Explanation of the „Rotating Snakes“-Illusion
This illusion comprises of the perception that the actually static circular areas around the black spots are rotating. This is only true for areas that one does not directly focus on and, interestingly, the circles also spin in different directions of rotation.
The „Rotating Snakes“-Illusion belongs to a group of visual phenomena that can be summarized as „physiological illusions“, i.e. illusions that are to be attributed to effects of excessive stimulation of a specific type (brightness, colour, size, position, tilt or movement). More concretely, the illusion is based on a variant of the „peripheral drift“, which can be defined as „an anomalous motion illusion that can be observed in peripheral vision“.
One prominent explanation building up on the human perceptual system is the peripheral-spatiotemporal-integration hypothesis (originally referring to patches with sawtooth gradients of luminance) which was proclaimed by Faubert & Herbert in 1999.
For the explanation of the hypothesis it is important to point out, that people nearly without exception perceive a sensation of movement in the direction from darker to lighter colours. The illusion also works for black and white versions, so only the existance of luminance gradients (differences in lightness), not the colours as such are of relevance.
For the explanation it is also important to bear in mind that the illusion works in several circumstances. One can look at different locations around the stimulus (i.e. move one´s gaze), or firmly look at something outside the stimulus and blink or the stimulus can be sequentially displayed at differing locations while one´s gaze is at one point. The latter aspect, by the way, shows that the perception of motion here is not triggered by efferent (=information originating from central nervous system) eye-movement signals.
Their hypothesis describes how, on the one hand, the illusion of movement starts in the first place and, on the other hand, how it is maintained. In a nutshell, the authors found three interacting factors or aspects that are important for the explanation of the phenomenon: First, transients are introduced due to eye movements or blinks, second, the processing latencies in the visual systems vary depending on luminance and third, aspects of spatiotemporal integration of the luminance signals in the periphery.
In more detail the ideas are as following. The latency of transmission of visual information varies with luminance. This means, the lighter the information the faster the transmission. Due to the differing lightness in the image (green is lighter than black, for example) there are differences in the arrival time of the information at units in the first layer. In the second layer, probably composed of units that respond to moving images, this differences in timing are integrated and result in motion signals (compare perception of movement). Further in the process the signals are integrated on even larger receptive fields. Given that in the areas of the retina that are away from the fovea the convergence of information is especially high, motion signals become stronger. This is why the illusion only works in peripheral vision.
For the illusion to be maintained the information then has to be refreshed by blinks, or, as more currenty studies indicate, micro-saccades.