Visual sensation is the direct stimulation of sensory receptors and the subsequent transmission of this sensory information to the central nervous system. This is accomplished through the following process: reception, where the detection of a stimulus occurs, transduction then converts this stimulus energy into electrochemical energy. The electrochemical energy is sent along the neural pathway to the visual cortex through transmission, and selection then isolates particular features of that stimulus. The final message is interpreted and organised. (Grivas, Down, Carter, 1996, pg. 79-84) Visual sensation is purely physiological, and is consequently fully automated by the body. Visual perception is not only physiological. It is also psychological. It is the process by which visual sensations are organised and given meaningful interpretation. This means that previous experience, expectations and memory influence the way that an object is perceived. For example when a cloud is seen without reference to other factors, the experience is sensation. When that cloud is seen and then linked to an external fact, such as that the cloud is a cumulonimbus and will result in heavy rain, then the experience is perception. Visual perception principles are rules that we apply to the incoming stimuli from our environment. These rules help to organise stimuli, and are thought to be present from early childhood, so are not consciously performed. These principles are divided into three major groups: perceptual constancies, Gestalt principles and Depth perception.Perceptual constancies refer to the stability of an object s perceived size, brightness and shape, although the image cast on the retina is going through constant change. Perceptual constancy comprises of size constancy, shape constancy and brightness constancy. Size constancy is the ability to judge the size of an object although the size of the image cast on the retina is changing. For example, when a basketball is moving closer to the eye, the image of the ball cast on the retina is enlarging, but it is known that the actual size of the basketball is not.Gestalt principles refer to a perceptual organisation where the whole stimulus is greater than the sum of its components. The Gestalt principles are based on the simplicity law which states that every stimulus is seen in such a way that the resulting structure is as simple as possible. Gestalt principles include figure-ground separation, closure, similarity and proximity. A reversible figure ground image is when part of the stimuli can reverse roles, so that the figure can become the ground and the ground the figure. These patterns are unstable because the figure and the ground are not easily discernible. For example in M.C. Escher s drawing, SYMMETRY DRAWING 60 (Two Lizards) (see appendix one), the orange and green lizards can reverse, becoming either the figure or the ground. This drawing can be perceived as the orange lizards being the figure or the green lizards being the figure. Both are equally likely.Irvin Rock (1995) conducted an experiment to determine the frequency of figure-ground reversals among different aged subjects. This experiment was carried out by showing RUBIN S VASE (see appendix two) to each of the subjects, and ascertaining what they saw. This experiment showed that children under the age of five could not spontaneously reverse the figure, and that high school aged children infrequently reversed. This was a far cry from a reversal every five to ten seconds, which was the typical outcome using the traditional method. This traditional method involved describing (or showing) the two images in the reversible figure before the experiment was carried out (Irvin Rock, 1995, pg. 123).Depth perception involves perceiving objects in three dimensions, and judging distance by using information from the two dimensional image formed on the retina. The judgment of depth and distance is accomplished by two forms of information: monocular and binocular. Monocular cues require information from one eye only, and are based on primary information (within the body), and secondary information (the stimulus itself). Monocular cues include accommodation, relative size, linear perspective, interposition/overlap, height in horizontal plane, texture gradient, light and shade, aerial perspective/haze, motion parallax/relative motion. Binocular cues require information from both eyes and include convergence and retinal disparity. Linear perspective is a cue to depth, based on the assumption that parallel lines converge as they get farther away. This convergence of parallel lines to a single point is also the way in which we view the world. For example in M.C. Escher s drawing, STILL LIFE AND STREET, (see appendix three), the converging lines of the street, and buildings, lead the viewer to believe that the drawing has depth, even though it is actually only two dimensional. Psychological features influence the way in which the incoming stimulus is interpreted. No two people are subject to the same psychological factors, and it is for this reason that perception varies from one person to another. These features include attention, habituation, prior experience, perceptual set, prejudice/set, hypothesis testing and motivation.
Attention is the ability to focus the consciousness on one stimulus while disregarding others. Attention is very selective, acting similar to a filter, so therefore greatly governs what stimulus we perceive from the external environment. A person s attention is most likely to be focused upon stimulus that is new or that has changed. Usually the most significant stimuli in an environment are focused on. For example, when learning to play tennis almost all attention was focused on hitting the ball. Once greater skill was obtained, this function required less attention, allowing more concentration to be focused on factors that are more important, such as anticipating your opponent s next shot.Habituation is the opposite of attention; it is the gradual weakening of response to repeated stimuli. When the stimulus is repeated the amount of attention it receives diminishes. However, if an important change occurs in the stimulus, then a break from habituation occurs. For example, when you travel in a car, the white lines in the middle of the road soon lose your attention because they are repeated so often, however, an orange reflector will still be noticed as you pass by it. Although visual perception is remarkably accurate, mistakes in perception do occur. Perceptual fallibility is most noticeable when there is a striking difference between perception and the external reality. Visual illusions deliberately set out to try to exploit the organisational cues of visual perception, often so much that what we see is not the same as the stimuli that is entering our eyes. (Charles, Edwards, Rogers, 1995, pg. 131)One such example of these illusions is an impossible figure. These figures cannot physically exist in a three-dimensional state, but can only be represented in illustrations; such as in M.C. Escher s drawing, MAN WITH CUBOID (see appendix four). First perception of this cube suggests a simple three-dimensional structure, but when an attempt is made to construct a mental map of the complete object, the structure reveals itself impossible. Julian Hochberg (1970) set out to devise a theory that would help explain these impossible figures. His findings stated that eye movements are necessary to construct a mental map of the visual stimulus, and that these eye movements then help us to piece together the elements into a complete scene. (Grivas, Down, Carter, 1996, pg. 138) From this theory, it is evident that the impossible figure is viewed as possible because of a manipulation on Gestalt principles. When the figure is seen as simple as possible, as a whole, it appears to exist. Nevertheless, when broken down into its more complex elements, it is seen to be impossible. In class, another visual illusion called the M ller-Lyer illusion was demonstrated using an empirical research activity. The aim of the experiment was to measure the extent to which this illusion occurs. Subjects estimated the length of line A (arrow ends) compared to line B (feather ends). This was accomplished through the use of a sliding apparatus, where line A was moved over line B, until they appeared to be of equal length. The results obtained showed that subjects underestimated the length of line B, predicting it to be 8.1 cm, not the actual 10cm. In conclusion, it can be said that equal length lines are perceived as different lengths, simply because of the different shaped line ends (see appendix five). Visual perception and sensation are two distinct processes, nevertheless, both of these processes must work in unison to gain a complete visual picture of the external environment. Visual sensation is a common feature to all able bodied humans, and is the process by which we receive external stimuli. This stimulus is then given meaningful interpretation using visual constancies, Gestalt principles and depth cues. Prior experience and other psychological factors allow each individual to perceive a stimulus in a unique way. The same principles that help us to perceive stimuli can also sometimes lead to inaccurate interpretations of that stimulus. This fallibility is due to internal factors such as visual principles that we apply to incoming stimuli. This leads to the incorrect interpretation of the M ller-Lyer illusion and impossible figures. It is these aspects, both learned and innate, that enable us to give meaningful interpretation to an otherwise confusing external environment. REFERENCES 1. CHARLES, EDWARDS, ROGERS (1995), Psychology a course for VCE units 3 and 4, Melbourne, Oxford University Press. 2. GOLDSTEIN, E.B (1989), Sensation and Perception Third Edition, Pacific Grove, California, Brooks/Cole.3. GRIVAS, DOWN, CARTER (1996), Psychology VCE units 3 and 4, Macmillian Education Australia PTY LTD.4. RATHUS, A.SPENCER (1990), Psychology Fourth Edition, Moore and Moore Publishing.5. ROCK, I (1994), Perception, New York, Scientific American Library.6. Encyclopaedia Britannica CD-ROM (1997), Michigan Avenue, Chicago, Encyclopaedia Britannica Inc.7. http://members.xoom.com/ksperry/escher.htm8. M.C. Escher Evolutions (1999), Rohnert Park, California, Pomegranate Publishers.