Monday, December 2, 2013

Or "How the Brain Handles Visual Information About Movement"
  • "Why don't things look shaky when I jump or run like when I try the same thing with a video camera?"
  • "Why can I track something moving, but I can't just move my eyes like that if there's nothing to watch?"
  • "Why do I get sick when I play video games for too long or try to read in the car?"
Well, given that the answers to these questions are all related to one another and have to do with how the brain processes visual information, I'm going to go ahead and combine them into a single post.


Image Stabilization & the Vestibulo-ocular Reflex
Why don't things look shaky when I jump or run like when I try the same thing with a video camera?

The world around us doesn't look shaky. We can move around as much and as violently as we like and what we see will still remain stable enough for us to see and process the things around us. If you were to record those same type of movements with a standard videocamera, however, the resulting footage is often extremely shaky and it can be difficult to figure out what's happening.

Exhibit A - Cloverfield

Filmmakers avoid shaky footage by using mounts which isolate camera movement from that of the operator by mechanically counter-acting operator movement. Which, coincidentally, is what your body does as well. Don't just thank your eyes, though, thank the vestibular system in your inner ear.

The vestibulo-ocular reflex (VOR) is responsible for keeping images steady on your retina, and it works by taking information provided by the inner-ear about your head's location and orientation and using it to counteract head movement by moving the eyes in the opposite location.


This works because the semicircular canals in your ears contains a viscous fluid that lags as your head rotates (imagine turning a cup of tea and noticing that the tea leaves seem to stay in the same spot as you turn the cup itself). Special hair cells detect the movement of the liquid, and use this to provide the brain with information about your orientation and movement in space.

Each canal roughly corresponds to a dimension in which the head can move, (pitch, roll, and yaw) and because the brain pays attention to the movement of the fluid in these canals, and not its specific location or orientation, the VOR can work even in the absence of gravity.

So what is responsible for keeping track of which way's up? That would be the otolith organs, a system of sensory cells and fluid-filled chambers also in the inner ear. Calcium carbonate crystals, called otoliths, are suspended in the aforementioned liquid in these chambers, and it's their inertia that's responsible for helping us track where our heads are.

When we're upright, the weight of the otoliths and fluid presses down on the sensory hair cell receptors and keeps them from moving around, but when we're tilted, the pull of gravity and the movement of fluid within the ear tells our central nervous system how we're oriented.

Visual Fixation and Saccades
Why can I track something moving, but I can't just move my eyes like that if there's nothing to watch?

Under normal conditions, your eyes are actually moving around all the time; scanning, stopping, and refocusing as your brain takes in the world around you. These rapid movements are called saccades, and we literally have no conscious control over them.

This is why it's impossible to move your eyes completely steadily in a single movement unless have some sort of target to track or upon which we can focus. The brain will actually readjust the size and speed of saccades in order to ensure that we are seeing things properly.


So why do you see the world as a single, seamless "frame" even though your eyes are jumping around all the time? Transsaccadic memory; a neural process that takes what you're actually seeing and recombines it into what you "see" so that everything looks stable. Your perception of reality literally does have a big of lag, not just due to neuronal signaling speed, but as a result of the amount of time taken to process incoming information as well. (And this doesn't just apply to vision either.)

You know when you look at a clock and it seems like it's stopped? This is why.

When we're tracking a moving object, however, it's a completely different process called smooth pursuit. Most people can smoothly track objects moving horizontally and vertically-downwards at speeds up to 30° of our field of view per second; any faster than that and our eyes start "skipping" forward to catch up ("catch-up saccades").

Smooth pursuit is a two-step process in which the first step is to "catch" the object you're going to track. It lasts about ~100 milliseconds, during which your eyes will still dart about a bit until they can fix on the target's velocity and direction of movement. The second stage consists of the automatic correction and control of eye movement to closely follow whatever we're looking at. Stage two lasts until the thing we're tracking stops moving in that particular direction (and we have to catch it again), or we get bored and stop watching it.


Motion and Simulation Sickness
Why do I get sick when I play video games for too long, or try to read in the car?

Motion sickness can arise in any situation where what you see is inconsistent with what you feel. The going hypothesis at the moment is that motion sickness is a side-effect of an evolutionary defense mechanism against neurotoxins, as the brain's area postrema is responsible both for resolving conflicts between what we see and what we feel, and for making us throw up if we think we've been poisoned. (There are, however, some alternative theories.)

Motion sickness occurs under three conditions:

1. Motion is felt but not seen
These are situations in which your vestibular system detects that your body is in motion, yet your eyes only display a single, relatively steady scene (e.g. text on a page while a car is driving on a bumpy road, the interior of a boat or plane when the vehicle is actually moving up and down or banking, etc.).

2. Motion is seen but not felt
Visually induced motion sickness (VIMS) occurs when what you see is inconsistent with what you feel. Historically, it's been reported most often as a result of watching films or video with unsteady shots, but with the advent of space flight, the rising popularity of video games, and the growth of virtual reality technologies and applications (In the form of "Simulation Sickness"), we are seeing it in more places, more often.

3. The visual and vestibular systems do not agree
This final type is rare, as it requires both systems to report contradictory movement, but has been observed in people exposed to artificial gravity, and may occur in situations such as when one is driving down a bumpy road (i.e. moving slowly forward but rapidly up and down).

In each of the situations described above, the brain is being told two different things from two different locations, and comes to the only logical conclusion it can: It's clearly a hallucination, which must have been caused by something we ate, therefore we should throw it back up so we don't die.

E-GAD, Brain. Brilliant!

So, given the fact that we are not, in fact, ingesting significant amounts of neurotoxic substances on a regular basis (at least, I'm not, I have no idea what you people do in your spare time) and that motion sickness has an actual, detrimental affect on people's abilities to travel, enjoy certain forms of entertainment, and to do their actual jobs, there's a real incentive to find ways in which motion sickness can be treated and prevented in the real real world and in virtual spaces as well.

At present, the most common treatments are the use of medication, or behavioral suggestions like "Look out the window", but this does very little to actually eliminate motion sickness or prevent the problem from popping up in the first place. Luckily, we've got organizations like NASA on the case (which has come up with a neat trick involving stobe lights and glasses) and game developers working in virtual reality who are able to experiment with how things like field of view affect motion sickness, and develop guidelines that help users become more accustomed to a VR environment.
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