The Stationary Eye

Image via

Image via

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.


The Brain as a Computer

image of brain as a computer

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At my first meeting with vision scientist and neurobiologist, Colin Blakemore, I asked him if the brain is like a computer and his response, “The brain is a computer,” made me smile. He had turned my metaphor into the thing itself that I was trying to represent.

Later I conducted some research and looked into the etymology of the word ‘computer.’ The term came into use in the 17th Century to mean a human being who “computes.” It is formed from the Latin verb computare, meaning ‘to count, sum up; reckon’.

In Chinese, on the other hand, the etymology of the word seems to have originated from the commonalities between the brain and the modern day computer. The Chinese word for computer, 電腦 translated literally is: “electric brain.”

Perhaps the boundary between human and machine is not so distinct after all?

At our next meeting at his office at the Centre for the Study of the Senses, University of London, I asked Colin about the analogy of the brain as a computer, how useful this is and where the limits lie.

He began by exploring the definition of what a computer is:

If we justify the computer as something that takes data in, in some form, processes it, stores some of it, acts on the rest by controlling something or displaying it, then certainly the brain is a computer… but the mechanisms are quite different.

He went on to explain how when we look at a scene, we are able to extract meaning from it:

You’re looking at a building with a tree in front of it, you see and understand the whole building even though what you can really see are two half buildings with a tree cutting them in half. Nevertheless you never think of them in that way, you think of them as a building. So, we’re very good at filling in the missing information.

One dramatic difference between the brain and the computer is the rate in which information can be processed. The brain seems to also have very strict bottlenecks of information processing:

Typically, working memory – the things that we’re holding continuously as you move around the world for short periods of time… the content of working memory is very small, it’s a few tens of bits of information as opposed to the gigabytes of RAM on a computer. And yet the brain is being flooded with the same kind of rate of information input that computers are.

The limits of the capacity of our visual attention is also very small. Our brains therefore have to be selective:

One way that is achieved is through attention. It looks as though what goes into working memory is largely determined by what we are attending to… When we look at a scene, most of what’s going into our eyes is not being attended to.

Since much of the visual input into our eyes is not attended to, what then is the relationship between what our eyes receive and what we perceive?

Upside Down Retinal image

Descartes Treatise of Man

Diagram from Descartes’ Treatise of Man (1664), showing the formation of inverted retinal images in the eyes, and the transmission of these images, via the nerves so as to form a single, re-inverted image (an idea) on the surface of the pineal gland. Via Plato.Stanford.Edu

During my first dialogue with psychologist J.Kevin O’Regan we discussed the inverted retinal image – the tip of the iceberg of the defects of our visual system. We discussed how for centuries the upside down image on the retina confused many scientists and philosophers from Kepler, Leonardo Da Vinci and Descartes through to George Stratton who conducted an experiment in 1897 with inverted glasses to test for perceptual adaptation.

We also discussed the analogy of the eye as a camera and how the ‘seat of vision’ was shifted from the eye to the brain.

“Once Kepler had discovered the image at the back of the eye, we realised that the eye was very much like – what we call a camera today – because that didn’t exist at the time. And so it was useful for people studying vision to realise that vision happened through this optical instrument, which was the eye. Because before Kepler’s time and in the middle-ages, people thought that vision happened in the eye. They thought that the organ of vision was the eye… After Kepler’s discovery, they realised that the eye was not the ‘seat of vision’ it was just an optical instrument and that optical instrument gathered the information and then that was sent to the brain and it was actually the brain that was the ‘seat of vision.’”

He went on to talk about how we use the eye as a tool to explore the world. And that we cannot know about the defects and the imperfections of this tool we use to see the world.

“One never sees the blind spot. You never see your retina. You see the world.”

Since we are aware of these defects of our visual system, I asked Kevin if it made him less confident about what he perceived as being real in the external world.

“Our vision of the world is a construction that we have deduced from the senses that we have at our disposal, our eyes and hands… What we mean by the world is a construction that we have constructed on the basis of the information we have gained but the world could be completely different – the real world. But maybe it makes no sense to call that other world ‘the real world,’ because really what we mean by real is what is real for us. The only thing that really is of consequence to us is what affects our senses.”

J.Kevin O’Regan was the former Director of the Laboratoire de Psychologie de la Perception at the Université René Descartes, Paris, one of France’s most influential experimental psychology laboratories. He is most cited today as the originator of the sensorimotor approach to consciousness. He is also one of the discoverers of the much-discussed phenomenon of “change blindness,” and well known for his work on eye movement in reading. He is currently working on a 5-year European Research Council Advanced project on Consciousness at the Laboratoire de Psychologie de la Perception.