See Inside Scientific American Mind Volume 25, Issue 1

Filling in the _________

Your brain fills in all kinds of visual gaps
Women on the beach.

YOUR DIRTY LITTLE MIND: Notice the naked nubile young women behind the orange occlusion (as if you hadn't already!)?Turn to page 23 to see the photograph without the masking.
SCIENTIFIC AMERICAN MIND (orange overlay); GETTY IMAGES (girls on beach)

An amazing fact: most of what you see is a confabulation of your brain. Sounds hard to believe, right? After all, you are reading this page. So how inaccurate could your visual system be? Well, it is not that our eyes themselves are inaccurate … just that our brain makes stuff up based on the sparse data it gets from our eyes and then leads you down the garden path.

Let's start with your retina. A terrible imaging device, by any engineer's estimation. So full of design flaws that it is essentially proof positive against intelligent design. The neural circuits that make up the human retina, and the blood vessels that feed them, sit between the eye's lens and the photoreceptors—which lie against the back of the eye. Light must travel through all this semitransparent machinery before photoreceptors can transduce it into neural signals. How dumb! Shouldn't the photoreceptors lie just behind the lens to meet the incident light without obfuscation? Well, yes. The retinal layout of some animals such as squids and octopuses is indeed organized in this way. Yet because your brain conjures up most of what you see anyway, trivialities such as pristine optical transmission are not that important.

Case in point: you only have high-resolution vision in the very center of your eye—about 0.1 percent of your entire visual field. You are legally blind to objects more than a finger width or two from the center of your vision. But it doesn't feel that way. Instead you perceive your entire visual field as a high-resolution and perfectly formed image that is always in focus: a load of pure baloney provided compliments of your brain. In fact, very near the part of your retina where your photoreceptor density is highest lies a region devoid of sensory cells, in which you are completely blind. It is fascinating and counterintuitive, so let's discuss the illusion that makes you think you see in this blind spot, as well as several other algorithms used by your brain to achieve filling in.

Further Reading

A New Visual Illusion: Neonlike Color Spreading and Complementary Color Induction between Subjective Contours. H.F.J.M. van Tuijl in Acta Psychologica, Vol. 39, No. 6, pages 441–445; December 1975.

Perceptual Filling in of Artificially Induced Scotomas in Human Vision. V. S. Ramachandran and R. L. Gregory in Nature, Vol. 350, pages 699–702; April 25, 1991.

Dynamic Noise Backgrounds Facilitate Target Fading. L. Spillmann and A. Kurtenbach in Vision Research, Vol. 32, No. 10, pages 1941–1946; October 1992.

Neural Correlates of Transformational Apparent Motion. Peter U. Tse in Neuroimage, Vol. 31, No. 2, pages 766–773; June 2006.

Microsaccades Counteract Perceptual Filling-in. X. G. Troncoso, Stephen L. Macknik and Susana Martinez-Conde in Journal of Vision, Vol. 8, No. 14, pages 1–9; November 4, 2008.

Nature abhors a vacuum.
—François Rabelais, 1534
(after Aristotle)

Nature just loves a vacuum.
It's most of the universe
—Neil deGrasse Tyson, 2013

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