New Animal Study Sheds Light ‘Visual Masking’ Phenomenon

New Animal Study Sheds Light ‘Visual Masking’ Phenomenon

A strange phenomenon called visual masking can reveal the timescale of perception, but the underlying mechanisms are not well understood.

The colored plots illustrate neural activity recorded in mouse visual cortex (V1); each row of tick marks represents the spikes of a different neuron; researchers can predict the target side from the neural activity with near perfect accuracy, animal subjects get many masked trials wrong due to how brain regions downstream of V1 process this information. Image credit: Gale et al.

The colored plots illustrate neural activity recorded in mouse visual cortex (V1); each row of tick marks represents the spikes of a different neuron; researchers can predict the target side from the neural activity with near perfect accuracy, animal subjects get many masked trials wrong due to how brain regions downstream of V1 process this information. Image credit: Gale et al.

Did you ever wish you could unsee something? It turns out your brain is capable of that.

Unfortunately, it’s a limited superpower: in visual masking, we don’t consciously perceive an image if another image is shown in quick succession.

But the timing of those images matters. The first image needs to flash on and off fairly quickly, and the second image needs to follow rapidly (on the order of 50 milliseconds), for the masking to work.

To be clear, while the first image doesn’t last in your field of vision for very long, it’s definitely long enough that you would be aware of it without the second image, or mask.

Scientists discovered this phenomenon in the 19th century, but why and how the human brain does this remains a mystery.

“This is an interesting observation, where what is present in the world is not accurately reflected in your perception,” said Dr. Shawn Olsen, a researcher at the Allen Institute.

“Like other visual illusions, we think that it tells us something about the way the visual system works and ultimately about the neural circuits that underlie visual awareness.”

In their new study, Dr. Olsen and colleagues delved into the science behind this odd optical illusio — and showed for the first time that it happens in mice too.

After training mice to report on what they saw, the researchers were also able to pinpoint a certain region of the brain that’s necessary for the visual masking illusion to work.

“Our study narrows down the parts of the brain responsible for awareness of the world around us,” said Dr. Christof Koch, also from the Allen Institute.

“What are the steps between the rain of photons on your retina and actual conscious perception of what you are seeing?”

When the rain of photons impinges on our retinas, the information takes a prescribed path from our eyeballs through several different regions of the brain, ending in higher processing areas of the cortex, the wrinkled outermost shell of the brain.

From previous studies of visual masking, scientists know that neurons in the retina and parts of the brain early in that pathway are activated even when a person is not aware that they’re seeing an image. In other words, your brain is seeing things without your knowledge.

To explore where unconscious sensation turns into conscious perception and action, the scientists first trained 16 mice to turn a tiny LEGO wheel toward the direction of a quickly flashed image in exchange for a treat if they chose the correct direction.

They then added a different masking image on both sides of the screen, directly following the target image.

With the addition of the mask, the animals could no longer do the task correctly — implying they were no longer aware of the original, target image.

Because visual masking had never been tested in mice before, the authors had to create the task for them, meaning the images and how they were shown differed from those used in previous human studies.

To confirm that the optical illusion they showed the rodents is relevant to us, they also tested it in 16 people.

Human perception (or lack thereof) and mouse perception of this specific visual masking illusion turned out to be very similar.

The researchers then used a special technique known as optogenetics that can quickly suppress the activity of cells or entire regions of the brain with a flash of light.

They aimed this suppression at the mouse primary visual cortex, known to be the first part of the cortex where visual information from the eyes enters the higher cortical areas of the brain.

By turning off the primary visual cortex at the instant the masking image was shown, but after the target image, they were able to block visual masking entirely — the mice went back to correctly pinpointing the location of the first image even though a masking image was present.

That result means that conscious perception is happening either in the visual cortex or in higher areas of the cortex downstream of it.

“That fits with the general sentiment in the field that the cortex is the seat of conscious perception in mammals, including us,” Dr. Koch said.

Although the study narrowed down the region responsible for conscious perception to the cortex, there are still many areas of the cortex that could be involved.

Further studies would need to silence these other regions to test their effect on the visual masking task.

“We’re starting to put some bounds on where masking is occurring,” Dr. Olsen said.

“We think this is a good paradigm to follow up on to track down the other regions that are listening to the primary visual cortex and essentially fusing the streams of target and mask information in the brain.”

The findings were published in the journal Nature Neuroscience.

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S.D. Gale et al. Backward masking in mice requires visual cortex. Nat Neurosci, published online November 13, 2023; doi: 10.1038/s41593-023-01488-0

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