Every few decades there’s a major new neuroscience discovery that changes everything. I’m not talking about your garden variety discovery. Those happen frequently (this is the golden age of neuroscience after all). But no, what I’m talking about are the holy-moly, scales-falling-from-your-eyes, time-to-rewrite-the-textbooks, game-changing discoveries. Well one was reported in this last month—simultaneously by two separate labs—and it redefines the primary organizational principle of the visual system in the cortex of the brain. This may sound technical, but it concerns how we see light and dark, and the perception of contrast. Since all sensation functions at the pleasure of contrast, these new discoveries impact neuroscience and psychology as a whole. I’ll explain below.
The old way of thinking about how the wiring of the visual cortex was organized orbited around the concept of visual-edge orientation. David Hubel (my old mentor) and Torsten Wiesel (my current fellow Brooklynite)—who shared the Nobel Prize in Physiology or Medicine in 1981—arguably made the first major breakthrough concerning how information was organized in the cortex versus earlier stages of visual processing. Before their discovery, the retina (and the whole visual system) was thought to be a kind of neural camera that communicated its image into the brain. The optic nerves connect the eyes’ retinas to the thalamus at the center of the brain—and then the thalamus connects to the visual cortex at the back of the brain through a neural information superhighway called the optic radiations. Scientists knew, even way back then, that neurons at a given point of the visual scene lie physically next to the neuron that sees the neighboring piece of the visual scene. The discovery of this so called retinotopic map in the primary visual cortex (by Talbot and Marshall) was of course important, but because it matched the retinotopic mapping of the retina and thalamus, it didn’t constitute a new way of thinking. It wasn’t a game-changing discovery.
Hubel & Wiesel discovered that the cortex did a fundamentally different thing than the retina and the thalamus. The cortex interpreted the meaning of the information, as opposed to just continuing to pass the image along from level to level. It broke the image down into features that were critical to object recognition and survival. The first step in their amazing research journey was their discovery that neurons in the primary visual cortex were not only retinotopic, but also orientation-specific—they preferred specific edge-tilts in the visual scene. This showed that the cortex of the brain—the brain part most associated with higher mammals and especially with human intelligence—could analyze the visual scene and pull out specific features from the images transmitted by the eyes. This started a revolution in brain science—which lasted for decades and inspired thousands of young scientists like me—to search the other parts of cortex to determine how sensory and cognitive features were processed in the different regions. Lots of other things have happened since, but to my mind, nothing of equal impact has occurred to change the way we look at the principles of cortical organization. Until now.
Now, two separate labs, Jose-Manuel Alonso’s lab at State University of New York School of Optometry*, and David Fitzpatrick’s lab at the Max Planck Florida Institute of Neuroscience, have made a discovery that I believe to be of equal importance as Hubel & Wiesel’s fundamental discovery. They found that the way that light and dark visual features are processed in the cortex is fundamentally responsible for how edge-orientation selectivity is derived. They have in essence shown that Hubel & Wiesel’s discovery of orientation-tuning is not the primary job of cortex. Instead, the cortex’s primary organizational principle is to create a giant switchboard that pits light vs dark at every position in the visual scene. From this new organizational principle everything else follows: including orientation selectivity, and selectivity to direction-of-motion. It’s all predicted by the way the retina eventually wires into cortex. Wow. Other sensory systems may be similarly wired so that the contrast features of sound, touch, taste and smell, are also organized by the antagonistic push-pull of their fundamental properties. Right? I don’t know: nobody does. But now we have a new conceptual hammer with which to find out. It’s an exciting time for brain mapping and you can bet that my peeps are lining up to start driving some nails.
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*Full disclosure: I am a professor at SUNY Downstate Medical Center—a different institution within the same state university system.