Journal Club: Demonstration of frequency tuning and other properties of the cochlea ex vivo using an in vivo-like OCT imaging chamber.

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Today's journal article

Alonso RG, Gianoli F, Fabella B, Hudspeth AJ. Amplification through local critical behavior in the mammalian cochlea. 

Why I picked this article

Our ability to hear soft and subtle sounds isn’t just passive detection; the cochlea amplifies the sounds that it senses using an energy-dependent system. This research investigates the concepts in auditory physiology: the "active process" that amplifies sounds inside the cochlea. 

Defining the core feature of the cochlea as:

  • Amplification of weak sounds 
  • Sharp frequency tuning 
  • Compressive nonlinearity (i.e. louder sounds are processed differently than softer ones) 
  • Spontaneous otoacoustic emissions (the ear literally produces sound as the result of the amplification process)
This research created a way to study the mammalian cochlea secured in a chamber, to study the above four properties of the cochlea in detail. 

Some of the research findings

Model: 

  • Mongolian gerbils (3-4 week-old) 
  • The cochlea was dissected in an oxygenated buffer, and the ends of the cochlea were sealed with adhesive. 
  • The cochlear disc was placed in a custom-made chamber in a way "The outer compartment was exposed to atmospheric pressure, whereas the inner compartment was closed and housed the inputs from two speakers as well as the front of a microphone"
  • The cochlea was immersed in an artificial inner ear solution and kept at 38°C (the body temperature of the gerbil)
  • Electrodes were used to apply an artificial mimic of endocochlear potential. 
  • acoustic stimuli were generated by two earphones (ER-3C, Etymotic Research)
  • Optical coherence tomography (OCT) with a standard imaging system (GAN621, Thorlabs) equipped with an objective lens (LSM02-BB Thorlabs) was used. 
  • The optical path of OCT was horizontal compared to the cochlea, mounted vertically. 

Figure 1c. experimental model for studying the cochlea used in this study. Alonso et al. 2025. 

Findings:

  • Sharp frequency tuning was demonstrated in the gerbil cochlea ex vivo in this comprehensive setup. 
  • Both the cochlear microphonic response and vibration of the basilar membrane showed frequency tuning. 
  • The sensitivity curve showed clear non-linearity in the response. 
  • At the characteristic frequency (i.e. most sensitive sound frequency for the location), there was a clear compressive non-linearity. 
  • The sound generated by the cochlea, or the distortion-product otoacoustic emissions, was observed in a similar way to previous observations from the bullfrog.  
  • When focusing on the basilar membrane vibration, the region of increasing vibration was where the outer hair cells were located. 

This research shows that, when placed in a carefully designed recording chamber, even a small segment of the mammalian cochlea outside the body still displays all four features. The active process still occurs locally and appears to be tuned near a "Hopf bifurcation", similar to observations reported previously in non-mammalian species.

Haruna's takeaway

The effort that has gone into this research to set up a chamber to keep the cochlea alive and in as native an environment as possible is enormous. The end result of the setup was the controlled angle to get the best visualisation by OCT, and the application of small sounds in a consistent way, collectively to collect sensitive data. Data from this research, I'm sure, is very useful to feed back into the modelling of the cochlear micromechanics. I was also not aware of the studies on bullfrog inner ear that this publication references, so I think I need to go and read them. 

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This is Haruna's 38/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.