Your Brain Keeps a Hidden Stash of Everything You Don’t Consciously See. Scientists Just Found It.

What we think we perceive in the present moment is far from everything we actually perceive. We’re guided by more than just the things we’re aware of. Even someone in deep concentration who believes they’re focusing on nothing else may not realize that their brain is constantly shifting back and forth between the conscious information they’re focusing on and unconscious information that never reaches awareness.The human brain is thought to switch between processing conscious and unconscious information extremely fast, about 15 times per second, and only a fraction of the information it processes ever reaches conscious perception. Neuroscientists Janis Hesse and Doris Tsao of the University of California, Berkeley, wanted to learn why some neural activity becomes consciously available and some does not, so they studied what happens when the brain is faced with conflicting images. To do this, they needed a situation in which the physical stimulus stayed the same throughout the experiment while perception changed. With this setup, any differences in neural activity could be attributed to perception rather than to the stimulus itself.One way to create such a situation is to show a different image to each eye. In binocular rivalry, each eye is shown a conflicting image, and the observer can consciously perceive only one image at a time. In a related phenomenon known as binocular flash suppression, the image first shown to one eye is suddenly suppressed, and becomes invisible, when a second image flashes before the opposite eye and is perceived instead. Neurons that respond more strongly to one type of image fire more when that image is perceived, even though both images are physically present. What especially puzzled the researchers was why these same neurons, while responding to the perceived image, still hold onto information about the suppressed one.“Previous studies have not revealed a pure neural representation of conscious perception, i.e., one uncontaminated by suppressed representation and closely resembling the representation of a perceptually-equivalent unambiguous stimulus, in any recorded region of the brain during rivalry,” Hesse and Tsao wrote in a study recently posted to the preprint server bioRxiv, which is still awaiting peer review. In other words, scientists had never before found a part of the brain that holds a “clean” version of what we consciously see, free from the image we’re suppressing.To investigate, the researchers implanted electrodes in the brains of macaques, fitted the animals with 3D glasses (the kind with red and blue lenses that let each eye see a different image), and showed them a series of images. Because each eye saw a different image, binocular flash suppression separated the physical stimulus, including the suppressed image, from conscious perception. The macaques’ brains prioritized the newly flashed image and pushed the other one aside. Human subjects who were tested with the same technique confirmed that the second image took over their perception.The images were faces, so the researchers could track how face-selective neurons responded. In the inferotemporal cortex, which is critical for visual recognition, Tsao and Hesse found that the macaques’ brains kept alternating between states of perception and suppression, even though the stimulus stayed the same and only perception changed. Each state could be read out from the activity of these neurons, although the consciously perceived face was easier to detect than the suppressed one. The researchers were especially surprised that the second image, after briefly dominating the first, did not stay at the forefront of perception. After the initial flash, neural activity kept rapidly alternating between the perceived and suppressed images. This effect turned out to come from the same neurons switching which stimulus they represented. The same alternating pattern appeared in the prefrontal cortex, which is involved in higher-order cognition. So rather than being discarded, suppressed stimuli are temporally interleaved with everything else the brain processes.“Neural activity not only allows discriminating between two different suppressed stimuli but allows reconstruction of the suppressed stimulus with high fidelity,” the researchers said. “Thus, the suppressed stimulus is not represented as a weak residual trace but occupies the same high-dimensional face space as the conscious stimulus during complementary [times].”It turns out that what we consciously see really is only part of what the brain perceives.Elizabeth Rayne is a creature who writes. Her work has appeared in Popular Mechanics, Ars Technica, SYFY WIRE, Space.com, Live Science, Den of Geek, Forbidden Futures and Collective Tales. She lurks right outside New York City with her parrot, Lestat. When not writing, she can be found drawing, playing the piano or shapeshifting.