Wednesday, May 12, 2010

I bet you can't stop your eyes from moving.

I’m fascinated by the control we have over our eye movements, because we first use our eyes to explore the world around us. Our control over what our eyes do largely commands how we complete complex behaviours like driving a car.

Some of our eye movements are automatic and made reflexively in response to some visual event. Imagine, for example, standing on a train platform and looking across the tracks to a friend on a platform opposite you. If a train flies past between you and your friend, your eyes can’t help but to flick back and forth, following parts of the carriages. This is an eye reflex called optokinetic nystagmus, or OKN for short.

To induce OKN in the lab, I get people to sit in a pitch black room and stare at the centre of a computer monitor while I track their eyes with the eye tracker. I tell participants that their goal is to stare directly at the centre of the monitor. But then I display 800 little white dots on the screen, all moving at the same speed from left to right at a rate of a few centimetres per second. Because there is nothing in the centre of the monitor for participants to keep their eyes fixed on, their OKN kicks in. My participants end up following a single dot smoothly for a short period of time until their eyes flick in the opposite direction to start following a new dot - this pattern repeats over and over regardless of how much the participants try to keep their eyes still.

Testing OKN in the eye tracker (the ghostly face at the far right is Morgan).

Below, I’ve graphed examples of what the data end up looking like, but let me quickly explain the graph before you click on the picture to see it in full size! Along the x-axis is time, and the y-axis plots how much the eyes move left and right. If someone kept their eyes perfectly still, the graph would be a straight line along the value “640”. Values higher than 640 represent eye movements to the right, and lower values are movements to the left. Each coloured plot represents about 2 seconds of OKN from a different participant in a different condition.

Eye data showing less and less OKN for different conditions -- click the image to make it bigger and see the paragraph above the photo for more detail about the graph.

You can see that, over time, participants eyes smoothly move right, and then really quickly flick back to left, making a saw-tooth like pattern. This pattern is most obvious for the red plot, and becomes less for the purple, and even less for the blue plots. The decrease in OKN over these examples is because of an experimental manipulation I employed to help people keep their eyes still - even from this small amount of data it’s obvious that it worked!

The results from my experiments will hopefully inform our knowledge about how motion and eye movements affect our perceptual organisation of the world.


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