 The brain has an extremely difficult job. It is responsible for governing behavior in response to a highly variable and uncertain world. In terms of vision, it sees only patterns of light, so there's a difference between what your eyes receive and what your brain sees. One of the reasons this photograph is interesting is the subtle conflicting features that tell us on the one hand we are looking at a child, but on the other she has adopted a very adult posture and is holding an object we don't associate with children. Your brain is combining these clues with the image to make judgments about the child. Context affects how you see this image. If I tell you the photographer is the child's mother, you might draw certain conclusions about their relationship and perhaps make social judgments. If I then inform you that the photograph is called candy cigarette, those conclusions might become entirely different. In fact, the visual system must rely on context at all levels because vision is a problem that mathematicians call ill-posed. This means that a variety of possible causes may be responsible for the same visual scene just because of geometry and light. Your brain evolved to find food and escape predators, tasks which require fast decisions, not to assess artistic images, so your brain must use typical features in the world to develop approximations in order to interpret visual scenes. Your brain is probably misinterpreting this image because light typically comes from the sky. Now what do you see? The fact that your brain can solve these problems is what allows you to see depth in this painting even though it is a 2D image. In order to recognize an object, your brain must be able to separate those features that define it from those that do not. A shape, for example, like a square, is composed of four edges meeting at right angles. You still recognize this as a square even if it is large or small, or rotated by some angle. We say that the square is invariant to these transformations. The very first neurons in the visual pathway respond best to bright spots on dark backgrounds, or dark spots on light backgrounds in certain locations of the visual field. Many neurons in primary visual cortex respond to edges, and one hypothesis is that they do so by receiving input from a set of neurons, each of which responds to, for example, bright spots on a dark background arranged in a line. If all of these neurons respond, they send a signal to another neuron that will respond when all of its input neurons are firing. These types of cells are called simple cells. There are also neurons in primary visual cortex that respond to lines and edges in a way that is invariant with respect to translations, movements from side to side, or up and down, at least within a small window. These types of cells are called complex cells, and one hypothesis for how these cells arise is that they receive input from a set of simple cells. One theory for how the brain develops responsiveness to complicated features in the natural world is by building hierarchies of these operations, alternating sets of simple and complex cells that construct features and develop invariances to them. If this is true, we would expect to see regions of cortex that respond to larger and more complicated stimuli and are invariant with respect to more complicated changes. Lines become shapes, shapes become objects, objects become faces. This idea is consistent with much that has been found in the visual pathway. You have neurons in your brain that respond to specific people independent of, for example, what they are wearing or the surrounding environment. This computational structure is very powerful and has been used to develop computer algorithms. As with the photograph candy cigarette, the context in which you observe something affects your interpretation of it. The objects on the screen probably do not mean much to you as they are. They probably appear as abstract shapes. If I provide a context that your brain interprets as occlusion, however, the very same features appear to have a definite identity and meaning. One of the most important questions for visual neuroscientists is how the brain processes context, but we know context can have important effects. In the famous Kana's a triangle, you see the three identifying points of a triangle and your brain fills in the edges, even though there are none present. The percept you have of an edge completing the triangle is an illusion. The brain does have an extremely difficult job. It has to help us make decisions in a highly variable and uncertain world. In order to solve the ill-posed problem of vision, our brain has to use the features of the natural world. In what context do objects appear? What features are often grouped together? It can't resolve these questions based purely on what your eyes receive. Artists can manipulate our visual system to tap into context, from the gist of a visual scene to the emotional resonance we might associate with a time or place so that a painting or photograph becomes more than the sum of its parts.