Figure 2. OKR gain is inferred from a piece-wise fit to the slow phase of tracked eye movements.

(a) We present a single pattern of horizontally moving bars to evoke OKR and crop it by superimposing a permanently dark area of arbitrary size and shape (left). Its velocity follows a sinusoidal time course, repeating every 10 s for a total of 100 s for each stimulus phase (right). (b) OKR gain is the amplitude of eye movement (green trace) relative to the amplitude of the sinusoidal stimulus (grey trace). The OKR gain is often well below 1, e.g. for high stimulus velocities as used here (up to 12.5°/s). (c) Even small stimuli are sufficient to elicit reliable OKR, although gains are low if stimuli appear in a disfavoured part of the visual field. Shown here are responses to a whole-field stimulus (top) and to a disk-shaped stimulus in the extreme upper rear (bottom, Figure 2—figure supplement 2).

Figure 2—figure supplement 1. Saccades are cropped before OKR gain is measured.

(a) As shown in Figure 2b, optokinetic gain is the ratio between the amplitudes of eye movement and stimulus movement. To extract these, raw eye traces must be processed. (b) OKR eye movements consist of a slow phase, gradually tracking stimulus motion, and intermittent saccades. (c) After pre-processing data to detect and remove saccades, we fit a piece-wise sinusoidal function with a single amplitude to the remaining slow-phase eye traces. The amplitude of the best fit determines OKR gain.

Figure 2—figure supplement 2. Even at remote stimulus locations, fish exhibit reliable OKR behaviour; trials with mixed behaviour are excluded.

(a–d) show 60 s samples of eye position traces obtained during visual stimulation (dark grey traces). Saccades were cropped, as demonstrated in Figure 2—figure supplement 1. Positive angles correspond to more rightward eye positions. Coloured traces show best fit to data (see Materials and methods), along with the resulting OKR gain. Fish were presented with either (a) whole field visual stimuli, or (b–d) smaller disk-shaped stimuli, as in Figure 3. (a) Whole-field stimulation elicits reliable OKR, even though high stimulus frequencies keep its gain below 1. (b) The same is true for disk stimulus in a preferred region of the visual space (stimulus D7, azimuth −90°, elevation +39°, that is lateral and dorsal to the fish), although the smaller stimulus evokes a smaller OKR gain. (c) Disk stimuli presented in a disfavoured part of the visual field (stimulus D3, azimuth −90°, elevation +74°, almost dorsal to the fish) evoke significantly lower gains, but still reliable OKR behaviour. Data in (a,c) correspond to trials shown in Figure 2c. (d) As it is possible to encounter behaviours other than OKR, or in addition to OKR, we inspected all trials before data analysis. Trials in which no pure OKR was present, such as the one shown here, were excluded (stimulus D3, same as in c).