When I ride in a car on even moderately rough roads, the ‘image’ that I see doesn’t bounce. The car bounce would have to be quite bad to give me the feeling the picture was jittering.
If I had a video camera sitting on the seat, the recorded image would bounce all over the place.
What’s different about me that the image my brian receives stays still? I know the seat cushions things a little but that’s not nearly enough. Is it a ‘physical’ thing; somehow my body and eyeballs conspire to automatically adjust to keep a still focus on my retina? Or does my brain do some sort of post-processing to remove the jitter???
I believe I once tried (as a passenger) resting my chin on the dashboard and IIRC things did bounce quite a bit, so I am guessing I have some sort of physical de-bouncer, but that seems quitre impressive.
Your brain is used to processing unstable images from your eyes. Even if you’re standing still and looking at, say, a sunset on a calm lake, your eyes are constantly moving around a few times a second (in saccades ).
The brain has a mechanism to filter out the motion and synthesize a coherent image from all this (filling in the blind spot in the process), and apparently that’s enough to compensate for ordinary vehicle vibrations.
The latest science fiction series from Robert J. Sawyer, his WWW trilogy of Wake, Watch, and Wonder, discusses vision and saccades a lot. Because only the fovea of the eye is the only part with really high-resolution vision, and it’s so small that it only covers a tiny area of the visual field (about twice the width of your thumbnail held at arm’s length), the eyes have to flit around and aim the high-resolution patch at different parts of the visual field several times a second, and the brain incorporates this data into a stable perception.
So, presumably, if your actual head isn’t vibrating too much, the vibrations aren’t any more significant than your ordinary eye movements, and the “confabulation across saccades” phenomenon takes care of it.
I was hoping to hear more about the “de-bouncer” and how we stabilize our head.
Having fooled around with recording video while mountain biking, I have found the difference in image shakea and movement between a bike mount and a helmet mount simply amazing. The body seems to do a lot of seemingly autonomous corrections in an attempt to keep the head traveling in a smooth straight line.
There is a system which automatically compensates for any movement of the head (can’t remember the name of it now). It utilizes the balance mechanism within the ear and can directly trigger the muscles of the eye.
Try to slowly move your head from left to right, but with your eyes remaining in the same position relative to your head (so your viewpoint also slowly sweeps around left to right).
You’ll find this is actually quite difficult to do. It’s much more natural to either stick on a single object and compensate for head movement, or “saccade” directly to something else of interest.
Plus your neck, and the weight of your head provides stability. Try leaning your head against the inside of the window as the car moves, and the quality of vision deteriorates quite considerably.
I am just guessing, it is because when you are driving, you are keeping a sharp eye on the road, and you would have been used to looking ahead, like when you are going on top of a steep hill or coming down from a steep hill, then your eyes automatically look for the road when it is not visible right, so when your car bounces, your eyes are on the road unconsciously, i mean, if you take a camera and record your eye movements when you bounce, you will notice that your eye looks down at the road unconsciously.
You’re thinking of the inner ear. It’s full of fluid, and used for balance too. When you move your head the fluid in your inner ear has to be accelerated to the new velocity of your head. Tiny hairs IIRC detect the fluid sloshing around as a result. Then they send signals down your nerves to your brain where your brain uses this data figure out your position relative to down and keep you balanced.
Also there’s nerves that are directly connected from the inner ear to the eye muscles, entirely bypassing the brain. That send little movement corrections to your eyes so that if your head moves your eyes compensate and maintain your gaze in it’s current direction.
That’s why if you spin around in a circle fast enough you get dizzy, and balancing becomes hard. By spinning round you get the fluid in your ears sloshing like crazy sending all these weird balance singles to your brain that don’t make sense.
With your inner ear disabled you’re left balancing manually.
A long time ago your ancestors had to chase down food, and escape from predictors. Such chases were high energy, and rapidly changing direction, bouncing through the water and air. Further they weren’t too bright, but they still needed to see where they going without a bouncing distraction.
Because of that your ancestors evolved organs to automatically correct for changes in movement.
Thank you, but I have studied the vestibular system.
I was trying to think of what the name of the specific reflex was. Probably I should have guessed it: it’s the Vestibulo-ocular reflex.
The really interesting thing about saccades is that, by tracking a person’s eye movement while they’re looking at a scene, you can actually reconstruct a crude image of what they’re seeing, with the parts that are most interesting being most clearly resolved.
Here (PDF) is a decent paper on vision and eye movement.
Related idea: I can’t find it right now, but I also remember reading that the way our head sits on the end of our neck is adjusted for shock absorption and tracking even while running at high speed, presumably to enable us to see prey effectively. Apes have trouble keeping their heads steady when they’re walking bipedally and have to work harder at it, while ours kind of “floats” on the top of our spinal column and requires relatively less muscular effort and adjustment.
