How the Prism Goggles Beanbag Toss Works

Here we are, Part III of our debriefing about the science demonstrations that you saw at the 2015 ISC cocktail party. Here, let’s delve into why you struggled to hit the bucket so much during the prism goggles bean bag toss.Don’t forget to check out Part I and Part II!

Prism Goggles Bean Bag Toss

People often compare the brain to a computer – something that stores information and makes calculations. The human brain, however is far superior to a computer in many ways.  One great advantage of the brain over computers is the brain’s ability to adapt and change in response to its environment. This adaptation is facilitated by physical changes in how neurons in the brain connect to each other and communicate; new connections are made and some are lost, while some connections are strengthened and some are weakened.

The ability of the brain’s ability to change is called “plasticity.” Plasticity allows us to make new connections between neurons whenever we, for instance, learn something in school or learn a new skill.  Since learning takes a long time (think about learning multiplication tables!), people might think that plasticity only happens after days and days of practice. This is often true, but there are exceptions. For instance, when it is raining outside, and you hear that first thunderclap, it might jolt you out of your seat.  However, as the storm goes on, and thunder continues to clap, you get used to it and eventually you might be able to ignore it.  This is because your brain has learned that whatever is making that sound is not going to hurt you, and through plasticity, has rearranged your neural connections to block out the scariness factor when you hear thunder.  The prism goggles game demonstrates how your visual system can adapt in the same way.

How does plasticity work?

High-fiving is difficult with prism goggles on!

Have you ever tried to learn a new motor skill, like playing an instrument, drawing, or playing sports? We are always told to practice to improve our skills. But why? How does practice allow us to improve?

Our brains allow us to improve our ability to make a voluntary movement in two ways: 1) by correcting our movements if we make errors, which we call “feedback correction,” and (2) by anticipating potential obstacles that might prevent us from moving correctly, which we call “feedforward control.”

When we first learn a new piece of music, or a new dance move, we often make a lot of errors (like pressing the wrong keys on the piano or tripping over your feet while dancing) and our bodies do not move the way we want them to. How do we get better? You brain adjusts your movements using feedback from what actually happened (this is feedback correction).  In other words, the brain receives sensory information from the arms and legs, telling it what the limbs are doing, then your brain compares this information to what it originally wanted you to do. The brain then uses this comparison to change what it is telling the limbs – “a little more to the left, a little faster here, etc” – and the movement hopefully gets better and better the more you practice.

Feedback correction is a constant task, and it applies to every movement you make. When you are holding an object in our hands, let’s say an egg, your brain is constantly adjusting the amount of force you are using to hold the egg, to keep it from either dropping or getting crushed.

Feedforward control, by comparison, does not correct errors, but anticipates them. When a football quarterback is about to throw the ball, he has to take all sorts of factors into account before making that throw, such as how strong the wind is, the position of his body, and in what direction and how far the ball needs to be thrown. The quarterback receives information on all these parameters from sensory signals like visual cues, sounds and other sensations, and he corrects how he will throw the ball in anticipation of those perturbations. On the other end of the field, the person receiving that throw is also using feedforward control. He is anticipating the direction of the throw, the speed of the ball and the weight of the ball by watching how the ball moves, and he anticipates how much muscle force he will need to catch the ball without dropping it.

Think of these two processes in the context of bowling.  Your goal is to get a strike, right?  So, you line up in the middle of the lane and you try your best to roll your ball down the middle.  You release the ball, and things are going great.  But, just before it reaches the pins, the ball suddenly spins to the right and hits just the three pins on the right.  You end up with a 7.  In your mind, you’re thinking, “hmm…I tried to roll the ball down the middle, but it ended up curving to the right!  What do I do??”  What would you do on your next roll?

You’d probably start farther to the left to compensate for the spin you put on the ball.  And it works out!  You roll the ball down the left side of the lane, and at the last second, it curves to the right and hits the first and second pin.  You recover with a spare.  Here, you planned to throw the ball down the middle, but the ball went to the right.  That feedback told you that you needed to adjust.  You did, and you got a spare.  This is an example of feedback correction.

You go home, go to sleep, wake up, go to work, and your friends call up and tell you that they want a rematch.  You go the bowling alley again, and your first turn comes up.  You recall that the night before, your rolls were curving to the right.  So, in anticipation of that happening again you start off on the left side of the lane.  As expected, the ball curves right into the center and you get a strike.  The anticipation that led you to start on the left side of the lane is feedforward control.

The cerebellum is not only responsible for learning new motor movement (as you learned in Part I), but it is also the structure of the brain responsible for both feedback and feedforward control. A good way to think about the cerebellum is as a thermostat, in that the rest of the brain gives it orders on what temperature that it must maintain, and it is able to anticipate how to keep the temperature steady, while it is also able to correct for deviations in temperature.

The Prism Goggles Game

Prism Goggles fit over your eyes and contain two prisms that offset one’s visual field by 10 degrees (that is, everything you see will be shifted 10 degrees over). If you put the prism goggles on and try doing a voluntary movement such as throwing a ball, you will have a lot of trouble throwing it in the right direction because your visual field has been changed (just like in the bowling example, you are having trouble getting strikes because your rolls are all of the sudden curving to the right). Over time though, as the brain adapts to the change in visual information, and you will start to get better at catching the ball within just a few minutes, until it feels like you don’t even have the prism goggles on (like how you began rolling the ball from the left side of the lane and started getting strikes again). This improvement is possible because of the cerebellum engaging in both feedback correction and feedforward control. You will probably miss at first but then, the cerebellum updates the system so that next time, you throw at a better angle (feedback correction).  Subsequently your brain expects that you would need to throw a different angle relative to where your target is because it has learned what the prism goggles are doing (feedforward control). In one last application of feedback control, if your bag successfully lands in the bucket, the cerebellum keeps says, “Good, keep throwing at that angle!”

Will Beischel is a graduate student in psychology at the University of Michigan.