The Stationary Eye

Image via cambridgeincolour.com

Image via cambridgeincolour.com

As I shared my findings on vision with my collaborator, Andrew Hunwick, he recounted a film he post-produced years ago with a close-up shot of an eye. They noticed how the eye was constantly moving, even when it was fixated on something. The involuntary movements of the eye looked unnatural and it didn’t fit in with the narrative of the film. So in order for it to appear normal, they resorted to pixel tracking the iris to stabilize the eye.

It is interesting to think that we don’t really think of our eyes moving around so much. When it does, we think it is abnormal, pathological even. Perhaps this is largely to do with the fact that the world appears stable to us most of the time. Surely, if our eyes are moving around constantly, then the image on our retina must also be moving around too. Why then don’t we get the impression of the world moving around also?

Before we tackle this conundrum that has baffled many scientists for years, I spoke to vision scientist and neurobiologist, Colin Blakemore about how our eye samples the world around us.

Even if you discount the jumps that are occurring three times a second, the eye is never stationary anyway. Even when the jumps have stopped, then it’s always tremouring, maybe 30 times a sec and drifting – so it goes to a new position, then immediately begins to drift, and it’s shaking while it’s doing it. It’s really quite scary when you think about the information that our brains have got to understand the world because it comes in the form of these little jerky, buzzing, drifting pictures.

And that’s not all. We do not take in with our eyes the whole visual scene. We only absorb detailed information about the part of the scene that we are attending to. Which is generally in the middle of where we are looking.

The retina has a specialised region in the centre called the fovea or the macula. Which is quite small, but it’s very different from the rest of the retina. It’s very densely packed with photo receptors and other nerve cells, so it’s processing information at very fine grain detail and has very good colour vision – except for blue.

However, as we move through the world, we don’t notice that blue information is missing in the centre of visual field. Colin explains:

If you look at the sky, look directly at the blue sky and in fact you’re receiving no blue information from the bit that you are looking at. So the fact that you know that it’s all blue and it looks uniformly blue is a very good example of the way in which, perceptually, we interpolate and fill in – we draw information from other parts of the field to make broad assumptions about what’s going on but don’t necessarily have the information that we appear to have in our perceptual experience.

The more I enquire, the difference between what our eye receives and what we perceive seems to get further and further apart.

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