Top down, bottom up

5 G

The concept of vision is absolutely insane as the processing of visual data in organic life is both top down and bottom up in terms of taking in photons, exciting electrons and abstracting the data set in order for the brain to process. What's even more astounding is the fact researchers are now discovering how life discerns what is real vs imaginary, something done in real time 24/7.

The checker shadow illusion. Although square A appears a darker shade of gray than square B,
in the image the two have exactly the same luminance.

Picture an apple, any apple.

As long as you don’t have aphantasia—the inability to visualize things in your mind’s eye—this suggestion triggers brain activity that’s surprisingly similar to what happens when you see a real-world apple with your eyes. Such neural overlap is economical because both cases require the brain’s visual system to carry out many of the same tasks. But it also raises a question: How does our brain tell reality and imagination apart?

Neuroscientists are now beginning to understand the brain circuit that handles this distinction. In a recent study in Neuron, researchers identified a brain region that generates what they call a “reality signal.” This signal is then evaluated by another region—one that, when it functions abnormally, has been linked to schizophrenia. Understanding this reality-monitoring circuit could help scientists understand, and possibly even treat, schizophrenia and other disorders that impair the ability to discern reality.

This “bottom-up” processing is only one part of the story. Information can also flow “top-down,” with higher-level cognition influencing perception. You can see this in action with ambiguous illusions, such as the Rubin vase or Yanny-Laurel audio clip, where your expectations determine what you see or hear. Or maybe you’ve been eagerly awaiting a visitor and keep thinking you hear a knock even when there’s no one at the door.

“Perception isn’t just a passive process,” says Philip Corlett, an associate professor of psychiatry at the Yale School of Medicine, who was not involved in the new study. “Stuff coming down from cognition sculpts what we perceive, and then stuff that comes up from perception might change what we believe.”

Rubin's Vase