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?

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"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.
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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 ...

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