The Eyes Have It

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The Mind’s Eye

Our sight, the most complicated sense we possess, has been fooling the mind behind our eyes for millions of years. We believe that we have wide angle, high detail color vision, but the reality is our accurate color vision is concentrated inside a tiny part of our visual field. The rest is simply movement sensing, mostly black-and-white awareness.

The brain fools itself into pretending that we can see as well away from the center of our vision as we can at our point of focus. This allows us to use much less mental processing capacity than would otherwise be needed. We fixate on the details of our focus and safely ignore the surroundings until our concentration moves in that direction.

The picture you hold in your head that you assume to be an accurate live feed from your eyes is really a composite image created in your mind’s eye from the information you are recieving from your senses. This would be similar to looking at a wall where a photographer is putting up photos to create a collage of the pictures taken from the window.

For more perspective about the mind’s eye, the January, 2005 issue of New Scientist magazine contains an article about a Turkish painter who has always been blind.

How We See the World

As you look at an object, light is reflected off the object into your eye through the lens. You detect this light when it falls on the retina at the back of the eye.

In the middle of your retina is your fovea. The fovea is the focus point of your vision and it has much greater image resolution. The fovea is just big enough to see a circle about the size of your thumbnail at arms length. It contains many more color detectors or cones, than the rest of your eye, allowing it to distinguish more detail.

The rest of the retina contains a few cones but is mostly rods. These are simple light detectors and only able to separate black and white. However, we have these rods in vast numbers; this gives us very efficient peripheral vision, which enables us to easily see any object that moves near us.

Figure 1: The left picture shows the difference between how much light you detect in the middle with your fovea and the rest of your retina on the outside. A series of snap shots (fixations) like this are combined in the brain to create the scene as we see it, shown in the right picture.

Note: At night there is too little light to detect color, so you will see better looking slightly away from an object and using your rods for ‘black and white’ detection.

Two Types of Vision

Your eyes work like digital cameras, although much better than any camera currently available. They are fully automatic and have two primary modes, video and photographic. The photographic mode takes snap shots to create images of stationary objects. The video mode follows anything that is moving.

Motion Tracking

Our eyes are attracted by movement and can continuously follow a moving target in a series of very fast tiny movements, micro-saccades. This is called smooth pursuit. We focus on the target and ignore all the surrounding detail. The target is always clear but everything else can become blurred as our brain creates the image.

The Photographic Eye

When we look at something standing still, the focus point of our eyes moves from detail to detail in a motion called saccadic jumps. At each stop, our eyes take a snap shot called a fixation.

Every fixation is passed to the brain, where they are combined with others to create the stable image of the world that we see.

Although both methods use the same system of eye control, saccades, the difference in response time using micro-saccades makes smooth pursuit much faster vision.

Saccadic vision is our main form of learning about the world around us. We can create a map of the path of our fixations and saccadic jumps, known as a scan path. As this scan path (left from Yarbus, 1967) shows, we tend to concentrate on details we consider important, for example, the eyes, nose, and mouth of someone’s face.

Interestingly, in certain rare cases of head injury where emotional attachment to other humans has been damaged, the saccadic map of the patient looking at a face is much more spread out, showing that it is a sub-conscious attachment to recognized and important details that determines the location of each fixation.

Do TryThis at Home

Follow these simple instructions and try these two different methods for yourself.

To Try Smooth Pursuit:
Stretch your arm out to one side and point your finger. Hold your head still and keeping your eyes on your finger, move your arm as fast as you can across your front.

To Try a Saccadic Jump:
Hold both arms out in front of you as far apart as you can, but so you can still see them both. Now focus on one hand and keep your head still. Now flick your eyes to the other hand.

In smooth pursuit you are able to keep your eyes on the moving finger, but you may notice that the background becomes blurry as you follow the movement. Saccadic jumps differ from smooth pursuit, as even though you are effectively making the same movement, there is no blurring as your eyes move from one hand to the other.

Blinded by…

You have seen in the demonstrations above that when we make a saccadic jump there is no blurring as our eyes move between fixations. This is because during a saccadic jump, our eyes stop sending information to the brain. It is what allows us to look at the world around us and avoid it being one big blurry mess as our eyes move around. (Saccadic suppression: We stop sending visual information to the brain during a saccadic jump. This stops any perception of blurring from the movement of the eyes. Discovered in 1903 by EB Holt.)

We make many saccadic jumps every second, so our eyes only send information to the brain for a fraction of each second. Current theory is that saccadic suppression starts about 50ms (milliseconds) before a jump and lasts for at least 125ms, possibly more; suggestions have been made as high as 400ms on occasions. Even at 125ms minimum, we can only make a maximum of 420 jumps per minute or eight per second, although the average is nearer four or 240 per minute.

This defines our reading speed limit; as our eyes move from word to word, fixating on each, we automatically stop ourselves from reading any faster.

This article is an extract from the Road To Reading series, published by Really Easy Reader, in an effort to explain the real science of reading.

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