Figure 1. The superior and inferior optokinetic reflex (OKR) are asymmetric in adult mice.

(A) Schematic of behavioral setup to elicit the vertical OKR. The mouse is situated so that one eye is centered in a hemisphere. Stimuli are projected onto the hemisphere’s concave surface via reflection off of a convex mirror. Eye movements are tracked using an infrared-sensitive camera and a corneal reflection (see ‘Materials and methods’). (B) Example video frames demonstrating that the eye traverses between superior, neutral, and inferior positions in the presence of vertically drifting sinusoidal gratings. Red arrows mark the infrared corneal reflection. (C, D) Example of OKR in response to full contrast (C) superior and (D) inferior unidirectional drifting gratings (10°/s). For each epoch, a continuous 60 s stimulus was flanked by 20 s of a static grating (shaded regions). Ticks above the plots mark the time of fast nystagmus either in the superior (magenta) or inferior (gray) direction. Examples from one animal. (E) Rate of vertical fast nystagmus for superior and inferior stimuli on each epoch for N = 5 mice. Horizontal line represents median, box boundaries are the interquartile range (IQR), whiskers represent most extreme observation within 1.5× IQR. (F) Cumulative vertical distance traveled during slow nystagmus in response to superior (magenta) and inferior (gray) drifting gratings (mean ± SEM). (G) Example of OKR in response to a vertically oscillating sinusoidal grating. The eye position is shown in green, and the stimulus position is shown in lavender. Saccades (‘fast nystagmuses’) have been removed to reveal the asymmetry between superior and inferior OKR. For each epoch, animals viewed eight oscillation cycles lasting a total of 120 s, flanked by 20 s of a static grating (shaded regions). (H) Average gain of slow nystagmus during the superior versus inferior stage of individual oscillations. Each small dot is a single oscillation. The region of magenta (or gray) indicates that gain was greater for the superior (or inferior) stage of the oscillation. Points that fall on the line indicate equivalent gain for both stimulus directions. Large dot and whiskers represent univariate medians and 95% confidence intervals (via bootstrapping), respectively. Significance value indicates whether the points tend to fall unevenly on one side of the unity line (two-sided signed-rank). (I) Eye position (green) and stimulus position (lavender) averaged across all oscillations and all animals (mean ± SEM). Starting eye position is normalized to 0° at cycle onset. The average ending eye position is displaced in the superior direction (two-sided signed-rank). N = 5 mice for all experiments; n = number of trials. *p<0.05, **p<0.01, ***p<0.001.

Figure 1—figure supplement 1. Example of sinusoidal vertical optokinetic reflex (OKR) before saccade removal.

Eye position (green) is plotted across time as a full-field grating oscillates vertically (lavender). The eye trace includes saccades (i.e., ‘fast nystagmuses,’ as indicated by tick marks: magenta for superior, gray for inferior). Saccades tend to occur in the opposite direction as the ‘slow nystagmus’ and do not facilitate image stabilization. However, saccades are necessary to keep the eye centered in its orbit. Removing these resetting saccades from the eye trace isolates the slow nystagmus component and reveals the asymmetry between superior and inferior OKR (Figure 1G).

Figure 1—figure supplement 2. Baseline vertical eye movements in head-fixed mice (see also Figure 8—figure supplement 4).

Vertical eye movements were measured in response to static gratings to calculate eye drifts for baseline subtraction. (A) Example raw eye trace over 22 s of a static grating. The calculated position of the eye drifts downward over time, which could reflect true eye movements or a calibration error in our recording configuration. These two possibilities cannot be disambiguated (see ‘Materials and methods’). The magnitude of eye position drift during static gratings is approximately 18-fold less than the magnitude of the eye movements elicited by high-contrast drifting gratings. (B) Distribution of instantaneous eye velocity across N = 5 animals for the 20 s prior to the onset of all drifting grating stimuli (unidirectional and oscillating gratings at high [full] and low [20% relative] contrasts) used to evoke the optokinetic reflex (OKR). On average, there is a slight bias toward inferior (i.e., downward/negative) eye velocities during this baseline period, with a median velocity of –0.0787°/s. (Bi) Full distribution. (Bii) Same data, zoomed in on 0° to reveal the inferior bias. (C–F) Absolute vertical position of the eye without drift correction (C) prior to stimulus onset (when the drift was calculated as in [B], includes data from high (full) and low (20% relative) contrast, oscillating and unidirectional experiments), and during (D) high-contrast oscillating gratings, (E) superior unidirectional gratings and (F) inferior unidirectional gratings. Absolute eye position is similar to that measured during baseline only for oscillating gratings. The eye moves to more extreme positions during unidirectional stimuli. For this reason, the baseline subtraction was only applied to eye movements measured in response to oscillating gratings. For all histograms, arrows mark the median of the distribution. Yellow arrow in (D–F) marks the median of the distribution shown in (C). See Figure 8—figure supplement 4 for further data on eye drift in response to low-contrast gratings. *p<0.05, **p<0.01, ***p<0.001.