Optomotor behaviour in Xenopus laevis tadpoles as a measure of the effect of gravity on visual and vestibular neural integration.

The ability of aquatic vertebrates to maintain their position requires integration of visual and vestibular sensory information. To understand better how aquatic animals integrate such information, we measured the optomotor behaviour of Xenopus laevis tadpoles raised in growth chambers in microgravity (< 10(-3)g), normal gravity (1 g), hypergravity (3 g) and on a slowly rotating clinostat (simulated microgravity). The goal of this research was to determine how development in an altered gravitational force field affects the visual- and vestibular-dependent behaviour of tadpoles. This research represents the first time that the optomotor behaviour of an organism raised from fertilization in microgravity has been tested. Significant differences were observed in the optomotor behaviour among the four gravity treatments. When first exposed to normal gravity, the microgravity-raised tadpoles exhibited the strongest (or most positive) optomotor behaviour, while the 3 g centrifuge tadpoles showed no optomotor response. Some abnormal behaviours (such as erratic swimming, lying motionless and abnormal swimming posture) were observed in the tadpoles raised in altered gravity on the initial day of testing. One day later, the tadpoles raised in hypergravity did not differ significantly in their optomotor behaviour from control tadpoles raised in normal gravity. However, tadpoles raised in microgravity still displayed an exaggerated optomotor response. One week after the tadpoles had been introduced to normal gravity, there was no longer a significant difference in optomotor behaviour among the different gravity treatments. This convergence of optomotor behaviour by tadpoles from the different treatment reflects the acclimation of their vestibular systems to normal gravity.

BLIND MEXICAN CAVE FISH (ASTYANAX HUBBSI) RESPOND TO MOVING VISUAL STIMULI

In apparatus for measuring optomotor behaviour, blind Mexican cave fish, Astyanax hubbsi, increase their swimming velocity upon rotation of a striped cylinder, i.e. in response to a solely visual stimulus. The fish follow the movements of the stripes at (i) rotation velocities between 60 degrees s-1 and 80 degrees s-1, (ii) light intensities of less than 20 lx and, (iii) stimulus widths subtending an angle of less than 1 °. Extirpation of the vestigial eye structures does not affect the response to the moving visual stimulus, which indicates that the response is mediated by extra-ocular photoreceptors. An optomotor response can be reliably evoked in a round test aquarium. Fish do not respond when the test aquarium contains environmental cues, such as bars on the wall or when a section of the round aquarium is divided off. This indicates that the fish obtain information about their environment from different sensory sources and that the visual stimulus is effective only when no other means of orientation are available. We suggest a modified theory of the optomotor response, which emphasizes the crucial role of the environment in eliciting the response and which permits behaviours more complex than just following the stimulus.

Fractionation of Drosophila Populations According to Optomotor Traits

1. The optomotor control of orientation and locomotion in the fruitfly Drosophila melanogaster requires the conveyance of information from distinct movement detectors in the visual system to distinct movement effectors in the motor system. Abnormalities of the optomotor control system have been found occasionally in Drosophila. 2. The abnormal flies can be isolated from population samples by appropriate fractionation according to the magnitude and the sign of the optomotor responses. A cyclically operating machine was used to fractionate two inbred strains, w+ and wα, which possess different alleles on the white-locus of their X-chromosomes. 3. Movements of an artificial visual environment elicit similar orientation-control responses, but antagonistic locomotion-control responses in the two strains. The responses depend on various parameters and may even change with habituation to the stimulus. However, the application of selection pressure through eight generations has little if any effect on the different optomotor behaviour of the inbred strains.