Figure 1 shows three components of the eye position during intorsional rotation
of the random pattern with a constant velocity of −44 deg/sec
(approximately 17 seconds) followed by no rotation of the random
pattern (5 seconds) and extorsional rotation of the random pattern with
a constant velocity of 44 deg/sec (approximately 28 seconds) in
experiments 1 and 2 (44ER). Torsional OKN with a negative (intorsional)
slow phase and positive (extorsional) fast phase was observed during
intorsional rotation of the random pattern, and the signs of tOKN were
the opposite during extorsional rotation of the random pattern in both
conditions. However, the slope of the slow phase of tOKN (slow-phase
velocity) in experiment 2 was steeper than that in experiment 1, and
the tOKN slow-phase beat duration in experiment 2 was shorter than that
in experiment 1
(Fig. 1) . Mean ± SD of the tOKN slow-phase gain
for each subject in experiments 1 and 2 are summarized in
Table 1 . Similarly, Mean ± of the tOKN slow-phase beat duration
for each subject in experiments 1 and 2 are summarized in
Table 2 . No significant difference in the slow-phase gain of tOKN was observed
between experiments 1 and 2 during the session of subject A with a
constant velocity of ±52 deg/sec (52AR). However, the slow-phase gain
of tOKN in experiment 1 was significantly smaller than that in
experiment 2 during each of the three sessions, when a constant
velocity of ±44 deg/sec was used (44AR,
P < 0.05, two
tailed
t-test, assuming unequal variance).
In the other 10 eyes, the majority (10/10 for the intorsional slow
phase, 7/10 for extorsional slow phase) showed a significantly smaller
mean tOKN gain in experiment 1 than in experiment 2 (P < 0.05, two tailed t-test, assuming unequal variance) in
the first session. When we counted 44AR and 52AR as two eyes, the grand
mean of the gain in experiment 1 was significantly smaller than that in
experiment 2 (0.037/0.051 for intorsion, 0.041/0.052 for extorsion,
experiments 1/2, respectively; P < 0.002, two-tailed
paired t-test) and the majority (11/12 for intorsional slow
phase, 10/12 for extorsional slow phase) showed a significantly longer
mean duration in experiment 1 than in experiment 2 (P < 0.05, two tailed t-test, assuming unequal variance). The
grand mean of the duration in experiment 1 was significantly longer
than that in experiment 2 (840 msec/585 msec for intorsion, 724
msec/543 msec for extorsion, experiments 1/2, respectively; P < 0.0001, two-tailed paired t-test).
In experiments 1 and 2, horizontal and vertical positions were
stabilized well at the center of the random pattern, but the
stabilization of the visual axis in experiment 1 was better than that
in experiment 2
(Fig. 1) . The mean SDs of the horizontal/vertical
components were 0.42°/0.57° (
n = 12) in experiment 1 and
0.93°/1.02° (
n = 12) in experiment 2, respectively. The
differences in the mean SDs were significant between these two
experiments (
P < 0.001, two-tailed paired
t-test,
n = 12). To test the effect of the
stability of the visual axis on tOKN, we investigated tOKN during
fixation on a visual target at 6.5° right or left from the center of
the rotating random pattern in seven eyes (experiment 3). We averaged
the SDs of the horizontal–vertical components during fixation on a
target at 6.5° right and left. Mean SDs were 0.60° and 1.02°
(
n = 7), respectively. Although the visual target
superimposed on the rotating random pattern decreased vertical OKN in
experiment 3, the stability of the visual axis in this condition was
significantly lower than that in experiment 1 (with a visual,
P < 0.05, two tailed paired
t-test,
n= 7).
Figure 2 shows three components of eye movements during experiment 3 (52AR).
Torsional OKN with a positive slow phase was induced by the random
pattern rotating with a positive constant angular velocity (52
deg/sec). Vertical OKN was not completely suppressed by the visual
target superimposed on the rotating random pattern. Upward and downward
slow phases of vertical OKN were observed during fixation on the target
at 6.5° to the left and right, respectively. The mean gain of
vertical OKN (vertical slow-phase velocity/vertical component of the
random pattern velocity at the target) for each subject was less than
0.2. That was clearly lower than that reported without a visual target
(0.8
17 ; 0.35–0.80
12 ). Mean slow-phase gain
and beat duration of tOKN in experiment 3 are summarized in
Tables 1 and 2 , respectively. Grand means of the slow-phase gain (0.034) and
beat duration (812 msec) of tOKN in experiment 3 were not significantly
different from those (0.037 and 825 msec) in experiment 1
(
P > 0.3, two-tailed paired
t-test,
n= 14) but were significantly different from those (0.051 and 564
msec) in experiment 2 (
P < 0.002, two-tailed paired
t-test,
n = 14).
Four of 11 eyes showed directional asymmetry of the tOKN slow-phase
gain in the same direction both in experiment 1 and experiment 2
(P > 0.05, two-tailed t-test, assuming
unequal variance). The gain during the extorsional slow phase was
larger than that during the intorsional slow phase in three of the four
eyes, but the other eye showed the opposite asymmetry. The grand mean
of the intorsional slow-phase gain was not significantly different from
that of the extorsional slow-phase gain (P > 0.5,
two-tailed paired t-test) for each condition (experiment 1,
experiment 2). Only one eye (44AR) showed directional asymmetry of the
tOKN slow-phase beat duration in the same direction, both in experiment
1 and experiment 2 (P > 0.05, two-tailed t-test, assuming unequal variance). Additionally, six eyes
showed directional asymmetry of the tOKN slow-phase beat duration in
either experiment 1 (three eyes) or experiment 2 (three eyes). In
total, the mean intorsional slow-phase beat duration was longer than
the mean extorsional slow-phase beat duration in four eyes in
experiment 1 and in three eyes in experiment 2. However, the other eye
in experiment 2 showed opposite directional asymmetry. Therefore, the
directional asymmetry in the grand mean of tOKN slow-phase beat
duration was observed in experiment 1 (P < 0.01,
two-tailed paired t-test) but not observed in experiment 2
(P = 0.118, two-tailed paired t-test).