SENSORY,ATTENTIAL AMD PERCEPTIONAL PROCESSES

Activity . P ERCEPTUAL C ONSTANCIES The sensory information that we receive from our environment constantly changes as we move around. Yet we form a stable perception of an object seen from any position and in any intensity of light. Perception of the objects as relatively stable in spite of changes in the stimulation of sensory receptors is called perceptual constancy.

Here we will examine three types of perceptual constancies that we commonly experience in our visual domain. Size Constancy The size of an image on our retina changes with the change in the distance of the object from the eye. The further away it is, the smaller is the image. On the other hand, our experience shows that within limits the object appears to be about the same size irrespective of its distance.

For example, when you approach your friend from a distance, your perception of the friend’s size does not change much despite the fact that the retinal image (image on retina) becomes larger. This tendency for the perceived size of objects to remain relatively unchanged with changes in their distance from the observer and the size of the retinal image is called size constancy. Shape Constancy In our perceptions the shapes of familiar objects remain unchanged despite changes in the pattern of retinal image resulting from differences in their orientation. For example, a dinner plate looks the same shape whether to individual; these are called personal illusions.

In this section, we will describe some important visual illusions. Geometrical Illusions In Fig. . the Muller-Lyer illusion has been shown.

All of us perceive line A as shorter than line B, although both the lines are equal. This illusion is experienced even by children. There are some studies that suggest that even animals experience this illusion more or less like us. Besides Muller-Lyer illusion, several other visual illusions are experienced by human beings (also birds and animals).

In Fig. . you can see the illusion of vertical and horizontal lines. Although both the lines are equal, we perceive the vertical line as longer than the horizontal line.

Fig. . : Vertical-Horizontal Illusion to be important. In the absence of these, the light points will not appear as moving.

They will appear either as one point, or as different points appearing one after another, without any experience of motion. Experience of illusions indicates that people do not always perceive the world as it is; instead they engage in its construction, sometimes based on the features of stimuli and sometimes based on their experiences in a given environment. This point will be further made clear in the section that follows now. S OCIO -C ULTURAL I NFLUENCES ON P ERCEPTION Several psychologists have studied the processes of perception in different socio- cultural settings.

The questions they try to answer through these studies are: Does perceptual organisation of people living in different cultural settings take place in an uniform manner? Are the perceptual processes universal, or they vary across different cultural settings? Because we know, people living in different parts of the world look different, many psychologists hold the view that their ways of perceiving the world must be different in some respects. Let us examine some studies relating to perception of illusion figures and other pictorial materials.

You are already familiar with Muller-Lyer and Vertical-Horizontal illusion figures. Psychologists have used these figures with several groups of people living in Europe, Africa, and many other places. Segall, Campbell, and Herskovits carried out the most extensive study of illusion susceptibility by comparing samples from remote African villages and Western urban settings. It was found that African subjects showed greater susceptibility to horizontal-vertical illusion, whereas Western subjects showed greater susceptibility to Muller-Lyer illusion.

Similar findings have been reported in other studies also. Living in dense forests the African subjects regularly experienced verticality (e.g., long trees) and developed a tendency to overestimate it. The Westerners, who lived in Fig. .

: Muller-Lyer Illusion A B Apparent Movement Illusion This illusion is experienced when some motionless pictures are projected one after another at an appropriate rate. This illusion is referred to as “ phi-phenomenon ”. When we see moving pictures in a cinema show, we are influenced by this kind of illusion. The succession of flickering electrical lights also generate this illusion.

This phenomenon can be experimentally studied with the help of an instrument by presenting two or more lights in a succession. For the experience of this illusion, Wertheimer had reported the presence of appropriate level of brightness, size, spatial gap, and temporal contiguity of different lights