2007 Sensory Physiology

PLACE PRINCIPLE

documented by Georg von Bekesy - awarded the Nobel prize in medicine in 1961

  • pressure wave travels the length of the cochlea virtually instantaneously
    (the speed of sound in seawater is ~1500 m/s, and the cochlea is the size of a pea)
  • in response to the pressure wave, the basilar membrane vibrates in a characteristic pattern called the travelling wave: the vibration always begins in the base of the cochlea, and moves toward a peak (whose location depends on frequency)
    • if you observe from one spot on the cochlea, the displacement would appear as a change in amplitude with time
    • one analogy is to picture a rope, tied at one end, that moves up and down when someone flicks the untied end
Travelling wave

PLACE PRINICIPLE:
(i.e., how the cochlea encodes frequency)

  • the resonant properties of the basilar membrane vary along its length, therefore the membrane deflects differently to pitch at each location:
    • LOW FREQUENCIES produce the largest deflections in the APEX (near the helicotrema) - this occurs because the basilar membrane is more flexible and has greater mass in the apex
    • HIGH FREQUENCIES produce the largest deflections in the BASE (near the round and oval windows) where the basilar membrane is more stiff and less heavy
  • as a result, one way to think of frequency coding is 1 IHC = 1 FREQUENCY
  • as a result of this process, complex sounds are separated into their component frequencies (Fourier analysis)
  • the mapping of frequencies to location is maintained throughout the auditory system, with each CNS nucleus carrying at least one tonotopic map

Travelling wave

S&G Figure 10.43

 

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