It is reasonable to question whether the elicited OKN was simply because the animal switched fixation from the red fixation spot to the green drifting grating. We are convinced that the animal did not switch fixation and that the OKN was in fact elicited by presenting the motion stimulus to the strabismic eye, for two reasons. The first reason is that the angle of strabismus remained the same for the duration of the grating presentation. As shown in
Figure 2, a switch in fixation is accompanied by a significant change in strabismus angle, and so if the animal were to switch fixation to directly view the eccentric grating stimulus, we would have observed an accompanying change in strabismus angle. The second reason is that when the animal directly viewed the drifting grating (i.e., drifting grating presented to the fixating eye), the OKN response was significantly more robust than the OKN response elicited when the grating stimulus was presented to the deviated eye.
Figure 4 shows an example of a single continuous section of data from monkey M2 illustrating both of these points. In this trial, the red fixation spot was presented to the right eye, and a leftward drifting grating (drift velocity of 10°/s; spatial location left 20°) was presented to the left eye. Although OKN responses are observed throughout the trial, two differences are noted between the shaded regions (marked in yellow) and the unshaded regions. In the unshaded regions, the right eye position (∼0°) is appropriate for fixating on the red fixation spot, and the left eye position is appropriate for viewing the drifting grating. In the shaded regions, however, while the left eye positions remain on the drifting grating, the right eye position moves away from the red fixation spot—indicating that a switch in fixation occurred, resulting in the left eye directly viewing the grating. This movement also results in an increase in strabismus angle. The second difference between the shaded and unshaded regions is with regard to the “quality” of the OKN. As can be seen in
Figure 4, the OKN response in the shaded regions is significantly more robust than in the unshaded regions. Quantitative analysis of OKN slow phases in the shaded and unshaded regions of
Figure 4 showed increased OKN frequency and decreased variability in slow-phase velocities in the shaded (direct viewing) region compared to the unshaded (deviated eye viewing) region (unshaded region left eye OKN frequency = 1.16 Hz, slow-phase velocity = −6.0 ± 2.0°/s; shaded region left eye OKN frequency = 1.90 Hz, slow-phase velocity = −6.5 ± 1.1°/s; difference in variability of slow-phase velocities statistically significant,
F-test,
F(17,10) = 3.3058;
P = 0.03). Therefore observations of the strabismus angle, position of the eyes, and the OKN characteristics together allowed us to identify data where the animal was viewing the fixation spot and the drifting grating was being presented to the deviated eye; and all further analysis of OKN properties was focused only on these sections (i.e., corresponding to the unshaded regions in
Fig. 4).