The optical illusion seen above makes the viewer feel as if they are falling into an expanding hole 鈥� and now we have a good explanation for why this happens.
Optical tricks can be caused by different elements of our visual and neural systems, even when the effects are similar. For example, the spinning circles of the Pinna-Brelstaff illusion are created by a communication delay between different regions of the brain that process vision. Meanwhile, another spinning circle illusion, Isia Leviant鈥檚 painting Enigma, has its origins much earlier in the vision process, being created by constant tiny, involuntary jerks of the eyes known as microsaccades.
To investigate whether the expanding hole illusion begins in the eye, rather than the brain, and at Flinders University in Adelaide, Australia, created a computer model of cells in the retina. Specifically, they looked at ganglion cells, which detect contrast and help adapt what we are seeing by lowering the brightness of very bright regions or increasing it for very dark ones. Incidentally, these ganglion cells are in part why smartphone pictures of the moon often disappoint 鈥� while our eyes can adapt to a great deal of dynamic range, digital cameras struggle.
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Ganglion cells work by signalling to the brain that a certain region is dark or light, but they also send signals to neighbouring cells. The model suggests it is this effect that creates the expanding hole illusion: certain ganglions perceive the dark centre of the image, and they trigger nearby cells to report a darkness even when they perceive an area just outside it, giving the illusion that the dark centre is larger than it actually is.
In addition, different types of ganglion cells are affected by smaller or larger numbers of neighbours, with smaller ganglion cells lying towards the centre of the retina and thus the centre of the illusion. The result is numerous confusing messages being sent to the brain, tricking us into perceiving change and motion where there is none.
鈥淭he neighbouring cells are somehow contributing to perception of what we see,鈥� says Nematzadeh. 鈥淚t鈥檚 changing all of the time.鈥�
at the University of Exeter, UK, says previous explanations for how the illusion works are 鈥渧ery high level and arm-wavey鈥� and involve the brain recognising the black centre as a physical hole, triggering mechanisms that have evolved to help us better understand 3D scenes, but which end up causing a disorienting effect when viewing a 2D pattern. He says the new explanation is simpler to understand, simpler to test and could widen our understanding of vision.
鈥淸More complex explanations] might be true 鈥� you can鈥檛 disprove them. But if you come up with an explanation that relies on fairly early visual processing stuff in your brain, then that鈥檚 鈥� for me 鈥� more useful,鈥� he says.
Nematzadeh hopes the computer model can be applied to investigating the origins of other optical illusions, while Troscianko thinks it could have wider applications: 鈥淚 might be able to use it to understand the patterns that we see in nature, which makes me very excited. Zebra stripes and butterfly wing patterns, and all of these kinds of things that are often very poorly understood.鈥�
arXiv
