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 fro

m 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.

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.

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.

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.

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

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 dimensio

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