Jumping spiders are not fooled by the peripheral drift illusion

Abstract

In the peripheral drift illusion, a static circular sawtooth pattern is perceived as if it were rotating. It is believed that this effect is a byproduct of how the neural substrate responsible for motion perception is organized. The structure of the motion perception circuitry is widespread across the animal kingdom, vertebrates and invertebrates alike, which in turn causes the illusion effect to be experienced by virtually all animals. Among invertebrates, jumping spiders possess a unique visual system. For them, the tasks of visual computation are split across 4 pairs of eyes, with motion detection, target recognition, and shape discrimination computed in completely segregated brain areas and visual field sections. In such an organization, it is unlikely that the circuitry for motion perception common to other animals is shared by jumping spiders. Consequently, jumping spiders should be immune to the peripheral drift illusion. To test this hypothesis, we placed jumping spiders on top of an omnidirectional treadmill and presented them with circular visual stimuli in their visual periphery. These were either composed of a sawtooth pattern, and therefore inducing the illusion, or of a sine-wave pattern of equal luminance and spatial frequency but not illusion-inducing. The stimuli could either be static or rotate around their center, either clockwise or counterclockwise. As jumping spiders perform distinctive full-body pivots when detecting a moving object in their visual periphery, we registered the frequency of this behavior to assess the illusory percept. We found that the spiders responded consistently to all moving stimuli, but did not react to the static illusion, therefore it was not perceived as in motion. The absence of the illusory percept in spiders opens many questions about the nature of their motion perception circuitry and casts doubts on how the illusion is widespread in the animal kingdom outside the common model species usually inquired about.