A New Tool for Investigating the Functional Testing of the VOR

Abstract

Peripheral vestibular function may be tested quantitatively, by measuring the gain of the angular vestibulo-ocular reflex (aVOR), or functionally, by assessing how well the aVOR performs with respect to its goal of stabilizing gaze in space and thus allow to acquire visual information during the head movement. In recent years, several groups have developed clinical and quantitative approaches to functional testing of the vestibular system based on the ability to identify an optotype briefly displayed on screen during head rotations. Although the proposed techniques differ in terms of the parameters controlling the testing paradigm, no study has thus far dealt with understanding the role of such choices in determining the effectiveness and reliability of the testing approach. Moreover, recent work has shown that peripheral vestibular patients may produce corrective saccades during the head movement (covert saccades), yet the role of these eye movements toward reading ability during head rotations is not yet understood. Finally, no study has thus far dealt with measuring the true performance of their experimental setups, which is nonetheless likely to be crucial information for understanding the effectiveness of functional testing approaches. Thus we propose a new software and hardware research tool allowing the combined measurement of eye and head movements, together with the timing of the optotype on screen, during functional testing of the vestibulo-ocular reflex (VOR) based on the Head Impulse Test. The goal of such tool is therefore that of allowing functional testing of the VOR while collecting the experimental data necessary to understand, for instance, (a) the effectiveness of the covert saccades strategy toward image stabilization, (b) which experimental parameters are crucial for optimizing the diagnostic power of the functional testing approach, and (c) which conditions lead to a successful reading or an error trial.

Keywords: compensatory saccades; dynamic visual acuity; functional vestibular testing; head impulse test; semicircular canals function.

Figure 1

Probability of correct answers as a function of logMAR increment of all subjects. Values are fitted with a psychometric logistic function. For increment of 0.5 logMAR and less, the probability of providing a correct answer decreases statistically significantly.

Figure 2

Timing performance of the system using a screen running at 60 Hz. (A) Head velocity/sensor output. (C) Head acceleration/sensor output and one running at 75 Hz. (B) Head velocity/sensor output. (D) Head acceleration/sensor output. The sensor trace is recorded using a screen mounted photodiode that transduces the presence/absence of the optotype on screen.

Figure 3

Results of the HITD test on a healthy subject. (A,B) Head and eye movement for CW and CCW rotations, respectively. (C) Plot of VOR gain values computed for each movement. (D) Percentage of correct answers for each acceleration bin.

Figure 4

Results of the HITD test on a patient with vestibular neuritis (left side deficit). (A,B) Head and eye movement for CW and CCW rotations, respectively. (C) Plot of VOR gain values computed for each movement, x indicates wrong reading. (D) Percentage of correct answers for each acceleration bin. Overall percentage of correct answers: 18% toward the affected side, 62% toward the contralateral.

Figure 5

(A,B) Head and eye velocity response, and optotype presentation from a normal subject and a patient, respectively, with (left) unilateral vestibular neuritis. (C,D) Head, eye, and gaze position, together with optotype timing for the responses in (A,B), respectively.

Figure 6

Head impulse testing device test 3 days (on the left) and 1 month (on the right) after the acute phase of a vestibular neuritis. (A,B) velocity plot, (C,D) position plot. The patient improved the percentage of correct answers from 3 to 39%.