Some photographers take advantage of the blurring effect produced when animals and humans move past the lens at high speed to produce extraordinary images, but if animals didn't have a sophisticated system – the optokinetic reflex – to stabilise their vision while mobile, this smeary perspective would be our permanent reality. Céline Gravot, from the Ludwig-Maximilians-Universität München, Germany, explains that most animals swivel their eyes in their heads to counteract the effects of motion: ‘You can experience this reflex when looking out of the window of a moving train; your eyes will move to follow the movement of the scene outside’, she says. But Gravot and her colleagues Alexander Knorr, Stefan Glasauer and Hans Straka were curious to find out how the swivelling motion of the eye responds to specific features of a scene. For example, would the reflex movement respond in the same way to dark objects – such as a flock of birds or squadron of planes – silhouetted against bright daylight as it does to brilliant stars against a dark sky?
As the vision of most mammals is too sophisticated to begin unravelling the answer to this question, the team focused on a simpler animal, the African clawed frog (Xenopus laevis) tadpole, which swivels its eyes to counteract the effect of its swimming motion. Designing a series of simple scenes – ranging from white dots on a black background, through various shades of grey, to black dots on a white background, in addition to crescent-shaped dots and small blocks – Gravot and Knorr swivelled the scenes before the eyes of stationary tadpoles while they filmed the animals’ eye motions and recorded the electrical signals produced by the tadpoles’ retinas.
However, the team was surprised when they realised that the tadpoles responded more strongly to the bright dots on the dark background than they did to the dark dots on a white background, swivelling their eyes through a wider angle in response to the white dots than the dark dots. In addition, the strength of the electrical signals generated by the retina also declined as the background became lighter and the dots darker. ‘We did not expect that’, says Gravot, as the only difference between the two scenes was the intensity of the spots relative to the background. The retina responded differently to both scenarios, ultimately altering the strength of the tadpoles swivelling reflex response.
‘This confirms again that the eye is a pretty smart sensory organ on its own’, says Gravot, who suggests that the eye may be making a rudimentary interpretation of the motion of a scene to produce the reflex. And she also suspects that the tadpoles responded less strongly to the motion of dark objects because they could be interpreted as food particles, where as bright objects moving against a dark background could be the result of visual motion generated by the tadpoles’ own movements, triggering a strong visual reflex in the animals. Gravot and her colleagues are now keen to discover how the tadpoles’ drastic lifestyle change, when they metamorphose from aquatic larvae to amphibious hunters, affects their visual processing.