Australasian Science Magazine

There isn’t just one area in the brain devoted to vision.

By By Isabelle Mareschal & Colin Clifford

Recent behavioural tests reveal that patterns we can’t even discern can deceive us into seeing things differently from how they really are.

Although most people think that we see with our eyes, most of the hard work is actually performed by our brains. The eyes themselves simply transmit information, via nerve impulses generated in the retina, to the part of our brain at the back of our head devoted to “seeing”.

There isn’t just one area in the brain devoted to vision, but rather many areas that radiate towards the front of our brain. Visual information from the eyes first arrives in the brain at the primary visual cortex, which encodes simple characteristics about images such as edges or bars. This information is then passed on to higher cortical areas where more and more complex image properties are represented.

A simple analogy might be to consider how a cartoonist creates an illustration. He first draws outlines of characters and then adds in details and colour, rendering the illustration more and more complex with every iteration. The brain can be likened to working in a similar manner: early cortical areas do the easy work, while later areas are lumbered with increasingly complicated tasks.

Recent research suggests that the first stages of visual processing in the primary visual cortex mainly occur unconsciously, but we actually have no idea what the neurons that convey information about vision are doing. These areas contain a multitude of neurons – approximately 150 million in the primary visual cortex – that never work independently. Rather, when one neuron is stimulated by nerve impulses from the eyes, a large group of its neighbours are also activated.

It is this group activity of cells that can sometimes lead to visual misrepresentations because of the complicated interactions between cells that can act to mutually excite or suppress each other. The result of these interactions can sometimes lead to visual illusions, when our brains tell us we are seeing something that isn’t actually correct.

Two examples of illusions we tested on our subjects are the tilt illusion (Fig. 1a) and White’s illusion (Fig. 1b). In the tilt illusion, even though the centre bars are perfectly vertical, most subjects (and hopefully most readers) see them as being tilted.

In White’s illusion, the two gray bars in the centre of the striped squares appear to be of different shades of grey, even though they are actually exactly the same. However, because of the light and dark bars around them, they appear to be very different. You can convince yourself that these bars are the same by placing paper over the stripes so that you can only see the central targets; they should now look the same.

In both examples, these misperceptions occur because the surrounding stripes excite cells that interfere with those stimulated by the central target.

We wanted to investigate whether these illusions can occur even when people are unaware of the surrounding context. If this were the case, it would suggest that the illusions arise in early visual areas.

To examine this, we used a tilt illusion display like the one in Figure 1a, and designed a surround so that subjects could not see any of the oriented bars. To achieve this we made a movie where the orientation of the bars in the surround was changed very rapidly so that it looked as though the surround was made of flickering noise (similar to looking at a detuned television). In the middle of this surround pattern, we briefly presented a central target of vertical bars and asked people to report if the bars looked tilted.

What we found was that even though the subjects could never see the orientation of the bars in the surround, they always perceived the centre to be tilted in the opposite direction to the indiscernible surround. This always happened very quickly, and could never be thwarted. Even though the subjects knew that the central bars were perfectly vertical they couldn’t prevent themselves from seeing them as being tilted. Try as we may, it is impossible to alter our perception of them.

Our research is a demonstration that what we think we see isn’t always “real”. Or, put in other words, the world can be quite different from how we think it is.

Perhaps the most striking example of how our brain recreates the world is the blind spot in vision (Fig. 2). This is a patch of the visual field from which the retina receives no input because of the optic nerve in the eye. However, we never experience a blind spot in our visual field because the other eye provides the brain with visual information. Even when we shut the other eye the brain effortlessly fills in what is missing, so that we never experience an incomplete view of the world. Because of this, everything falling in the blind spot appears “normal”; we always see the world as being perfectly coherent and complete.

Our brains do such a good job at filling in missing information, even outside of the blind spot, that a major danger is that diseases that result in visual field losses (e.g. glaucoma) can often go unnoticed.

You can experience the blind spot by shutting one eye and looking at the “R” in Figure 2. If you hold the paper at arm’s length, you should still be able to see the “T” in your periphery. Now slowly move the paper towards you and the “T” will disappear when it falls within your blind spot.

What emerges from our research, as well as other experiments studying visual illusions, is that a lot of what we assume to be real in the world is not always true. Information within our visual system is often distorted or incomplete, yet we always perceive an effortlessly coherent view of the world.

Generally, visual illusions are quite subtle and of little danger to our well-being. But by studying how and when they emerge we hope to gain insight into how the cells in our brains interact in wonderful yet complicated ways to create our illusion of reality.