Eye movements of flatfish for the changes of gravity (II).

In this study, we analysed the eye movements of flatfish for body tilting and compared with that of goldfish. The fish was fixed on the tilting table controlled by computer. The eye movements for body tilting along the different body axis were video-recorded. The vertical and torsional eye rotations were analysed frame by frame. In normal flatfish, vertical eye movement of left eye to leftward tilting was larger than that to rightward tilting. For head up or head down tilting, clear vertical eye movements were observed. On the other hand, torsional eye movements showed similar characteristics as goldfish. These results suggested that sacculus and lagena were important for otolith-ocular eye movements in flatfish.

Vestibulo-ocular reflex and gravity in fish.

An otolith organ on ground behave as a detector of both gravity and linear acceleration, and play an important role in controlling posture and eye movement for tilt of the head or translational motion. On the other hand, a gravitational acceleration ingredient to an otolith organ disappears in microgravity environment. However, linear acceleration can be received by otolith organ and produce a sensation that is different from that on Earth. It is suggested that in microgravity signal from the otolith organ may cause abnormality of posture control and eye movement. We examined function of otolith organ in goldfish revealed from analysis of eye movement induced by linear acceleration. We analyzed vertical eye movements from video images frame by frame. In normal fish, leftward lateral acceleration induced downward eye rotation in the left eye and upward eye rotation in the right eye. Acceleration from caudal to rostra1 evoked downward eye rotation in both eyes. When the direction of acceleration was shifted 15 degrees left, the responses in the left eye disappeared. These results suggested that otolith organs in each side transmitted different signals.

What kind of eye movements will goldfish show in space?

Six goldfish (1 normal, 1 with otoliths removed on both sides, 4 with otoliths removed on one side) were flown in space. The behaviors of the fish were recorded with a video camera on Mission Elapsed Time (MET) Day-00, 02, 05, 08, and 12. On MET Day-00, fish with otoliths removed on one side showed rolling behavior toward the operated side. No rolling behaviors were observed after MET Day-8. Five fish showed backward looping behaviors during the mission. After the space experiment, torsional eye movements and vertical eye movements were examined by body tilting. The experiments showed that the sensitivity of eye movements were low for head up tilting and tilting to operated side.

Eye movements of goldfish depending on the relationship between acceleration vector and otolith organ.

Prior experiments demonstrated that the acceleration deviating 15 degrees to the right (left) from the longitudinal body axis could not produce vertical eye movement in left (right) eye of the goldfish. From these results, we expected that vertical eye movement of goldfish for the acceleration perpendicular to the longitudinal body axis might different between right and left eye. However, in this experiments, there were no clear difference in magnitude of vertical eye movements for the acceleration shifted 15 degrees around the left-right body axis. On the other hand, the response of right eye was larger than that of left eye for the acceleration applied from left to right of the body. These results suggest that stimulation from medial to lateral and lateral to medial of the otolith organ has different effect on vertical eye movement in each eye of goldfish.

Function of otolith organ in goldfish revealed from analysis of eye movement induced by acceleration.

An otolith organ on ground behave as a detector of both gravity and linear acceleration, and play an important role in controlling posture and eye movement for tilt of the head or translational motion. On the other hand, a gravitational acceleration ingredient to an otolith organ disappears in microgravity environment. However, linear acceleration can be received by otolith organ and produce a sensation that is different from that on Earth. It is suggested that in microgravity signal from the otolith organ may cause abnormality of posture control and eye movement. Therefore, the central nervous system may re-interprets all output from the otolith organ to indicate linear motion. A study of eye movement has been done a lot as one of a reflection related to an otolith organ system. In this study, we examined function of otolith organ in goldfish revealed from analysis of eye movement induced by linear acceleration or the tilt of body. We analyzed both torsional and vertical eye movements from video images frame by frame. For tilting stimulation, torsional eye movements induced by head down was larger than that induced by head up for larger tilt angle than 30 degrees. In the case of linear acceleration below 0.4 G, however, no clear differences were observed in both torsional and vertical eye movement. These results suggest that body tilt and linear acceleration may not be with equivalent stimulation to cause eye movement on the ground.

Functional asymmetry estimated by measurements of otolith in fish.

It is widely accepted that the incidence of space adaptation syndrome (SAS) is due to a mismatch of sensory information from various receptors to the central nervous system. We investigated the functional asymmetry of vestibular organ, which may caused sensory conflict in space, by measuring the weight difference of otolith between left and right side in goldfish and carp. In the goldfish utricular otolith, the maximum difference was 0.8 mg and the mean difference was 0.091 mg. The percentage of weight difference to the heavier otolith was calculated. The maximum difference was 20.57% and the mean was 3.035%. A difference exceeding 10% was found in only 2 goldfish. In the carp utricular otolith, the maximum percentage difference of weight was 24.8% and the mean was 3.491%. A difference exceeding 10% was found in only 3 carp. The maximum difference of saccular otolith was 11.8% with the mean of 6.92%, and that of lagenar otolith was 32% with the mean of 5.6% in goldfish. The close relationship of utricular otolith weight between both sides suggested that the otolith asymmetry might not be the main factor inducing SAS at least in goldfish and carp.