Visual perception is the interpretation of the stimulus of light, when it reaches the eyes. This is an important sense that we use to gather information about the world around us. However, sometimes the images that are perceived differ from the actual reality of the situation. For example, an optical illusion occurs when the brain perceives the information gathered by the eye in a different way that does not correlate to physical measurements. For example, in the famous Necker cube illusion, lines that are drawn in 2 dimensions, but the brain perceives them as a 3--dimensional cube. This is an example of a cognitive illusion, which means that assumptions about the world and what you are looking at, leads to unconscious inferences and thus, you may see something that is not necessarily there. Physiological illusions occur when excess stimulation occurs and the intense or repetitive activity can alter perception. For example, the Hermann grid illusion has the appearance of gray spots at the intersections of light and dark areas. These are examples of experiences using the sense of sight.