Spatial frequency and the optomotor response.

(A) Preliminary experiment to find the spatial frequency (in cycles per degree) of the sinusoidal grid stimulus most effective in eliciting an optomotor response when testing at the medium height (28-36m) with a stimulus speed of 5 mm/s. 24 7dpf AB wildtype zebrafish larvae were tested at each spatial frequency, with the grid moving first forward and then backward for two test traverses each 60s in duration. The optomotor index shown here is the ratio of total distance swum in the direction of the grid stimulus divided by distance moved by the stimulus. It is similar to the OMR ratio measure used elsewhere except that it is based on swimming over the whole trial period rather than only during OMR trajectories. The spatial frequency (0.0156 cpd) used in all subsequent experiments was close to the optimal value of 0.015 found here. The absolute grid period was varied in proportion to height to keep it as constant as possible. (B) A follow up experiment checked that the same spatial frequency is also effective at the low (4-12mm) and high (52-60mm) heights. There were 8 larvae in each experimental condition, with each individual larva experiencing a single height and all spatial frequencies. The frequency of 0.15 cpd was again at least as effective as either 0.01 cpd or 0.02 cpd, suggesting that any variation in perceived grid spatial frequency due to differences in the degree of refraction experienced at different heights did not have a material impact on the strength of the OMR in these experiments. (PDF)