Our visual system does not give a true representation of the external world. Why do we not see an exact representation of the external world and what role does prior knowledge play in perception?
"Our visual system does not give a true representation of the external world". Why do we not see an exact representation of the external world and what role does prior knowledge play in perception?
) Describe vision
Investigation of any physiological system is always an extremely complex task; the visual pathway is here no exception. However, in the past twenty years a lot of conclusive studies have been made in this area which now forms the basis of a firmer fundamental understanding of the processing of visual information from the retina through the cortex. Perception is a very interesting part of the body to study because everything we know from the external world must first come through our senses. This information will be received as sensations arriving at sense organs in the body, such as the eyes, the nose and the ears. This information will be assed on to the brain and the central nervous system (CNS) with the help of sensory receptors which detect the physical properties of the world around us, such as light and sound. Hubel (1988)
As our first contact with the external visual world is through our senses it is important to understand the physical make up of the eye and the visual pathways in the brain.
Visual System
The visual system interprets information from visible light to build a representation of the world that is surrounded by our body. With light we mean a narrow band of electromagnetic spectrum which has a wavelength of between 380 and 760 nanometres and is visible to humans.
However not all lights are visible to humans for instance it is impossible for humans to see other types of electromagnetic energy such as ultraviolet radiation, gamma rays or x-rays. We are able to see various things around us as the light is reflected from these things into our eyes. This reflected light creates an image in the eye which causes electrical signals to trigger a chain of events in the brain which leads to consciousness' perception. The light falls on the back of the eye as two small, upside down, two dimensional images, but what we see is a colourful three dimensional world. Wickens (2000)
To get a better understanding of the visual system it is essential to describe the various organs and their functions starting with the eye.
The eye
As the eye is a very delicate object it is protected externally by the eye lid which keeps out dust and dirt. It also closes in an automatic reflex action if something dangerous comes close to the eye and lies within the bony socket in the skull.
Light entering the eye is refracted as it passes through the cornea. It then passes through the pupil which is controlled by the Iris and is further refracted by the lens. The lens has the job to invert the light and project an image onto the retina.
The pupil is controlled by a ring of coloured muscles called the Iris. The colour of the Iris is genetically determined and her function is to regulate the size of the pupil. The Iris works as a shutter that can be closed down to regulate the amount of light entering the eye. If bright light enters the eye it will cause the Iris to regulate the size of the pupil to a minimum of 2 diameters but it will increase the size under dim light conditions. The lens which is situated right at the back of the pupil has the task of bringing light into focus on the retina at the back of the eye. The lens can change its shape to bring objects at various distances into focus. This process is called accommodation. Wickens (2000)
The retina
The retina is the light-sensitive, multilayered inner surface of the eye. It contains the rods and cones, as well as neurons that form the beginning of the optic nerve.
The rods and cones are visual receptors that transform light into neural impulses. The rods have poor sensitivity; they detect black, white, and grey colours and function well in dim light. The cones on the other hand have excellent sensitivity and enable colour vision. They function best in daylight or bright light.
Because of these two different functional systems it allows humans to see over a wide range of light intensities. Bruce (1996)
The actual photoreceptors are the rods and cones, but the cells that transmit information to the brain are the ganglion cells. The ...
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The rods and cones are visual receptors that transform light into neural impulses. The rods have poor sensitivity; they detect black, white, and grey colours and function well in dim light. The cones on the other hand have excellent sensitivity and enable colour vision. They function best in daylight or bright light.
Because of these two different functional systems it allows humans to see over a wide range of light intensities. Bruce (1996)
The actual photoreceptors are the rods and cones, but the cells that transmit information to the brain are the ganglion cells. The axons of these ganglion cells make up the optic nerve, the single route by which information leaves the eye. Wickens (2000)
The fovea defines the centre of the retina, and is the region of highest visual activity. In the fovea there are almost exclusively cones, and they are at their highest density. In addition, at the fovea all of the other cell types squeeze out of the way to allow the most light to hit the cones. This makes the fovea visible microscopically. Bruce (1996)
Optic Nerve
The optic nerve transmits electrical impulses from the retina to the brain. It connects to the back of the eye near the macula.
The blind spot is the region of the retina where the optic nerve leaves the eye. Because there are no rods or cones in this area, this is a spot where the eye is visually insensitive. The fact that we are unaware of our blind spots indicates that the brain must perceptually "fill in" the missing visual detail. Wickens (2000)
2 +3) Examine why our visual system does not give a true representation of the external world and also consider aspects of the visual world that the brain judges to be important and those which it can ignore.
In order to make sense of complex visual information that we gather around us we need to impose some form of organization incoming stimuli. For instance Gibson (1966) suggests the idea of horizon ratio and movement. He argued that if we see things from close they look the same size but when we see them in a further distance they appear to be smaller. But we know that they are all the same size. All objects of the same height or size, whatever their distance from the observer is cut by the horizon in the same ratio. However objects of different sizes, but at the same distance from the observer also have different horizon ratio. For instance if we look down from a small building everything appears to be a lot smaller than it actually is. But if we look at the same thing again from a different angle or distance it would appear a lot more different. Our brain knows that the size of cars or people do not change it is our vision that is tricking us.
Another aspect of perception Gibson (1966) mentioned is the texture gradient. He argued that the perceptual world is made up of surfaces of different textures and these different textures help us with depth and orientation. If we take a look on the figure below we get the impression that the texture appears to be flat it could be classified as an upright garage door but if we would compacted lines at the top which means put them close together it gives the impression of depth.
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Whereas direct perception theorists believe that cues are picked up directly from the visual array, constructivist would suggest that they are learned through past experiences with objects around us. Some of the so called 'cues' are overlapping which means that when one object blocks the view of an other the blocked object is perceived to be further away. Another cue is linear perspective where parallel lines, such as those at the side of the road or a railway appear to converge in the distance. A different one is called relative size where objects in the distance seem smaller than they actually are, therefore smaller objects are perceived to be further away. Also shadowing plays an important part. As light usually travels in straight lines, which means that surfaces where the sun shines on will appear to be relatively bright and surfaces away from the light source will be in shadow. Bruce (1996)
Constructivist theories believe that successful perception requires not only intelligence but also at the ability to combine sensory information with knowledge based on previous experience.
Another very important part of the visual system is called adaptation which is the ability of the visual system of contrast processing. In order to be able to see clearly it is important to have visual acuity which means the capacity to discern fine details. Visual contrast refers to the difference in brightness levels between adjoining areas. Pinel (1999)
Brightness and colour are important characteristics of human vision, but we also need to be able to detect contours in order to be able to recognise objects in our perceptual world. Contours can have various shapes such as curves or straight. A study by Hubel and Wiesel (1959) traced the path from the receptors in the retina through the ganglion cells and into the visual cortex. In this study they were able to identify several types of cells in the visual cortex, which appear to respond to lines and edges of particular orientations .They called these cells simple cells, complex cells and hyper complex cells. A simple cell and complex cells respond to a line or straight edge and fall to a specific part of the retina, hyper complex cells respond to lines in a particular orientation and of a particular length. Hubel (1988)
In the complex chain of perception there are three main thoughts we have to consider when trying to explain vision and the importance of the brain. The first one is external and physical and includes the travel of electromagnetic waves from the object to the eye. The second is the physical visual apparatus, from eye to brain, consisting of nervous tissue. The third, and most complex, is the interpretation of the visual stimulus and the creation of the internal model of the world that is used by the consciousness.
In the third step, the visual stimulus that we receive from the outside world is combined with information from the memory to create the picture. This is the most important part of vision, as all the interesting parts of the vision occur here. Hubel (1988) For instance we recognise certain things, we know when we see a chair that we can sit on it, when we see an apple we know that we can eat it.
The picture we receive from the external world must be constructed from incomplete information, in fact inferred from clues.
The three-dimensional world is sensed by the two- dimensional retina, emphasizing the central role of depth clues. Wickens (2000) The picture depends on the unconscious recognition of objects, so that what we remember of properties can be transferred to the new object based on the visual hints. Recognition is what gives vision its reality, showing the central role of mind. Wickens (2000)
In the figure below a cubic frame is seen in front of the eight disks even though there is no cubic frame at all. Interestingly, the bars of the frame are seen between the disks even though these regions are physically uniform. The appearance of these illusory bars is the outcome of visual processing in which possible shapes are recovered from fragmented images. Sometimes we are able to see a cubic frame behind the light wall through the eight circular windows. This example demonstrates an important function of our visual system as the brain makes us see a thing as if it exists; it is filling out the missing gaps.
Another very important part that the brain plays in perception is that it turns the upside down picture that we receive on the retina into the right way again. This is a significant difference between seeing and perceiving. Seeing describes the physical side of vision whereas the brain gets involved with perceiving in incoming visual information. Pinel (1999)
As there has been a lot of physiological research, some important research also has been done in the cognitive aspect, such as in the process involved in perception. Some questions occurred such as how we can make sense of the visual stimuli that comes to the sensory system and how are we able to recognize objects and also how we understand the relationships between them?
There are two main types of theory of perceptual organization. One is proposed by Gibson (1966) and his theory of perceptual organization. He proposed the direct theory of perception. According to his view the human mind manipulates and transforms information through a series of processing stages. This theory suggests that there is sufficient information in the sensory stimulus to allow the individual to make sense of their environment without the involvement of stored knowledge. Gibson (1966) felt strongly that it was important to study perception in real world environment.
On the other hand the top down theories emphasize the involvement of context and prior knowledge in interpreting information and derived from the sensory system. In other words prior knowledge determines the interpretation of the sensory data. Neisser (1976)
4) Discuss some illusions and what they can tell us about how the visual system processes information.
Illusions are misperceptions as we are fooled by the cues such as overlapping, relatively height and relatively size.
This illusions show that our visual perception cannot always be trusted. The components of an object can distort the perception of the complete object. Our mind represents the final judge of truth. Some optical illusions are the result of either incongruent design elements at opposite ends of parallel lines or the influence of background patterns on the overall design. Other illusions on the other hand occur because of the adjustment of our perception at the boundaries of areas of high contrast or the inability to interpret the spatial structure of an object from the context provided by the picture. Wickens (2000)
Sometimes a view on an object may not contain enough information for the mind to make a conclusive interpretation. Where there are only two reasonable interpretations, the mind may alternate them, as if unable to make up its mind.
Illusions can also arise from contrast of brightness, as the perception strives to maintain line and shade. The well-known illusion shown above is an example. There are grey patches at every crossing, except for the one you are looking at directly.
Illusions of motion and colour are difficult to illustrate in text. Many colour illusions are due to physical causes, because of the poor spectral resolution of the eye, and also differences in illuminates and pigments.
The figure above was introduced by Kanitza, an Italian psychologist. Everyone sees a white triangle in front of the three black disks and inverted triangle. However, the white triangle actually does not exist. The contours of this triangle are illusory contours created by our brain. The illusory triangle looks brighter than the background. This is a result of brightness contrast caused by the black disks being partially occluded by the corners of the illusory triangle. The reason, why the illusory triangle looks brighter as a whole and not only as its corners is that the brain fills in a uniform surface with certain brightness.
All this evidence shows that our visual system does not give a true representation of the external world. Our brain function tells us what to do and fills in the gap that might not be seen by the visual system. Prior knowledge plays an important part as Gibson (1966) stated in his research and not to forget the cues we have to bear in mind when trying to explain the visual pathways.
Both theories the direct theory and the constructivist theory have been influential in describing vision. We need both to receive a three dimensional accuracy of the world and what is happening around us. We do not only relay on what we pick up directly from the visual array that surrounds us but we also rely on visual stimulus and also on stored knowledge in a problem solving approach to perception.
References
Bruce, V., Green, P.R. and Georgeson, M.A. (1996) Visual Perception: Physiology, Psychology and Ecology (3rd edn.). Hove: Psychology Press
Gross, C.G. (1999) Brain, Vision, Memory: Tales in History of Neuroscience. Cambridge, MA: MIT Press
Hubel, D.H. (1988) Eye, Brain and Vision. New York: Scientific American Library
Pinel, John (1999) Biopsychology. England: Allyn & Bacon
Wickens, Andrew (2000) Foundations of Biopsychology. England: Pearson Education Limited
Zeki, S. (1933) A Vision of the Brain. London: Blackwell
Gibson, J.J. (1966). The Senses Considered as Perceptual Systems. Boston: Houghton Mifflin.
Gibson, J.J. (1979). The Ecological Approach to Visual Perception. Boston: Houghton Mifflin.
Neisser, U. (1976). Cognition and Reality. San Francisco: W.H. Freeman.
Adam J & Bateman L (1983) A correlational analysis of symmetry between the arrowhead and featherhead Mueller-Lyer illusions. Perception 12:119-129
Christie PS (1975) Asymmetry in the Mueller-Lyer illusion: Artifact or genuine effect? Perception 4:453-457
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Rudel R & Teuber HL (1963) Decrement of visual and haptic Müller-Lyer illusion on repeated trials: A study of cross-modal transfer. Quarterly Journal of Experimental Psychology 15:125-131 [this is, to my knowledge, the first paper to suggest the 3-arrows arrangement I used at the top]
Barbara Widholm Introduction to Biological Psychology