Why does vHIT detect semicircular canal loss?

In a healthy subject a head rotation to the left activates receptors in the left horizontal semicircular canal, so the nerve fibres from the left canal generate nerve impulses which cause both eyes to rotate smoothly so that both eyes exactly compensate, or correct for, the head movement.  So both eyes stay looking at the target during the passive unpredictable head movement.  But if the person has a loss or deficiency in the left horizontal canal system, then the neural drive to the eyes will not be enough to drive the eyes to correct perfectly for a leftward head movement.  So the eyes will move with the head and the result will be that at the end of the head movement the eyes will have been dragged off target and the patient will have to make a corrective saccade to get back onto the target.  That is the overt saccade which the clinician sees.  Obviously a right side canal deficit will cause a loss for rightward head movements.  If just one side is affected and the other side is healthy then the corrective saccade will only occur for head rotations towards the affected side. That patient has a unilateral vestibular loss.  This is the most frequent kind of deficit. When both sides are affected, the patient makes  corrective saccades for both directions of head rotation – they have bilateral vestibular loss.