How to Record the Cochlear Microphonic (CM)

This video shows how to record the cochlear microphonic (CM) during an electrochleography (ECochG) measurement.

You can read the full transcript below.

What is the cochlear microphonic?

The cochlear microphonic is a response from the cochlea that mimics the input stimulus and is believed to be a response primarily of the outer hair cells.

Why measure the cochlear microphonic?

The presence of a cochlear microphonic along with an absent or abnormal ABR is used in the diagnosis of auditory neuropathy spectrum disorder.

Recording methods

Unlike the summating potential and compound action potential, the cochlear microphonic is an alternating current voltage that mirrors the polarity of the acoustic stimulus.

Therefore, in order to record this, it is recommended to measure the condensation and rarefraction polarities separately.

Alternatively, the test can be used by using an alternating polarity stimulus and the condensation and rarefraction components separated after the measurement by using the A-B buffer.

Test parameters

The NHSP 2013 UK guidance on cochlear microphonic testing recommends that a click stimulus is presented to the ear at 80 dB nHL with a stimulus repetition rate of 87.1. This is because the cochlear microphonic is a pre-neural response and thus not susceptible to neural adaptation seen when recording responses from the central auditory system.

When recording using a single polarity stimulus, a stimulus artifact can also be recorded. It is therefore recommended to always use insert earphones when recording the cochlear microphonic as these create a delay between the stimulus artifact and the cochlear microphonic itself, making it easier to see on the screen.

Cochlear microphonic recording and analysis

With the patient in a restful state and the ongoing EEG low, you can start the test. As the test continues, the cochlear microphonic components begin to come through from the noise. You should continue recording until the noise floor becomes sufficiently low. Recording 2000 sweeps is usually sufficient.

Because of the high repetition rate, the test time should be very short. If you’ve recorded using separate rarefraction and condensation clicks, then you should run a second measurement ensuring that you have a measurement from each stimulus polarity. 

The presence of the cochlear microphonic can be confirmed by sinusoidal segments that inverts in the two stimulus polarity waveforms, beginning within one millisecond of the stimulus and possibly lasting up to five or six milliseconds.

If this can be seen, then it is essential to confirm that this is not a stimulus artifact. This can be achieved by performing a control run using the same parameters but this time clamping the insert earphone tubing, stopping the sound from reaching the ear.

If the measured potential remains, it is due to the stimulus artifact and is not a true cochlear microphonic. If the response disappears, then this indicates the response is truly present.

This concludes this quick guide video on recording an ECochG displaying the cochlear microphonic.