Why are there corrective saccades when the VOR gain is in the normal range?

A recent paper in Frontiers in Neurology (by Korsager et al. Front Neurol. 8:81, doi.org/10.3389/fneur.2017.00081 ) reported very small saccades at high velocities. 

But saccades with normal VOR gain is exactly what you should expect!

Consider a healthy person who has a VOR gain of 0.9 (i.e very high, in the normal range)
but the gain is not 1.0, so during a head impulse his eyes will not exactly compensate for head velocity.  So at the end of the impulse his gaze will not be exactly on the fixation target, and so he must make a corrective saccade to get back to target. The result is a VOR gain of 0.9 and (small) corrective saccades.  The size of the corrective saccades will be larger at high head velocities as Korsager found (because the size of the fixation error will be larger, due to the higher velocity).

I was asked 3 questions

1. But why corrective saccades with normal VOR gain ( but less than 1.0) at high head velocities are not seen in every subject ?

• Many reasons, e.g. - patients vary in how accurately they fixate the target - their criterion. 
• In vHIT even tiny saccades are easily observed because the eye velocity during even small saccades is so large.
• VOR gain varies from trial to trial. The test is to detect a clinically significant reduction in VOR and minor variations in the performance of subjects are not of clinical significance.

2. Why it is sometimes on one side and not the other on the same subject , despite similar head velocities ?

Are the head angular accelerations exactly the same? I doubt it. The stimulus for the semicircular canal is head angular acceleration but vHIT uses head velocity as an indicator of head angular acceleration.  Realize you can get to a head velocity of 250 deg/s by very different head angular accelerations.

3. Why it is more commonly seen in symptomatic subjects?
A borderline patient can have reduced VOR gain, but still be in the statistically defined "normal" range. They will consistently make small saccades.  vHIT is giving you the measure of the absolute level of vestibular function.  Just as an audiogram shows the absolute level of hearing,  so vHIT is showing the absolute level of semicircular canal function – is it 100%  (VOR gain of1.0) or 90% (VOR gain of 0.9) or 70% (VOR gain of 0.7) etc.  The cut-off for “normal” is arbitrary and in such cases it is up to the clinician to use their judgement.

Ian Curthoys
30 August 2017
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