The mind’s eye
Vision is our most crucial sense. We rely on it for survival, but just how reliable is it?
It is tempting to think of our eyes as mini-cameras constantly filming the outside world. In fact, vision is nothing like that. The seamless view of the world is an illusion created by the brain after it has dismantled the input it receives from the eyes.
For a start, we do not look at a scene in a steady way. Instead, our eyes constantly flicker back and forth (involuntary movements known as saccades), scanning scenes in detail. From this constantly shifting input, the brain builds up a coherent mental picture corresponding to a scene. Try watching someone reading to see these small movements for yourself. Did they think their eyes were moving?
We now know that neurons in our brain specialise in recognising particular aspects of a scene, such as edges or dots or motion. Each neuron has a ‘receptive field’, an area around it that is sensitive to its favoured stimulus (like a detector attached to a security light, which can detect movement within a particular area of ground).
Then, in a computational task of staggering complexity, the brain integrates all these signals to create a visual impression of the outside world.
Another key difference between the visual system and a camera is the phenomenon of attention. There is so much going on in the world that the brain has to filter out unnecessary inputs. One way it does this is by focusing on (or ‘attending’) to a small area at any one time.
We are not very aware of this, partly because our peripheral vision is sensitive to movement, so if something noteworthy happens there we are quick to notice. But it means we take in much less of a scene than we might imagine.
A nice example is a study in which volunteers were asked to watch a videotape of people playing basketball. They were asked to count the passes made by one of the teams. Afterwards they were asked if they had noticed anything unusual.
Fixated on counting, almost half failed to spot a woman dressed in a gorilla suit who stopped to face the camera, banged her chest and walked off.
The other big difference between the brain and a camera is that the brain guesses more. When presented with incomplete information, it fills in the gaps, making assumptions about what should be there given the rest of the visual input it is receiving.
This filling in can be useful. The visual system is often trying to extract patterns. So when it finds one but with a bit missing, it fills in the missing space, so we get a complete coherent picture. But it sometimes leaps to the wrong conclusion. Can we believe our eyes? Not always.Lead image:
N Seery/Wellcome Images CC BY NC ND