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Former CIA Clandestine Operative
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Chris Hadfield
Retired Canadian Astronaut & Author
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This optical illusion plays tricks on your brain

Check out these mysterious optical illusions that affect our visual perception.

If you look at the cross in the middle for at least 10 seconds, colorful spots on the sides will begin to fade away, courtesy of the Troxler effect.

  • Troxler's effect or "fading" causes images to disappear from your field of vision.
  • Scientists don't have a full understanding yet of how this works.
  • The effect is linked to the way neurons are adapted by the visual system.


Why do we some times see things that aren't there? Delighted by stories of vengeful ghosts and spirits, we largely assume there's a world outside of our regular field of vision. Whether it's inhabited is certainly debatable but the curious psychological condition called Troxler's Fading or Troxler's Effect may explain one optical illusion.

This effect is named after a Swiss physician and philosopher Ignaz Paul Vital Troxler (1780-1866), who discovered it in 1804. It is essentially a trick of perception that describes what happens if you fix your gaze upon a single point in the visual field. It doesn't even have to be for a long time – 10 seconds would do. That can make images and colors disappear from your peripheral vision.

THE LILAC CHASER

"Lilac chaser".

Look at the black cross at the center of the image and the spots in this "lilac chaser" illusion will fade away in a few seconds. A grey background and the cross will remain unless you are among those who will also see a moving blue-green spot. You might even notie a bunch of green spots when you move your eyes away after a while.

HOW DOES IT WORK?

Research indicates the effect is related to how neurons important for perceiving stimuli are adapted by the visual system. Unchanging stimuli will eventually disappear from our awareness while our mind will fill the areas where they used to be with the background information (or color). A "sensory fading" or "filling-in" is linked to saccades – involuntary eye movements that happen even when the gaze appears settled. If we fixate on a point, an unmoving image or scene would fade from view in a few seconds thanks to the "local neural adaptation of the rods, cones and ganglion cells in the retina," explains the Illusions Index.

The effect is made stronger if the stimulus image is low contrast or blurred.

While studies showed the effect doesn't only occur in the eyes but partially in the brain, there's not yet a definitive explanation for everything involved in this unusual visual phenomenon.

Another example image of the Troxler effect. Look at the center of the image for about 10 seconds.

And another fun example:

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Our ‘little brain’ turns out to be pretty big

The multifaceted cerebellum is large — it's just tightly folded.

Image source: Sereno, et al
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  • A powerful MRI combined with modeling software results in a totally new view of the human cerebellum.
  • The so-called 'little brain' is nearly 80% the size of the cerebral cortex when it's unfolded.
  • This part of the brain is associated with a lot of things, and a new virtual map is suitably chaotic and complex.

Just under our brain's cortex and close to our brain stem sits the cerebellum, also known as the "little brain." It's an organ many animals have, and we're still learning what it does in humans. It's long been thought to be involved in sensory input and motor control, but recent studies suggests it also plays a role in a lot of other things, including emotion, thought, and pain. After all, about half of the brain's neurons reside there. But it's so small. Except it's not, according to a new study from San Diego State University (SDSU) published in PNAS (Proceedings of the National Academy of Sciences).

A neural crêpe

A new imaging study led by psychology professor and cognitive neuroscientist Martin Sereno of the SDSU MRI Imaging Center reveals that the cerebellum is actually an intricately folded organ that has a surface area equal in size to 78 percent of the cerebral cortex. Sereno, a pioneer in MRI brain imaging, collaborated with other experts from the U.K., Canada, and the Netherlands.

So what does it look like? Unfolded, the cerebellum is reminiscent of a crêpe, according to Sereno, about four inches wide and three feet long.

The team didn't physically unfold a cerebellum in their research. Instead, they worked with brain scans from a 9.4 Tesla MRI machine, and virtually unfolded and mapped the organ. Custom software was developed for the project, based on the open-source FreeSurfer app developed by Sereno and others. Their model allowed the scientists to unpack the virtual cerebellum down to each individual fold, or "folia."

Study's cross-sections of a folded cerebellum

Image source: Sereno, et al.

A complicated map

Sereno tells SDSU NewsCenter that "Until now we only had crude models of what it looked like. We now have a complete map or surface representation of the cerebellum, much like cities, counties, and states."

That map is a bit surprising, too, in that regions associated with different functions are scattered across the organ in peculiar ways, unlike the cortex where it's all pretty orderly. "You get a little chunk of the lip, next to a chunk of the shoulder or face, like jumbled puzzle pieces," says Sereno. This may have to do with the fact that when the cerebellum is folded, its elements line up differently than they do when the organ is unfolded.

It seems the folded structure of the cerebellum is a configuration that facilitates access to information coming from places all over the body. Sereno says, "Now that we have the first high resolution base map of the human cerebellum, there are many possibilities for researchers to start filling in what is certain to be a complex quilt of inputs, from many different parts of the cerebral cortex in more detail than ever before."

This makes sense if the cerebellum is involved in highly complex, advanced cognitive functions, such as handling language or performing abstract reasoning as scientists suspect. "When you think of the cognition required to write a scientific paper or explain a concept," says Sereno, "you have to pull in information from many different sources. And that's just how the cerebellum is set up."

Bigger and bigger

The study also suggests that the large size of their virtual human cerebellum is likely to be related to the sheer number of tasks with which the organ is involved in the complex human brain. The macaque cerebellum that the team analyzed, for example, amounts to just 30 percent the size of the animal's cortex.

"The fact that [the cerebellum] has such a large surface area speaks to the evolution of distinctively human behaviors and cognition," says Sereno. "It has expanded so much that the folding patterns are very complex."

As the study says, "Rather than coordinating sensory signals to execute expert physical movements, parts of the cerebellum may have been extended in humans to help coordinate fictive 'conceptual movements,' such as rapidly mentally rearranging a movement plan — or, in the fullness of time, perhaps even a mathematical equation."

Sereno concludes, "The 'little brain' is quite the jack of all trades. Mapping the cerebellum will be an interesting new frontier for the next decade."

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