Dr Jonica Newby

What's the difference between me and a frog? Well, look into my eyes. See? They're jiggling. The frog's aren't. How come?

NARRATION

The answer is so intriguing, I had to cross the world to Phoenix, Arizona and meet the neuroscientists in the laboratory of illusions.

Dr Jonica Newby

Wow, and so our eyes are never still?

Dr Susana Martinez Conde

They're never completely still. Even when we think they are completely still they are producing these tiny motions.

NARRATION

These tiny motions are called microsaccades which means little jumps. Since microsaccades were first discovered a century ago, their role has been hotly debated. In the 80's scientists even concluded they were useless.

Dr Stephen Macknik

They actually published a paper with that as the title: 'Small saccades serve no useful purpose'. Well it was a bit premature I think to come to that conclusion.

NARRATION

So how did this team finally crack the case? Well, naturally, by reaching for an illusion.

Dr Jonica Newby

OK, you at home, are you watching the TV screen? Because I want you to try this one along with me. We're going to make something that's right in front of you disappear... You ready?

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Please stare at the red dot, while noticing the blue circle. Keep staring at the red dot and eventually the blue circle will disappear.

Dr Jonica Newby

Starting to fade... Oh, disappeared completely, and then back again. Did it work for you too?

NARRATION

If not, try again later on our website. So what's this got to do with microsaccades?

Dr Jonica Newby

Well, now I'd like to invite you to visit the inside of my eyeballs.

NARRATION

When light enters our eyes, the photons hit the cells in the retina at the back of the eye. They then send a message to the brain 'photon incoming' and voila, we see! The trouble is, if the photons don't change, the cells get bored and stop sending a message and the image disappears. This is called adaptation and it's not confined to vision.

Dr Susana Martinez Conde

It has happened to all of us to go around the house looking for our glasses only to realize after a while that we have been wearing them all the time. That is because after a while your touch receptors adapt.

NARRATION

When animals first evolved, visual adaptation didn't matter so much. Their priority was to detect things that moved like prey or a predator. But what about if you need to look at something still for a long time? Not much good if it disappears. So you've probably guessed it. Maybe microsaccades evolved to stop our retina cells being bored. This theory was put forward a while ago, but remained fiercely contested until Susana and Stephen came up with an ingenious experiment to settle the matter.

Dr Jonica Newby

Is this a fancy eye tracker?

Dr Susana Martinez Conde

This is top of the line. It records your eye position 1,000 times per second.

Dr Jonica Newby

A thousand times a second, my goodness.

Dr Susana Martinez Conde

So press the button whenever the blob disappears.

NARRATION

This is another version of the disappearing blue line illusion you did earlier. The scientists ran hundreds of people through this regime and found well the same as me.

Dr Susana Martinez Conde

So you can see that when the microsaccade production falls right here, the stimulus becomes invisible, when the microsaccade production peaks it becomes visible.

Dr Jonica Newby

So if my eyes weren't jiggling, things I was actually staring at would disappear?

Dr Susana Martinez Conde

If your eyes weren't moving at all, you would only be able to see things that move. Far from being useless, microsaccades, they've proven, are a marvel of evolution.

Dr Stephen Macknik

The image now shifts across your retina every single time that you make the microsaccade. And what that does, is it shakes up the system, it takes the edges that are being adapted and moves them to another neuron that's now not adapted so it continually refreshes the visual image in the neurons of your eye and of your brain. So the micro-saccades keep everything visible for you while you're fixating. And without them, we'd be blind all the time.

Dr Jonica Newby

So that's the difference between us and a frog. The poor frog can be surrounded by flies, and unless one moves, he'll starve. Whereas we humans, we can see the flies while they're sitting still.