In our everyday lives we are used to perceiving the world via the five senses to such a great extent that we tend to take for granted the assumption that what we see, hear, taste, smell and feel is what the world is really like. This is quite natural as beside perception, there seems to be no other way of gaining knowledge of the things that surround us. This often leads to an impression that our perception of the world is something that is imposed externally and thus exists as an objective and independent entity.
Although there is most certainly some sort of physical reality underlying sensory experiences, the above concept is little more than a mental shortcut. Perception is, in fact, different from sensation. Although occurring spontaneously, perception is a quite complicated process involving the brain, and it is this process that plays the crucial part in the conceiving of empirical knowledge. Raw sensation on its own would not be of much use to a person, as illustrated by the case of patients with a disorder called visual agnosia. Such people fail to make sense out of what they see, for as a result of their condition they have lost their ability to recognise objects. Consequently they find themselves utterly confused and are unable to lead a proper life independently.(1)
In fact, it is the brain that accounts for the major part of the perceptual processes. Another clinical example demonstrating this is patients whose optic areas in the brain have been damaged; they reported partial or total blindness but nevertheless responded to basic visual stimuli(2) - another case of sensation without conscious perception.
The first step in the interpretation of sensory information involves data filtering, including their reduction. As it has already been pointed out, perception has a vital practical purpose. It is therefore important that the information from the senses be relevant, especially in the case of non-human animals whose survival is constantly at stake. For this reason in the process of evolution animal species have developed sensitivity to those forms of energy which aided their survival.(3) No animal, including human, can actually perceive the world in all of its forms. In this sense perception only deals with a certain blend of physical reality.
Selective attention is yet another limiting factor to the amount of data ultimately perceived. A lot of the information comes from the senses but is not paid attention; as a result it is not consciously perceived. This is another example of perception being selective in accordance with the context and the practical needs.
As was already demonstrated by the case of visual agnosia, recognition of objects is vital if visual perception is to make sense. Visual perception can be thought of as not "seeing" but "seeing as"(4); the incoming sensory information undergoes organisation and interpretation. These processes are to a great extent influenced by previous experience.
In general, we tend to put together the perceptual elements to form a global, coherent picture. For example, we would normally recognise basic shapes in diagrams even if some elements are missing from them; also, we perceive elements which appear close together in space and/or time as making up an entity.(5)
It is this tendency towards pattern recognition that enables us to see the world as made up of objects rather than mere details. This is clearly an advantage. However, visual perception might become confused in the cases where more than one interpretation is available, for example in deliberate optical illusions such as the famous Leeper's ambiguous lady.
Another important feature is seeing in three dimensions, which is achieved by comparing information from each of the two eyes. (A similar process is involved in locating the source of sound.) Moreover, 2D representations of reality, such as photographs, pictures and television images, are perceived with a sort of third dimension added to them. This is brought about by interpreting depth cues such as overlapping and the apparent size of objects. This makes the images look still more realistic. Without this ability of the brain, visual art could not produce such dramatic impressions.
Whereas the features listed above are common for most people (and, perhaps, non-human animals as well), an individual's perception is also related to the knowledge attained from their previous experience. The memory contains a great deal of information, or schemata, about the different features of the outside world. The brain spontaneously places the incoming sensory information into the context of these, so as to make the perceptual image coherent and expectation-matching as far as it is possible. A good example of this is the Ames distorted room. In this experiment, a room of an odd shape was constructed which could be viewed through a special peephole, in which it appeared as a normal rectangular room, but the people in it looked as either giants or dwarfs. Most of the people judged the people to be of an odd size because their perception of the room matched their expectations, and they knew that there are people with height well above or below the average. The one person who judged the room to be odd was a wife who saw her husband through the peephole.
As the main purpose of language is communication between people, it encodes information in the form of sensory data, auditory and visual that is. Perception therefore takes on a new semantic aspect, whereby a certain sequence of uttered sounds or written characters corresponds to a particular concept, either definite or abstract, which otherwise has nothing to do with the word itself. Thus when we read or listen to speech, we literally perceive more information than there is. Again, a language only makes sense to those who know it; other people would perceive the phonemes or the letters but not the meaning. This is yet another example of how different people perceive the same things in different ways.
One more thing to be mentioned is evaluation. Every person has their own attitudes, and these certainly play a part in how the person perceives the world. Nothing can look beautiful or ugly, sound pleasant or unpleasant or taste delicious or disgusting, unless so judged by the perceiver. Humans possess a unique ability of aesthetic appreciation, originating from their natural creativity. Every individual has their own taste and preferences for certain kinds of art, which is why what some people think is beautiful is condemned by others.
From the facts described above it becomes obvious that there is a significant, even crucial effect of perceiver on perceived. What the world is like to a person depends at least as much on themselves as on physical reality. Indeed, it would be most meaningless to try to infer what the world is "really" like. If we try and eliminate from the image of the world all the categories, forms and interpretations applied by the mind, we will not even end up with a swarm of chaotically moving elementary particles - we would still need to perceive these!
Suppose a person is born without any ability to perceive at all. Not only will they be unaware of the existence of the world; moreover, without any inputs to the thinking processes their consciousness will be absent altogether, so they will also be unaware of their own existence - and would they then exist at all? Thus one is justified in saying that perception accounts not only for the world looking the way it does, but also to a great extent for ourselves being what we are.
2 Weiskrantz, R. cited in: Gross, R: Psychology: The Science of Mind and Behaviour TOP OF PAGE
3 Gross, R: Psychology: The Science of Mind and Behaviour TOP OF PAGE
4 Fodor, J.A., Pylyshyn, Z.W. How direct is visual perception? TOP OF PAGE
5 Gross, R: Psychology: The Science of Mind and Behaviour