While people commonly think of illusions as misperceptions, they are actually examples of how sophisticated how our perceptual systems are. Without conscious thought, we are deftly efficient at extrapolating sensory cues, which in turn allows us to take huge neural processing shortcuts. Interestingly, getting AI to be able to also see illusions, just like we do, would be a key Turing Test step in achieving Artificial General Vision Intelligence. Here are four of the most fascinating illusions born out of neuroscience discoveries investigating how we visually perceive reality.

1. The Leviant Traffic Illusion

This scientifically crafted image is called 'The Leviant Traffic Illusion'. If you look closely at the center you will likely perceive rapid motion in the circular sections.

Although there are specialized regions of the visual cortex dedicated to processing perceived motion, regions which process static contrasting light patterns can also come into play. This effect occurs because the rings are 'isoluminant', that is, they match the mean brightness of the black and white rays. If the luminance level varies from the mid-range, the motion effect quickly fades away.

A particularly interesting thing about this illusion is that the perceived motion is multistable, allowing it to appear as a global rotation either clockwise or counterclockwise.

It can also be seen as local bidirectional motion, with the alternating circles moving in opposite directions.

These alternating effects represent your brain working hard to leverage subtle visual cues - what would usually be meaningful information about real motion in the world around you.

2. Simultaneous Brightness Contrast

This very simple image with two circles exactly the same shade of grey mystified vision scientists for over a century. It reveals our capacity to process what's known as 'simultaneous brightness contrast', which Chinese ceramic painters discovered and worked into their craft over 800 years ago.

This effect was long believed to be a high-level brain process tapping into past learning experiences of how the world works. That was until recently, when MIT researchers studied blind children in India and found that they were susceptible to this illusion the moment their sight was initiated after surgery.

Through further experiments they revealed that this innate brightness estimation actually takes place before visual information reaches the brain’s visual cortex (likely pre-processed via retinal neurons). The Müller-Lyer and Ponzo illusions were then also found to have the same underlying mechanisms.

These discovery was made possible by 'Project Prakash', whose mission is to save children from preventable blindness while answering deep scientific questions.

3. Chaz Firestone Balls

This is a particularly strong illusory effect, making us see different colored balls. This 3D illusion was created by David Novick, Professor of Engineering Education and Leadership at the University of Texas. In his own words,

“A three-colour confetti illusion with spheres, which appear to be yellowish, reddish, and purpleish, but in fact have exactly the same light-brown base colour (RGB 255,188,144). Shrinking the image increases the effect.”‍

4. Expanding Black Hole

This perceptual illusion of an expanding black hole (spoiler - it's a static image), was used to probe a new discovery in neuroscience this summer.

The illusory effect isn't simply a perceptual interpretation, it literally evokes a biological response - your pupils actually dilate to let in more light (in 86% of people). This also imparts an impression of visual optic flow - like the feeling of moving into a tunnel.

This collaborative research by scientists in Oslo and Japan, shows that the pupillary light reflex can depend on the perceived environment, rather than physical reality. Which is surprising, because our pupils do not change aperture while dreaming, regardless of the dream.

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