Three-dimensional vector analysis of the human vestibuloocular reflex in response to high-acceleration head rotations. I. Responses in normal subjects

Abstract

1. The kinematics of the human angular vestibuloocular reflex (VOR) in three dimensions was investigated in 12 normal subjects during high-acceleration head rotations (head "impulses"). A head impulse is a passive, unpredictable, high-acceleration (3,000-4,000 degrees/s2) head rotation of approximately 10-20 degrees in roll, pitch, or yaw, delivered with the subject in the upright position and focusing on a fixation target. Head and eye rotations were measured with dual search coils and expressed as rotation vectors. The first of these two papers describes a vector analysis of the three-dimensional input-output kinematics of the VOR as two indexes in the time domain: magnitude and direction. 2. Magnitude is expressed as speed gain (G) and direction as misalignment angle (delta). G is defined as the ratio of eye velocity magnitude (eye speed) to head velocity magnitude (head speed). delta is defined as the instantaneous angle by which the eye rotation axis deviates from perfect alignment with the head rotation axis in three dimensions. When the eye rotation axis aligns perfectly with the head rotation axis and when eye velocity is in a direction opposite to head velocity, delta = 0. The orientation of misalignment between the head and the eye rotation axes is characterized by two spatial misalignment angles, which are the projections of delta onto two orthogonal coordinate planes that intersect at the head rotation axis. 3. Time series of G were calculated for head impulses in roll, pitch, and yaw. At 80 ms after the onset of an impulse (i.e., near peak head velocity), values of G were 0.72 +/- 0.07 (counterclockwise) and 0.75 +/- 0.07 (clockwise) for roll impulses, 0.97 +/- 0.05 (up) and 1.10 +/- 0.09 (down) for pitch impulses, and 0.95 +/- 0.06 (right) and 1.01 +/- 0.07 (left) for yaw impulses (mean +/- 95% confidence intervals). 4. The eye rotation axis was well aligned with head rotation axis during roll, pitch, and yaw impulses: delta remained almost constant at approximately 5-10 degrees, so that the spatial misalignment angles were< or = 5 degrees. delta was 9.6 +/- 3.1 (counterclockwise) and 9.0 +/- 2.6 (clockwise) for roll impulses, 5.7 +/- 1.6 (up) and 6.1 +/- 1.9 (down) for pitch impulses, and 6.2 +/- 2.2 (right) and 7.9 +/- 1.5 (left) for yaw impulses (mean +/- 95% confidence intervals). 5. VOR gain (gamma) is the product of G and cos(delta). Because delta is small in normal subjects, gamma is not significantly different from G. At 80 ms after the onset of an impulse, gamma was 0.70 +/- 0.08 (counterclockwise) and 0.74 +/- 0.07 (clockwise) for roll impulses, 0.97 +/- 0.05 (up) and 1.09 +/- 0.09 (down) for pitch impulses, and 0.94 +/- 0.06 (right) and 1.00 +/- 0.07 (left) for yaw impulses (mean +/- 95% confidence intervals). 6. VOR latencies, estimated with a latency shift method, were 10.3 +/- 1.9 (SD) ms for roll impulses, 7.6 +/- 2.8 (SD) ms for pitch impulses, and 7.5 +/- 2.9 (SD) ms for yaw impulses. 7. We conclude that the normal VOR produces eye rotations that are almost perfectly compensatory in...

Human 3-D aVOR with and without otolith stimulation

“…The relatively low maximum gains we found contrast with the aVOR gains of near unity observed with very short duration, high acceleration movements (Aw et al 1996;Crane et al 2000). The decreased gain in our paradigm is probably not due to the attentional demands of the task (where each trial is at least 30 s), because the Fig.…”

“…Crane et al (1997) measured the yaw aVOR at a higher frequency (1.2 Hz), and found gains similar to ours at 1 Hz. With the higher accelerations attained with rapid head impulses, non-linear VOR pathways may be recruited (Minor et al 1999), leading to higher gains (Aw et al 1996). Rapid head impulses are also nonperiodic, and lead to a predominantly unilateral canal stimulation.…”

Vestibular and Saccadic Abnormalities in Gaucher’s Disease

“…Horizontal, anterior, and posterior semicircular canal functions measured with the HIT were impaired as the angular VOR gains were about half of the close to unity gain in normal subjects (Aw et al 2001;Fahey et al 2008;Weber et al 2008). Onset latencies of HIT were prolonged from (7-10) ms (Aw et al 1996) to~18 ms. Horizontal saccadic initiation failure (Harris et al 1999) caused the absence of horizontal refixation saccades in response to the reduced horizontal VOR gain. This precludes clinical observation of the horizontal refixation saccade in presence of deficient horizontal VOR gain (Halmagyi and Curthoys 1988).…”

A new and objective test to evaluate functions of the semicircular canals: A review of video head impulse test

“…Akut ve kronik periferal vestibüler rahatsızlıklarda baş savurma testi kullanarak saptanamayan gizli sakkadlar video baş savurma testi ile saptanabilmektedir. [25,26] Video baş savurma testi ile tüm semisirküler kanalların tek tek değerlendirmesine, beraberinde otolit organların VEMP ile değerlendirilmesi kombine edildiğinde tüm vestibüler duyu organlarının değerlendirilme imkanının olabileceği de savunulmuştur. [20,27,28] Sonuç olarak, video baş savurma testi, uygulaması kolay, fazla vakit almayan, sonuçları oldukça güvenli ve duyarlılığı yüksek yeni geliştiril-miş bir testtir.…”