholtz first theory of the role of BM as a spectrum analyzer providing a frequency-position map of sound Fourier components. base apex Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

15 Tonotopic Organization Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

Critical Bands & Pitch Determination Can think of the 3.5-cm long Basilar Membrane as being divided into 10 regions of 3.5 mm each providing sensitivity to ~10 octaves. The region of the basilar membrane excited by a pure tone of given frequency is wide: ~ 1.5 mm – “Critical Band”; region corresponds to just under 3 semitones (frequency range of about 18%), where 12 semitones = 1 octave. “just-noticeable difference” = ~ 1/10 th of a semitone (i.e., ~ 0.6% difference in frequency) Interplay between physiological effects of signal sent to brain and signal processing by the brain are complicated and important!

17 The Copernican Revolution Von Békésy's findings stimulated the production of numerous cochlear models that reproduced the observed wave shapes, but were in contrast with psychophysical data on the frequency selectivity of the cochlea. Davies (1983): a revolutionary new hypothesis there exists an active process within the organ of Corti that increases the vibration of the basilar membrane. displacement Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

18 Active amplification Careful measurements on living animal cochlea Same animal post mortem Johnstone et al (1986) What causes the amplification? Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

19 Inner hair cells 10,000 afferent (signals go the brain) Outer Hair Cells 30,000 Sparsely innervated Rows of Hair Cells in the healthy cochlea Hair cell 30  m 5m5m Hair Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

20 Hair cells are mechano-electrical transducers Both inner and outer hair cells work this way 500 nm 2nm diameter 1980 ’ s

21 The inner hair cells send signals to the brain that are interpreted as sound. What do the outer hair cells do? Outer hair cells exhibit electro motility they are also electro-mechanical transducers Slide from Ian Shipsey, Purdue U., presentation on cochlear implants