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Q16 - Cochlear Frequency Coding and Sound Pressure

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Human Sensory Organs

The Hungarian biophysicist, György Békésy, was awarded the 1961 Nobel Prize for his research on the function of the mammalian cochlea. He showed that each frequency of sound causes a specific part of the basilar membrane to vibrate the most. This point is determined by the resonant properties of the membrane, with narrower parts vibrating more at higher frequencies.

Neurons sense this vibration and encode the sound with two different mechanisms:

  1. Place code: Each neuron connects to the membrane in one place and signals the presence of the narrow frequency band that causes maximal vibration in that part of the membrane.
  2. Periodicity code: If the frequency is below 4 kHz, the timing of action potentials in the cochlear nerve is synchronised with individual cycles of the stimulus (this is called phase-locking).

The intensities of audible sounds are expressed in decibel sound pressure level (dB SPL):

$$dB,SPL = 20 \log_{10}\left(\dfrac{\text{Pressure of sound in Pa}}{2 \times 10^{-5}}\right)$$

Graph with x-axis frequency (Hz, log scale, 20-20000) and y-axis intensity (dB, 0-140ish). Two curves: an upper curve (“Legend 2 = Threshold of pain”) running roughly 110-135 dB across the frequency range, with a marked point around 130 dB near the low-frequency end. A lower curve (“Legend 1 = Threshold of audibility”) running from around 65 dB down to below 0 dB in the middle frequencies and back up at the high end, with a marked point around 50 dB near the low-frequency end, at the same x-position as the upper curve’s marked point. Fig.1. Axis 1 = Frequency (Hz). Axis 2 = Intensity (dB). Legend 1 = Threshold of audibility. Legend 2 = Threshold of pain.

Based on the information above, indicate with an X if each of the following statements is true (T) or false (F).

Q16.1. Sounds detected by hair cells towards the narrowest part of the cochlea are coded in a phase-locked manner.
Q16.2. For sounds detected near the base of the membrane, the number of fibres firing is proportional to the amplitude of the displacement of the basilar membrane.
Q16.3. What is the absolute refractory period of fibres in the cochlear nerve? Give your answer in milliseconds (ms).
Q16.4. Using dB SPL = 20·log₁₀(P / (2×10⁻⁵ Pa)), and reading the threshold-of-audibility and threshold-of-pain curves at 50 Hz from the figure: how many times greater is the pressure of a painful sound than a just-audible sound, at 50 Hz?
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Question reproduced from IBO 2019, Theoretical Exam A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Official answer key