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Sensations are in the form of raw neuralenergy and have little meaning of their own.

They must be organized and interpreted inthe process we call perception.

Some aspects of perception our common to

all of us and therefore we experience acommon reality but others are uniquedepending upon our learning and emotionalexperience.

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Therefore the first step in perception is

selection, choosing where to direct our

attention.

Three major factors are involved in the act of

paying attention to certain stimuli and toignore others

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1) Selective attention: In almost every situation thebrain is faced with excess of sensory information,hence it sifts between information, choosing the most

important and discarding the irrelevant . For e.g. A person with a prying nature manages to

focus on the conversation of his interest even if thereare dozens of other people talking . This process is

called selective attention. Or if you are in search of alight green coloured tennis ball in your veranda, youwill ignore all other coloured objects but will closelylook for any green coloured objects which you catchsight of.

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Feature Detectors: respond only to certain

sensory information . This information is

selected by specialized neurons in the braincalled feature detectors .

Feature detectors are found in the temporal

and occipital lobes and are especiallyresponsible in the recognition of faces.

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Most feature detectors are built in but at timesthey need interaction with the environment todevelop normally.

Habituation: The brain is more apt atrecognizing changes in the environment thanwith Information that remains constant.

Interesting examples include the attention wepay to a new song or a new book or even to newrelationships. But after a while our brain getsused to these new changes and habituates topredictable stimuli.

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After selection comes the next phase oforganization .

This particular process of perception formation

was explained by Gestalt psychologists whostudied how humans organized sensoryinformation and perceived things as a whole.

The most fundamental law of organization,

according to them is the tendency to differbetween figure and ground. The words on thisslide are the figures and the white space in thebackground is the ground.

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Continuity:We tend to perceive lines or

patterns that follow a smooth contour as

being part of a single unit. Closure:The tendency to see a finished unit

from an incomplete stimulus.

Similarity: Similar objects are grouped

together.

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We normally perceive the world as a fairly constantand unchanging place even when the sensation of theobjects around us might change.

For example the size of the image that falls on theretina changes as a person walks away from us, butwe do not perceive the person to shrink in size. Theshape of the pot changes when we look at it from

different angles but we do not perceive it to bechanging. The effect of light or dimness on a faircomplexion face does change its effect but ourperception about his/her complexion does notchange.

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All of this happens because human brain has thequality of perceptual constancy.

Following are the types of perceptualconstancies:

Brightness : example of a white paper in dimlight

Colour constancy: red apples seen in dim light Size constancy: the experiment with the Twa

people Shape constancy: example of the coin , the

Ames room illusion

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Our retina is two dimensional that is it can

see up and down, right and left.

How is it that we can perceive a threedimensional world i.e. one that has distance,

width and height.

We can perceive depth through sound or

odour or the movement of air but most of allwe assess distance through our vision.

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Texture gradient: The texture of objects is

larger and more visible up close and smaller

when far away. Linear perspective: Objects cast smaller

images on the retina when they are moredistant. As a result, parallel lines such as

railway tracks appear to grow closer togetherthe farther away they are from us.

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Superposition:Closer objects tend to be partiallyin front of or partially cover distant objects e.g.in the case of tall buildings.

Shadowing:The shadows cast by objectssuggest their depth.

Speed of movement:Objects farther awayappear to move across the field of vision moreslowly than do closer objects.

Aerial perspective:Water vapour and pollution inthe air scatter light waves , giving distant objectsa bluish, hazy appearance compared withnearby objects.

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Accommodation:The shape of the lens of theeye changes to focus visual images on the

retina from stimuli that are differentdistances from the eye. This is known as theprocess of accommodation and provideinformation about the distance of objects.

Convergence:When both eyes are looking atan object in the centre of the visual field ,they must angle inward more sharply for anear object than for a distant object.

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Retinal disparity:Because our two eyes are

almost 2 inches apart they do not see the

same view . This is the retinal disparity. Thebrain fuses the two images received by the

two eyes into one overall image.But theprocess provides important ideas about

depth.

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