Perceptual Constancies and the Role that they play in Our Lives.
Everyday, all over the world millions of people encounter numerous of images. It would be very easy to think of the eye in this process, simply as a recorder of the things around it, much like a camera. Although there are some simple similarities between the eye and the camera, this would be a very inaccurate way of describing the function of the eye. If our eyes simply recorded the world around us and produced pictures in our brain, our world would look very different to the world that we see at the moment. To understand the world around us we have to do more than just see things, we have to interpret them.
In this essay I will be focusing on the visual perceptual constancies, namely shape, size and depth; however, I will also mention light and colour constancies. Perceptual constancies allow us to comprehend the image that the retina in the eye perceives from the stimulus. According to R.L. Gregory perceptual constancies maintain perceived size and shape constant, regardless of the changes of the size and shape of images on our retina due to a movement in viewing position or a change in lighting etc. (Gregory 1974). The perceptual image is much more stable than the image received by the retina which can change in relation to light, distance and movement. It would be very hard to understand the world if we were just to perceive it through our retinal images. The main principals of perceptual constancy are, size constancy, shape constancy, colour constancy, light constancy and speed constancy.
Although all of the principals of perceptual constancies are very important to our perception of the world, one of the most important is size constancy as it plays a
vital role on our everyday life, enabling us to tell when things are far away or near to us. If we were to rely on our retinal image, looking at a friend walking towards us could be a very scary experience! The retinal image becomes larger the closer an object comes towards us, therefore, a friend walking towards us would appear to be getting bigger and bigger, if we were to take our retinal image at face value. Size constancy is obviously also closely related to distance constancy (Coren et al. 1994). ‘Scientists have observed that a change in the size of the retinal image is not
perceived as a change in the size of the object but rather as a change in the distance of the object’ (Bloomer 1976 p.50). This theory has been proven in a laboratory
experiment when a subject was put in a darkened room and asked to observe an object that was gradually increased in size and then decreased in size. The subject observed that the object seemed to be moving toward him and then away from him (Bloomer 1976).
This can be seen clearly in
Figure1. Even though we see one
tiny man in the picture and one
large man, the principal of size
constancy allows us to deduct that
Fig. 1 the seemingly smaller man is
actually the same size as the apparently larger man, even though the retinal image is about one third that of the larger image.
It was believed that our ability to keep size constant in receding or advancing objects was due to our familiarity with the object in question, meaning that because we are familiar with certain objects we can easily estimate the size of that object, no
matter what distance it is from ourselves. This theory however, cannot explain the fact that children and also animals also show size constancies (Vernon 1937).
Another factor that allows us to accurately estimate the size of an object is the context in which the object is in. It is very uncommon for us to see everyday objects out of their natural surroundings or in isolation. As a result of this we have become used to seeing objects in relation to other objects, we are so used to seeing cars for example in relation to people, or in relation to garages, that it is very easy for us to accurately measure the size of a car in our town. An example of this effect can be seen in Figure 2.
Both of the circles in the centre are
identical in size; however, due to the fact
that one of the circles is surrounded by
larger circles, it looks much smaller than
the other circle which is surrounded by
Our emotions also play a part in our perception of size. The emotive value of an object can cause us to ‘see’ it as being larger than it actually is. In a very famous study two groups of children were shown some coins and asked to estimate the size of the coins. One of the groups of children were from a wealthy background while the other group were from a poor background. All of the children overestimated the size of the coins; the poorer children however, greatly over estimated the size, while the wealthier children only slightly over estimated their size. (Bloomer 1976)
Another very important perceptual constancy is depth. There are many cues that we interpret as depth, from texture gradients to haze. The further away an object is, the less fine detail we can see. One of the most important cues that we use to see depth are shadows and shading. Shadows tell us what direction the light falling on an object is coming from, we are assured that an object is solid by its shadows. ‘Normally, shadows appear on the receding parts of objects, and indicate recession and hence solidity’ (Vernon 1963 p. 130) Changing the shadows or shading on an object can have a significant effect, as can be seen in Figure 3. The pattern of shadow
on an object, depends upon its position in
respect to the light, in this picture, simply
by changing the lighting from above to
below, one of the circles recedes instead of
protruding. This is even more obvious if
we rotate this image see Figure 4. Now
most of the circles seem to be receding, while only one of them is projecting towards
us. This effect is traditionally one of the
first taught to art students, the use of light
and dark in artwork is called chiaroscuro,
and it is used to give the illusion of depth
on a flat surface. (Bloomer 1976)
Shape or Form constancy is also a very important section of perceptual constancies as we live in a three dimensional world. Shape constancy allows us to
reflectance properties of an object’s surface pigment’(Coren 1994 p.504). Even
though under certain lights the hue of a colour will change we can still interpret it as the colour red or blue etc. However, just as we relate size to the other objects around it in size constancy, our perception of a colour depends on the colours surrounding it as well as the colour that it creates on the retina (Coren et al. 1994). The colour red for example will look much darker if it is surrounded by black and much brighter if it is surrounded by white. We experience colours in lots of different situations, and just as we do in the perception of shape, our memory uses these experiences to create the normal colour of an object, e.g. grass is green. This normal colour means a colour that has not been influenced or changed by shadows or different types of illumination. This makes it very easy for us to see colour but it does, however, soften the effect that colour has on us, we rarely notice the subtle shades of colours. A result of this is that we have come to think of shadows as black, we seldom notice that shadows have different shades depending on the colour of the object itself.
Light constancy is the process in which the lightness or brightness of a surface or an object appears to be unchanged, even though the lightness and brightness around the object changes. The light that our retina receives from an object depends upon two factors, the amount of illumination from an external source, for example the sun or a light bulb, and the amount of light reflected back by the object to us. The amount of reflected light depends on the colour of the surface or object: white reflects a lot of light while black absorbs a lot of light. Our perception of the brightness of a piece of paper depends on the situation in which we are viewing it. It will seem very bright if we view it in a well-illuminated room and it will appear very dull if view it in a dimly
light room. However while the brightness of the paper changes, it is still the same shade of white (Coren et al. 1994).
In conclusion, the perceptual constancies are vital to our understanding and survival in this world. It is hard to imagine a world where people shrink as they move away from you and become gigantic as they come toward you, where two-dimensional images are all that we can see and where objects change shape depending on the way in which you look at them. This is what the world would be like if we did not have visual perceptual constancies, if we simply saw the image which our retina picks up with out decoding it and turning it into an image which we can understand. Many of us never think about how we see things and take it for granted. It is so natural to us that we never even think about how it happens everyday in our lives and how different our world would be without the many process involved in forming our visual perceptions.
Bloomer, Carolyn M. (1976) Principles of Visual Perception. New York: Van Nostrand Reinhold Company
Coren, Stanley, Lawrence M. Ward & James T. Enns. (1994) Sensation and
Perception: International Edition.
Gregory, R.L. (1990) Eye and Brain: The Psychology of Seeing. Oxford University Press
Gregory, R.L. (1974) Concepts and Mechanisms of Perception. London: Duckworth
Vernon, M.D. (1963) The Psychology of Perception. Harmondsworth: Penguin Books
Vernon, M.D. (1937) Visual Perception. London: Cambridge University Press