(Note: This is part of a series of posts I’m writing in conjunction with my new book, The Tale of the Dueling Neurosurgeons.)
Guinea pigs are practically synonymous with experiments. Lab rats have become the workhorses of modern medicine. Genetics owes a huge debt to the humble fruit fly. There’s almost no branch of the life sciences, in fact, that hasn’t leaned heavily on one animal or another. Even psychologists—who study the human mind in all its uniqueness—have employed chimpanzees in studies, teaching them sign language or raising them as “children” to see how they handle suburbia. (Not well...)
The cat has received far less fanfare as a model organism, possibly because we don’t like to think about using house pets in experiments. And truth be told, reading about some of the experiments that cats have endured can stir up real discomfort. But it’s at least some solace that Felis domesticus has played a crucial role in several triumphs of twentieth-century neuroscience, and I’ve decided to honor their sacrifice with a trio of posts. Here goes.
The first breakthrough, by David Hubel and Torsten Wiesel in the 1950s, involved sight. Hubel and Wiesel were studying a part of the brain known as the primary visual cortex (in the very back of the brain, near the bump on your noggin). In particular they wanted to know how neurons there responded to different shapes, and for whatever reason—possibly because cats have large, well-developed vision centers—they chose to work on cats. Frankly, the experiments conjure up thoughts of A Clockwork Orange: they strapped each cat down in a chair with its eyelids propped open and forced it to stare at a projection screen. (Thankfully, the cats were anesthetized and felt no pain.) Wires that measured neuronal activity ran into the cat’s brain, and Hubel and Wiesel projected images onto the screen to see what got the neurons excited.
At first, nothing seemed to excite the neurons: they simply wouldn’t fire, and the duo wasted months on fruitless experiments. Finally, though, after a slide got jammed in the projector, one neuron went off like a machine gun: rat-a-tat-tat-tat. (This was a total mistake—and one of the best examples of serendipity in science history. You can read the full, hilarious story here.) After some desperate fiddling, Hubel and Wiesel realized that the neuron was responding to the sharp shadow that the edge of the slide was making on the screen. This neuron, in other words, dug lines.
Follow-up work revealed that this was in fact a general principle of vision neurons: no matter what shape our eyes are looking at—a spiral nautilus, the curve of a hip—when the input reaches our primary visual cortex, the neurons there determinedly break that image down into line segments. It’s as if everything we see around us is stick figures. Before these cats, no one had even suspected this possibility.
These clockwork kitties revealed something else critical as well: that we see moving images much more easily than still images. Why? Because, evolutionarily speaking, it’s probably more critical for animals like cats to spot moving things (dogs, mice, falling trees) than static things, which can wait. In fact, animal vision—including human vision—is so biased toward movement that we don’t technically see stationary objects at all. To see something stationary, our brains have to scribble our eyes very subtly over its surface so that it appears to move, at least a little. (Experiments by later scientists, who built on Hubel and Wiesel’s work, have proved that if you artificially stabilize an image on the retina with a combination of special contact lenses and microelectronics, the image vanishes completely.)
Overall, some observers claimed that neuroscientists learned more about vision during a few years of work by Hubel and Wiesel than in the previous two centuries, and they won a richly deserved Nobel Prize in 1981. The duo wasn’t done, either. The next post in this series explores how other, even more put-upon cats helped reveal how our brains learn to see in the first place during infancy.
(Coming up in part II: Kittens, stripes, and “critical windows” in brain development.)