Your webinar mentions calorics is testing one frequency and it is similar to testing one frequency on the audiogram, but I cannot relate this anatomically as with hearing.

Your question seems to relate to the various options for rotational testing i.e. measuring the VOR in response to a rotational stimulus.

The modern balance clinic offers various complementary ways in which to provide a rotational stimulus to the vestibular system such as calorics, rotatory chair and video Head Impulse Testing. A common question relates to which test option should one select under different circumstances if they are all a rotational stimulus.

The present question ties into this wider one, and an analogy sometimes given to convey the way in which the test options are complementary is the use of the audiogram – restricting oneself to one or another would be rather like only testing hearing using one pure tone frequency instead of several across the audiometric spectrum.

As a starting point this seems a good analogy, and the below figure shows the spectrum of (head) rotational frequencies over which the different vestibular tests operate.

But at some level the analogy does break down and perhaps this is the source of your confusion. Let us unpack matters a little.

The frequency of sound (or number of cycles per second) is coded by the cochlear in a tonotopic way – different locations on the cochlear respond best to different frequencies. However, the vestibular region of the ear that sense rotation (semi-circular canals) decompose the stimulus in different semi-circular according to direction, not frequency (different semi-circular canals respond best to rotations in their plane of orientation). 

So you might say the word “frequency” is context specific. In terms of frequency of head motion, we are by extension talking about the strength of the stimulus. A low frequency head rotation would produce a low acceleration (weak stimulus) and a high frequency head rotation would produce a high acceleration (strong stimulus) while in hearing the strength of the stimulus is denoted by intensity. So, in a sense one could argue that a better analogy between vestibular and hearing systems would be to relate frequency of head rotation to a sound intensity, not frequency. Then you might visualise how caloric testing (which simulates a low frequency head rotation and low acceleration) might be analogous to testing one’s hearing with only a low-level sound. Increasing the head rotation frequency such as with the rotational chair or vHIT increases the head acceleration, which is analogous to increasing the sound intensity. 

If you like feel free to watch the webinar which generated this question.

May 2017
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