The Optokinetic Response of Larval Zebrafish

Abstract

Abstract: : Purpose: Zebrafish have been widely used as a model for vision research, and the optokinetic response (OKR) has served as one of the tools for evaluating visual performance. In the present study we characterized the response properties of the OKR of larval zebrafish. Methods: Young zebrafish larvae (8–10 days) were immobilized in 2% methylcellulose on a platform in the center of a rotating, opaque drum lined with black and white vertical stripes located approximately 4 cm from the animal’s head. An Axiocam mounted on a Zeiss OPMI 1 stereomicroscope was used to view and record the fish’s response to the rotating drum. Photographs were taken at 4 frames per second, and the position of each eye in relation to the head was measured in Photoshop. Three complete saccades were analyzed and the averaged values were used to calculate the amplitude and speed of the OKR. Results: When a low spatial frequency drum (0.06 cycles/degree) was used, the amplitude of the OKR did not vary significantly over a range of drum speeds from 2.5 to 18 rpm. On the other hand, plotting the speed of the OKR as a function of drum speed gave rise to a bell–shaped curve, with a maximum OKR at about 10 rpm. Interestingly, the two eyes exhibited an asymmetric response to the rotating drum, i.e., the left eye was more responsive to a clockwise rotation, and the reverse was seen with counter–clockwise motion. Although this asymmetry persisted over the entire range of drum speeds tested, the situation was reversed when tested with a higher spatial frequency stripe pattern, i.e., the right eye became more responsive to a clockwise stimulus and vice versa. The amplitude of the OKR for both eyes exhibited an inverse relation to increasing spatial frequency of the stimulus, whereas the speed of the OKR showed a steep decline within the range of 0.08 to 0.14 cycles/degree. Conclusions: The data indicate that zebrafish are more responsive to objects with low spatial frequencies moving from behind the animal, and to high spatial frequencies of moving objects located in the frontal plane. It appears that zebrafish visual system utilizes different processing mechanisms for stimuli located in different regions of the visual field.

Keywords: eye movements: recording techniques • ocular motor control • visual acuity