Discussion
Previous paper claimed that relationship between the VOR gain and
horizontal eye position was inversed (10). This was similar to the
theoretical model that the VOR gain is calculated as\(\text{Cos}\left(\text{gaze\ angle}\right)*eye\ ball\ radius\)indicating the vertical movement of eye during the head impulses on
vertical canals (Figure 4). However, during clinical practices,
especially when performing vHIT with Oto system, patients hard to follow
the targets during the vertical impulses when head turned 45 degrees
away.
In our investigations, we combined the VOR gains from all different
vertical plane at different angles together, before compared it to the
theoretical model (Figure 4A). This comparison revealed that the 25° has
the best gain that most significant different from 0°. One explanation
we had was the range of pupil movement when staring at the 0° was
shorten by the canthus and eyelids (Figure 4B-F). Even instructed the
participants to open their eyes up, the relative distance of pupil shift
was cut. Moreover, most of the time, patients can’t open their eyes
large enough for systems to detect the real eye position, which usually
were underestimated the eye movement (Figure 4E). Even though pupil
movement would not be blocked at 45°, the VOR gain was decreased, which
agreed with previous data that suggested decreased oculomotor kinematics
at 45° affected (10). Thus, we proposed that different from “ideal”
situation and previous papers, 25° is a better gaze angle when
practicing vertical vHIT.
Similar to formerdata, ESC performed less well on vertical semicircular
canals (1). In contrast, Oto was considered as a gold standard system by
former investigations (1, 11). We compared the new device to either of
these systems. Our results showed that first of all, the pupil detection
system with the newly reformed goggle worked just as fine. Secondly,
different from Oto system, the new goggle allows the patients at LARP
plane, especially, to stare at the targets easier. This convenience not
only leads to a better result on the RP plane, but offers examiners the
freedom to pick whichever eye for VOR tests. In a word, this newly build
VG system performed as good as Oto, even better than ESC system.
With the limitations of our experiments, there are still several
directions that we would like to investigate in the future. First of
all, our hypothesis was performed with limited participants recruited.
It would be better to test this modified protocol with broader subjects.
Furthermore, we would like to test the VG system out in vertical
semicircular canals to verify the universality of our newly proposed 25°
gaze angle. Besides, in the previous publications, inverse relationships
between the VOR gain and distance was claimed (12, 13). This increased
gain was much lower than predicted calculation that indicated the
limitation of VOR in vHIT. In clinics, the chair usually was set at 1m
to 1.5m away from the target in a consideration of the vergence reflex
and accommodations. However, a unified standard of this distance is
still missing. In general, these experiments may help a new practice
protocol to be introduced in the future.