THE WORLD AS we perceive it is a stable world,and this stability is present early in life.FOR INSTANCE,a man size does not appear to change much as he walked toward us;A DINNER plate does not look like a circle when viewd from one side and an ELIPSE when viewd from another;AND LOCATION of the sound does not appear to change when we move our heads.STABILITY Of perception help us to adapt to the ENVIORMENT.it would be virtually imposible to operate in a world where sounds changed their locations as we moved our heads and objects changed their sizes and when viewd from different positions and distances.IMAGINE,what it would be like if your friend assumed a multitude of sizes and shapes.THE STABILITY of the enviorment as we perceive it is termed as PERCEPTUAL CONSTANCY.


SIZE CONSTANCY

THE size of the representation,or image,of an object on the retina of an eye depend upon the distance of the object from the eye;THE farther away it is,THE smaller the representation.YET WHEN you cross the street to speak to a friend,your perception of the friend`s size does not change much,EVEN through the retinal representation alter greatly in accordance with the geomatry of the situation.CONTRIBUTING TO this constancy is the great deal of additional information you have about the circumstances:YOU KNOW something about the distance of your friend from you ;YOU perceive the changes that took place in other objects as you approched;AND YOU know how large your friend supposed to be,the friend assumed`s size.



THE IMPORTANCE of the distance and background information in maintaining size constancy was shown in a classical experiment by E H HOLWAYIN the 1940.THEY USED ambiguous stimuls disk of light which could have no real assumedsize and they changed the amount of distance and background information available to the subject in the experiment.THEY FOUND that size constancy decreased as the distance and background information available to the subject decreased.IN OTHER WORDS, The subject perceived the size of the disk of light more in accordance with the size of the retinal representation when they lacked information about distance and background.


ONE INTERPRETATION of this result,might be that peoples somehow automatically use information about distance and background to ''CORRECT'' the size of their retinal representation,thus keeping their perception relatively constant.SO OUR KNOWLEDGE of a fimilar object the assumed size can be important factor in size constancy,especially under condition in which other information is not avaialble.


SIZE CONSTANCY AND ILLUSION,


SOME OF THE illusion have ben explained in term of MISPLACED size constancy,FOR INSTANCE,two horizontal bars are the same length,but we perceive the upper one as longer then the lower one.THIS ILLUSION is said to work because the railroad track converging in the distance provides a strong cues for depth.THUS WE receive information that that upper bar is farther away bthen the lower bar.SO IN THE illusion the horizontal bar are the same length but size constancy leads you to MAGNIFY the distant one.

BRIGHTNESS CONSTANCY,


VISUAL OBJECT ALSO appear constant in their degree of WHITNESS, GRAVNESS,OR BLACKNESS, even though the amount of physical energy reflected from them may change enormously.PEOPLES ARE not like PHOTOELECTRIC CELLS, simply registring the amount of the light being reflected from the surface.OUR EXPERIENCE of the brightness stay relatively constant despite great changes in the amount of physcial energy reaching our eyes.FOR EXAMPLE, objects or surface that appear white in a bright light are still perceived as white in dim illumination.SIMILARLY, what look black to us in dim light still look black in intense light.SO WE have brightness constancy in most situitions,when the illumition changes,it change over the whole feild:THE PHYSCIAL energy ratio between an object and its surrounds stay constant.IN OTHER WORDS, the unchanged brightness ratio give constant brightness experience.


EFFECTS OF LEARNING AND MOTIVATION

WE HAVE idea about perpectual process that are much the same for everyone,but we know that peoples differ in the ways they processsensory inputs to give rise to what they experience.TWO PEOPLES may have very different perception of the same television drama.BASED ON PAST experiences or any special trainning we have had,EACH OF us has learned to emphasized some sensory inputs or to ignore other.ELEANOR GIBSON, has defined perpectual learning as ''AN INEREASE in the ability to extract information from the enviorment as a result of experience or practice with the stimulation coming from it.GIBSON GIVES many example that shows how perception can be changed through learning.SHE CITES the competence of many peoples trained in various occuption to make perpectual distinction that untrained peoples cannot make.


DISTINGUISHING THE call of birds is one of her examples, A TRAINED ornithologist can do it,but most of us have great diffculty.IF YOU ARE lucky enough to live where there are plenty of birds,try shutting your eyes, and listening to the birds dawn and dusk.THEY MAKE THE DEFENING RACKET.you may be able to extract some features that enables you identify particular speaches of birds.BUT FOR THE most part, unless you have special tranning the whistless,trilling, and buzzing will blend together so that you may not be able to distinguish the call of the one bird from another or recognize call upon hearing them again.PERPECTUAL learning is needed before you do it.



IN THE LATE 1940 and throgh the 1950,MANY PSYCHOLOGIST turned their attention to the idea that MOTIVES and NEED influence perception.THIS VIEWPOINT was called the''NEW LOOK'' in perception.ALTHOUGH many of the newlook experiments were flawed and some of their ideas were criticized,THE GENERAL IDEA that individual differences in motives and needs affect perception persist,IN OTHER WORDS,we we may attend to and organize sensory input in a way that match our needs.FOR EXAMPLE, the peoples who are thirsty will try to pay attention to those events in the enviormenmt which will staisfy their needs.


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AN INTRODUCTION TO PSYCHOLOGY BY RICHARD A KING

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''A man is not finished if he is defeated, he is finished if he quits.''