From owls to butterflies to monkeys, there are many examples where the visual part of the brain has evolved to devote more resources to tasks that are particularly important for a species. Nocturnal animals like owls have enhanced night vision at the expense of good colour vision, for example. This type of trade-off is important because having a bigger brain comes at a cost for animals. A big brain is heavy, for a start, and also requires feeding. The energy consumption of the human brain exceeds that of any other organ, including the heart.
Most research on vision has concentrated on the small central part of our visual field. This is the part where we see best; the part tested with the letter chart by your optometrist. And it really is very small. Hold your fist directly in front of you at arm’s length – that’s about the area that we are talking about. The vision outside this central area is a good deal less sharp. Evolution has resulted in an elegant solution. Vision is good enough in the periphery only to attract your attention to an object. To explore it with higher resolution, you can then look at it directly if you choose.
That’s not the only use we have for peripheral vision, however. Imagine the scene: you are enjoying dinner with a friend and are engaged in a stimulating conversation. Without taking your eyes off your companion, you reach for a glass of wine, pick it up and take a drink. This feels effortless, but the brain needs to form an accurate picture of the shape and size of the glass and where it is located. (Grasping does also rely on touch, but we first judge the object’s size and shape with our eyes and then reach out and adjust our grasp to match what we have perceived.)
Because human arms and hands are below our eyes, reaching out often takes place below eye level – as does the manipulation of objects. We and our research colleagues wondered whether the visual brain might have adapted to this through evolution, resulting in a visual system that is more finely tuned below than above eye level.
We tested this idea using a task in which we asked six young adults to compare shapes presented on a computer screen to see if they could tell the difference between a perfect circle and a slightly distorted circle (vision studies like these typically use small subject groups because of extensive testing per head). As we expected, they performed best when they looked directly at these shapes.
We then repeated the test for shapes presented above and below eye level, and also to either side. Sure enough, our subjects were much better at judging the shapes when they were presented in their lower visual field. In fact, they were more than 50% better at this compared to when the shapes were either above or to either side.
If this was an evolutionary development, it would make sense that our lower-field vision was better than our other peripheral vision only to the extent that it allowed us to process the shape but not the type of the object. To see whether this was indeed the case, we repeated the experiments with a specific type of object: faces.
Faces are considered to be a particularly important class of visual stimuli for humans. Humans are social animals and accurate face perception is central to social functioning. Good face perception allows you instantly to recognise familiar people, distinguish friends from foes and read the emotions of people you encounter.
Studies have shown that humans and other primates have evolved specialised brain areas that are dedicated to processing faces. While it is important to be able to detect a face in your peripheral vision, there would be no advantage to being able to discriminate between faces more accurately in the lower visual field than in other peripheral areas. Most of our encounters tend to be face to face, after all.
When we tested the same people again using faces, this hypothesis turned out to be correct too. We found that our subjects were no better in the lower visual field than elsewhere, and were actually slightly better at discriminating faces which were shown to the left. This apparent curiosity is probably due to the fact that humans’ face-specific brain area is located in the right cortical hemisphere. As with other brain functions, the right side of the brain controls the left side of the body, so this is what you would expect.
As a control in our experiment, we also tested our subjects on faces with internal features removed – eyes, noses, mouths and so forth. Having reduced the face to a shape in this way, we speculated that our subjects would now perceive them more accurately in the lower field of vision than the other peripheral areas. This is exactly what we found.
It suggests there is an evolutionary advantage to processing an object shape below the centre of your vision – picking up berries on an ancient forest floor without taking your eyes away from potential predators, for instance. But there is little advantage, and a great deal of cost in terms of brain power, to extend this to all sorts of different object classes including faces.
So next time you enjoy a glass of wine over dinner, you might feel safe in the knowledge that your visual senses have evolved to allow you to successfully pick up the glass without directly looking at it. Just remember to keep it on the table below your eyes. Put it on a shelf above eye level and there’s a much greater chance that the wine ends up all over the table – or worse.