Many factors can affect perception of reality, including physical and mental states. Stimuli such as sound and light patterns can result in changes to a person's perceptions that he or she may not be able to control, but active reorienting of the mind can also produce changes in perception. Mental illnesses like schizophrenia may interfere with perception in a variety of ways. In some cases, actual patterns of thought can influence the way a person understands physical reality as well.
Physical reality is generally understood to objectively exist, but each person's perception of that reality can be different. This is because the mind does not directly access that objective reality, but rather interprets it so that the brain can use the information. One interesting case in which this is true is perception of steepness, where a person's physiological state can affect the steepness he or she sees. Even when a person knows that the hill is physically no steeper than a certain measurement and can accurately represent its steepness using physical methods, he or she can still believe that the hill is visually steeper than it is.
Sound can affect a person's perception of reality as well. Subsonic frequencies are often shown to create feelings of uneasiness in people that sometimes result in sightings of ghosts or other haunting activities. On a more direct level, constant loud noise can interrupt thought patterns and make accurate perception very difficult.
Mental states can affect a person's perception of reality as well. Consciously focusing the mind on an object, for example, can change the way that object appears to a person. This is partially because the human mind does not treat all visual information as equally relevant. Focusing on sound can yield a similar effect, though this is more difficult for many people to accomplish.
Illnesses can change perception and can often dramatically limit a person's access to objective reality. Mental illnesses that change the way stimuli are interpreted must by definition change perception. Both delusions and hallucinations can change a person's perception of reality, whether they are the result of drug use or mental difficulties.
One interesting factor that can change a person's perception is language. Many people believe that the way a person uses language can affect his or her understanding of certain aspects of reality, but some people believe that language can literally change how a person's views the world. This theory, which is sometimes known as linguistic relativity, is sometimes framed as changing conceptions rather than perceptions of reality, but results in the same mental processes either way.