Visual sensation and visual perception are the processes by which we see and understand our surroundings. The difference between these processes and the dividing line where sensation ends and perception begins is debatable, however some distinctions can be made.
Sensation is a physiological function, and begins with reception, when the cornea and lens of the eye focus light on the photoreceptors (rods and cones) in the retina of the eye. After this, transduction occurs, transforming stimulus energy into nerve impulses. The next step is transmission, which is the movement of the data in the form of nerve impulses from the receptors to the brain. As sensation is physiological, all people obtain the same sensation from a given stimulus.
Perception involves psychological processes, and the first automatic response is selection, which assists perceptual organisation and helps the brain sort the sensory elements into a whole picture. The next step is organisation, where elements are grouped into forms or shapes, followed by interpretation. Interpretation gives the stimulus meaning, and involves psychological characteristics such as attitude or prior experience and means that in some cases, our interpretation may vary from other people’s. For example, two people sense a round, flat, shiny object, one from a poor country, another from a rich country. The rich person might understand the object to be a music compact disc because he has had prior experience with them, but the poor person may not know what it was as he has never seen one before.
The visual perceptual principles are processes which are applied to incoming visual stimuli to help explain any inconsistencies that may occur when the perception process takes place. There are three main types, the Gestalt principles, depth cues and perceptual constancies.
The Gestalt principles are based on the concept that we perceive objects in the simplest possible manner (Grivas et al 1996 page108), and describes how the brain organises, groups and simplifies visual stimuli.
The depth cues are signals given from a visual stimulus (secondary cues), or from our own visual system (primary cues). These cues help us to perceive distance and depth, and can be either monocular (requiring only one eye) or binocular (requiring the use of both eyes).
The perceptual constancies are visual stimuli which are put into three categories – brightness constancy, size constancy and shape constancy, which are thought to be innate, or learned while still very young. Perceptual constancy is the ability to understand that an object remains constant although changes occur in the retinal image.
Brightness constancy occurs when an object is perceived to maintain a constant brightness, despite changes in lighting being present.
Size constancy occurs when, although the retinal image of an object may change, we still perceive it to be the same size. The brain recognises that although the retinal image is changing, the size of the object remains constant. An example is a train moving away from a station. The retinal image gets smaller as the train moves away, however we realise that the train is not actually shrinking. Instead, the brain uses size constancy and we understand that the train is moving away from us.
Shape constancy involves recognising that although the image being cast on the retina changes, the shape of the object remains the same. When an object is viewed from different perspectives, the angle of the retinal image changes, however shape constancy compensates for that and we understand that the object remains constant.
An example is a coin being flipped. As it spins, the retinal image suggests that the shape is changing because it changes from a circle to a half moon to a line, the automatic process of shape constancy enables us to understand that the coin remains the same shape.
Although every person, given normal vision will sense the same image on their retina when seeing the coin flipping, their perception of the image may differ. Psychological factors influence our perception and give us our individuality. We each have a perceptual set made up of expectations, prior experience and psychological states that directs our attention and determines how we interpret visual stimuli.
One psychological factor that influences perception is prior experience. Prior experience refers to the individual experiences which have some kind of personal significance. Each person has a different set of experiences, and those experiences will be interpreted differently by the individual according to such factors as upbringing, intelligence, personality and the values they hold.
For example, my dog plays with her biscuits. One day I asked my friend to give her one, and he laughed at her when she growled and pawed at it. He had never seen a dog do that before, however I have seen it many times, and expect her to do it, so my reaction was different to my friend’s.
In our class we conducted an experiment testing the prior experience theory. The teacher used a series of 15 pictures slowly transforming from a man’s face to a woman holding flowers. Half the class was shown the man’s face until we reached the ambiguous picture. The other half were shown the female holding flowers first, thus, our prior experience had influenced our perception. The results were that out of the group shown the man’s face as the first stimulus, 93% saw the man in the ambiguous stimulus, and out of the group shown the woman, 75% saw the woman in the ambiguous stimulus. From this, we can conclude that prior experience had an influence on what we perceived when shown the ambiguous stimulus (ERA – March 1998 – The effect of prior experience on perceptual set).
Another psychological factor that influences visual perception is motivation. Motivation is the process by which a person is in an increased state of arousal, influencing behaviour by activating, directing and sustaining it.
For example, if I am playing a casual game of football, and I see a girl I want to impress, I would play harder in order to get her to notice me. I want to impress the girl, so I am motivated to play to my best abilities.
In an experiment, McCleeland (1948) showed a range of subjects who hadn’t eaten between 1 to 16 hours blurred pictures of food. The subjects that hadn’t eaten for longer periods perceived food more often that those who had. From this, they concluded that motivated states like hunger, a motive created from a biological need for food, can influence our perception of visual stimuli. (Morris, 1990, page 113).
Psychological factors are not the only thing that can affect perception. The fallibility of visual perception refers to an incorrect interpretation of sensory information. There are two major reasons that visual perception is fallible. The first is psychological factors which provide everyone with an individual way of interpreting a stimulus, the second is via illusions, which mislead the perceptive process by distorting them with cues inherent in the stimulus.
The psychological factors which affect perception give an individual bias on what is perceived. These factors and their impact on perception can vary, however illusions are almost always perceived in a similar fashion. Illusions are unavoidable, and even when it is known and understood that an illusion is being viewed, it is still perceived. They usually only apply to two-dimensional settings, as when there is a third dimension, distance is incorporated giving us a constant perception.
A visual illusion occurs when an incorrect judgement is made which conflicts with the reality of the perceived object. The most common of these are geometric illusions, which are line drawings that produce visual perceptual errors. A well-known visual illusion is the Muller-Lyer illusion. This consists of two parallel lines, identical in length. One has arrow heads (an arrow pointing outward) on each end and the other is feather-tailed (an arrow pointing inward) at both ends. The feather-tailed end is perceived to be longer, despite measuring it with a ruler. The explanation for
this is that the feather-tailed line is perceived to be further away. A widely accepted explanation for this is put forward by Richard Gregory. He believes that we mentally make a three-dimensional figure out of the lines, so the two objects cast the same-sized retinal image, however we perceive them to be at two different distances. If two objects produce the same-sized retinal image, and one is perceived to be further away, the further object is perceived as bigger. (Grivas et al, 1996, page132).
From the above, it can be concluded that visual sensation is a biological function that starts with light triggering reception, transduction and transmission. Perception follows, which involves psychological processes: selection, organisation and interpretation.
There are three main visual perceptual principles that organise sensory data: the Gestalt principles, depth cues, and perceptual constancies. Although everyone receives the same image, perception may differ, and this is due to psychological factors such as prior experience and motivation. Perception is also distorted by illusions, which occur when the perception of a stimulus is in conflict with the real dimensions of the object. Despite these inaccuracies however, even if the real dimensions are known, our perception remains reliable.
ERA – March 1998 – The effect of prior experience on perceptual set.
Grivas, Down & Carter, 1996 Psychology VCE Units 3 & 4. Macmillan
Morris, 1990 Psychology An Introduction (7th Ed.). Prentice-Hall, Inc.
Worchel & Shebilske, 1986 Psychology Principles and Applications (2nd Ed.) Prentice-Hall, Inc.