Much better than a camera

It is sensitive to light levels that vary 100 trillion times in intensity, from near darkness to the glare of a sunny beach; it comes with an autofocus system that works flawlessly for about four decades; and it even knows how to make local adjustments to contrast, so that no part of the image will be overexposed when another part is still dark. What's more, it delivers the result pre-processed, clean and filtered, with color correction, and ultra-high resolution in the most important part of the image. It even has a cover that closes automatically at the slightest risk of scratching.

What camera is this? Your retina, at the back of each eye. As if that weren't enough, it even comes with a built-in internal clock that adjusts the detector's sensitivity according to the time of day and the brightness of the environment.

Just as comparing the brain to a computer does not do justice to biology, which over time makes the brain increasingly attuned to what it actually does in each user, a camera has yet to be created that can match the pair we have in our heads (which, being a pair, also allows stereoscopic, 3D vision). This is hardly surprising: technically, the retina, formed by cells organized in ten layers, is part of the brain. It is true that it protrudes into the eye sockets during development, but it is still part of the brain, formed from the same primordium that gives rise to the hypothalamus, the structure that controls the body's physiology, including hormone production.

As part of the brain, the retina connects directly to it and also has a circuitry worthy of the cerebral cortex. Much more than mere detectors of a camera's sensor, the retina has a rich and intricate internal circuit that takes about 40 milliseconds to pre-process the information that arrives in the form of light. Only then is the result reported to both the visual cortex, which allows the formation of an image accessible to consciousness, and to the colliculi, in another part of the brain, which reposition the eyes at the slightest sign of movement, without the need to think about it.

Just as the normal state of the brain is alternately awake or asleep, depending on the combination of modulators that bathe it, the retina also receives histamine fibers from its sibling, the hypothalamus, the same fibers that keep the brain awake. All that remains now is for someone to discover that the retina not only covers itself with the eyelids at night, but also sleeps...

Extracted from Suzana Herculano-Houzel (2025) Neuroscience of Everyday Life, originally published in Folha de São Paulo in January 2018