Name: Vlog #55 - How We Sense Movement
Uploaded: 2025-05-23T07:34:13.000Z
Duration: 5 min 11 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

I love superheroes, and I love good movies.

Put those two things together, and I go fucking bananas.

One of my favorite superhero movies is, of course, Sam Raimi's original Spider-Man, not only because it's brilliant storytelling, and it is, but because that movie really conveys the grace, the almost balletic movement of our hero through the New York City skyline.

What gives Spider-Man his agility is a combination of heightened reflexes and a precognitive spider sense.

Add to that super strength and some handy webs, and you have the best time this side of Superman.

Indeed, I've spent countless hours walking through cities, staring wistfully up at buildings as the other me, the Spider-Man me, swings here and there in deep parabolic arcs, twisting into triple flips, landing, I don't know, on the pole of some American flag atop the Empire State Building.

Though I've been known to loiter around the spider exhibit at the Natural History Museum, it's a real human tragedy that we don't get to have superpowers.

I mean, the best we can do is free running.

And by we, I, of course, mean other people.

And yet, there are times when walking through the densely populated spaces of Manhattan that I've marveled at my ability to move through a crowd and not crash into every other person.

Sometimes I can actually feel myself perceiving the speed and crisscrossing directionality of all the people in front of me.

And when I get to the other side, I feel like a regular Han Solo.

Why aren't we a mess in airports or on street corners?

When we look out onto a moving crowd, what we're actually seeing, beyond busy people talking on their cell phones, is subtle increases and decreases in the intensity of light reflecting off objects.

When light intensity increases, an object is moving towards you, usually.

When it decreases, it's moving farther away.

But the retina is also sensing directionality.

Neurons fire and encode as they sense edges moving over backgrounds in time.

And we have separate groups of neurons dedicated to perceiving things moving left or right or clockwise or counterclockwise.

To perceive your own sense of directionality in action, fix your eyes on the center of this animation.

What I'm about to show you is what's called the waterfall illusion or motion after effect.

Stare at a waterfall for long enough, and the neuron group, coding its downward motion, will, for lack of a better term, get bored and adapt to make that the norm.

Turn your head away to, say, a stationary rock, and the rock will seem to move upward in the opposite direction of the falls.

That's because the stationary thing, relative to the adapted normal, is actually up.

Or instead of circles moving inward, this pattern may look like it's moving outward.

Add to this, our eyes, on average separated by 6 centimeters, are remarkably accurate at triangulating an object as long as it's within 100 meters.

If it's beyond 100 meters, we have to rely on other tricks like perspective or motion parallax.

Taking all this information, the brain can calculate the velocity of a variety of moving things at once.

The retina, which we should never forget is the only visible part of the brain, carries information via the optic nerve to the lateral geniculate nucleus and into the visual cortex in the occipital lobe of the brain.

From here, the data splinters down two paths.

The two-stream hypothesis, made famous by David Milner and Melvin Goodale in the early 90s, holds that some information goes down the ventral stream to the temporal lobe for object identification and recognition so that we can figure out what it is that we're looking at.

The rest goes down the dorsal stream to the parietal lobe, where we process the object's spatial location relative to the viewer.

In other words, there's two streams, one for what and one for where.

What was merely color, edge, and brightness in the occipital lobe is a mapped spatial awareness by the time it reaches the parietal.

Now, our knowledge of how we process movement is only as good as our working knowledge of the brain, a grasp which is, to say the least, incomplete.

Better and finer technologies will give way to a better and finer understanding of the brain.

But really, understanding the brain is only a function of our desire to understand it or to treat certain kinds of brain damage.

What's incredible is that when functioning normally, we don't have to think about each and every complicated step from perception to cognition to action.

We can, like certain heroic wall crawlers, navigate our environment with a natural grace and a kind of balletic dance with the fast and chaotic movement of everything else.

Whatever life holds in store for me, I will never forget these words.

With great power comes great responsibility.