(Note: This is part of a series of posts I’m writing in conjunction with my new book, The Tale of the Dueling Neurosurgeons.)

As explained in a prior post, neuroscientists made the first real strides in understanding how vision works inside the brain by studying cats. (In short, no matter what we’re looking at, neurons in the primary visual cortex break the image down into line segments, as if the entire world consisted of stick figures.) But the neuroscientists responsible for this work, David Hubel and Torsten Wiesel, also used cats to study brain development, especially during infancy.

The results was startling. Cats raised in one environment were blind—literally blind—to any lines running the “other” way. Cats raised in a horizontal world, for instance, could see the seats of chairs just fine and would jump up onto them to nap. But they couldn’t see the chair’s legs at all and were constantly banging into them. The vertical-world cats had the opposite problem. They weaved around the chair legs like champs but could never find a cozy spot to snooze.

These experiments provided some of the first and best evidence for the existence of “critical windows” in brain development. The basic idea is that the brain, which is plastic when young, must be exposed to certain sights early in life or it will remain blind to those sights forever. In this case, because the vertical-world and horizontal-world cats never saw any differently oriented lines during their critical windows, their brains ended up dedicating all their vision neurons to the one orientation and neglecting the other.

Something similar can happen with human beings. No humans have ever been raised in such stark surroundings, thankfully. But those of us raised in modern cities tend to notice horizontal and vertical lines more quickly than lines at other orientations. In contrast, people raised in nomadic tribes do a better job noticing lines skewed at intermediate angles, since Mother Nature tends to work with a wider array of lines than most architects. (You wonder, though, where children raised in a city by Gaudi would fall!)

The idea of critical windows extends beyond just vision, of course: almost every system in the brain has a critical window when it needs to experience certain stimuli or it won’t get wired up properly. The most obvious example is language: if you don’t learn a language early on, it’s nigh impossible to become truly fluent. To be sure, different brain systems have different critical windows that last remain open for different lengths of time. But in some fundamental way, we’re all like Hubel and Wiesel’s kittens—able to process some things without effort, but crashing into those darn chair legs that our brains, through no fault of our own, simply can’t process.

(Coming up in part III: “left brain” versus “ ,” and the Nobel Prize for cats.)