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Figure 4—figure supplement 1 Differences between the OKR gains of directly stimulated left eyes and directly stimulated right eyes can be explained by a linear combination of biological and environmental factors, for example, biases of individual animals b3,k or asymmetries of the stimulus arena (see Materials and methods). Comparing data from the regular and rotated setups (Figure 4e–f), as well as data from fish immobilised upside-down (Figure 4g), including additional stimuli covering entire hemispheres (Supplementary file 1E), we can infer the underlying factors via multivariate regression of our linear model. We find that individual biases (grey) vary strongly from fish to fish and are broadly distributed from left to right. There are some constant biases across fish (green), both towards the left side of their visual field (b2) and towards one of the two LED hemispheres (b1); however, these biases are small and, given the large variability of individual biases, might be a result of the limited number of fish studied. Underlying stimulus types are listed in Supplementary file 1B and Supplementary file 1E. (b) Histogram of individual-fish biases shows no collective preference for either side.