Figure 9 illustrates the eye movement traces when three subjects with types 2,
3, and 4 MLN monocularly viewed a target, in the primary position. The
effects of either removing the target or stabilizing the retinal image
are shown respectively in columns two and three. For all three MLN
types, removal of the target reduced the intensity of the MLN and
decreased the mean slow-phase velocity. The application of a fbg of+
1.0 also modified the nystagmus.
In an effort to explore the effect of visual feedback more
systematically, three feedback gain (fbg) conditions were investigated.
These were 0.0, 0.5, and 1.0 fbg and were equivalent to normal retinal
image motion (fbg = 0), a 50% reduction in retinal image motion
(fbg = 0.5), and a stabilized retinal image state (fbg =
1.0).
Figure 10 illustrates how these three test conditions affected the class of MLN
slow phases for three subjects with types 2, 3, and 4 MLN. For subjects
with type 2 MLN, the +0.5 and +1.0 fbgs greatly modified the subjects’
oscillations which reverted to those seen normally during binocular
viewing (i.e., square-wave jerks). In contrast, the +0.50 and +1.0 fbg
conditions did not appear to change the incidence of the dominant MLN
slow-phase class for subjects with type 3 and type 4 MLN.
The response to the introduction of a change in visual feedback
appeared to be specific to each subject, and
Figure 11 illustrates each of the six variations that we found. In
Figure 11A the
subject who had type 3 MLN never exhibited any apparent change in the
right-beating MLN. In comparison, the monocular left-beating nystagmus
seen before the onset of feedback in a subject with type 2 MLN, changed
to square-wave jerk oscillations when feedback was introduced
(Fig. 11B) . This response, whereby the monocular oscillations either
partially or completely reverted to the binocular state was typical of
all subjects with type 2 MLN. The same response was also elicited when
the target was removed during binocular viewing. Of note, when the
feedback was introduced, there was often a delay of up to 3 seconds
before the onset of the square-wave jerks. Thereafter, for the subject
in
Figure 11B the left eye slowly drifted nasally 4° or so. In the
third feedback-related affect
(Fig. 11C) , the introduction of feedback
to a subject with type 3 MLN brought about a large increase in the
amplitude of the right-beating MLN. Once again, there was a measurable
latency—2 seconds in this case—before the onset of the
large-amplitude oscillations.
Figure 11D illustrates how the
introduction of a +1.0 fbg to a subject with type 4 MLN brought about a
shift in the eye position of approximately 8°. This shift was
mediated by leftward saccades. By comparison, a subject with type 3 MLN
(Fig. 11E) who also showed a gaze shift, achieved the new eye position
by a series of slow eye movements. Finally, a subject with type 4 MLN
(Fig. 11F) exhibited a change in the nystagmus intensity which is
superimposed on a large-amplitude, low-frequency slow eye movement.
The saccade-mediated gaze shift seen in
Figure 11D for the
subject with type 4 MLN was investigated further by examining how the
level of the fbg influenced the change in the eye position.
Figures 12A 12B 12C 12D illustrate that the shift was directly related to the
level of the fbg, so that the +1.0 fbg condition brought about a 6°
shift in the eye position.
Removal of the visual feedback brought about a gaze shift in the
direction opposite to that seen when the feedback was introduced. This
eye position change was achieved by either an extended slow phase
(
Figs. 13A 13B ) or a saccade
(Fig. 13C) .