PERCEPTION:

Perceptual Organization

Constructing Our View of the World

Consider the vase shown in Figure below for a moment. Is it a vase’! Take another look, and instead you

may see the profile of two people.

Now that an alternative interpretation has been pointed out you will probably shift back and forth

between the two interpretations. Reason for these reversals is this: Because each figure is two dimensional,

the usual means we employ for distinguishing the figure (the object being perceived) from the ground (the

background or spaces within the object) do not work. The fact that we can look at the same figure in more than one way illustrates an important point. We do not just passively respond to visual stimuli that happen to fall on our retinas. Instead, we actively try to organize and make sense of what we see. We turn now from a focus on the initial response to a stimulus (sensation) to what our mind make of that, stimulus. Perception is a constructive process by which we go beyond the stimuli that are presented to us and attempt to construct a meaningful situation

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The Gestalt Laws of Organization

Some of the most basic perceptual processes operate according to a series of principles that describe how we organize bits and pieces of information into meaningful wholes. These are known as gestalt laws of organization, set forth in the early 1900s by a group of German psychologists who studied patterns, or gestalts (‘Wertheimer, 1923). They discovered a number of important principles that are valid for visual (as well as auditory) stimuli:

Gestalt principles of Organization

Most common principles of gestalt theory are;

Proximity


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The law of proximity says that items which are close together in space or time tend to be perceived as belonging together or forming an organized group.

Similarity

This law says that same things are considered one thing. Similar items tend to be organized together.

Good continuation

The tendency to perceive a line that starts in one way as continuation in the same way.

Closure

It refers to perceptual processes that organize the perceived world by filling in gaps in stimulation.

Good Form

It is a type of closure. We fill in the gaps perceive form rather than disconnected lines.

Symmetry

It says that there is a tendency to organize things to make a balanced or symmetrical figure that includes all the parts. Feature Analysis: Focusing on the Parts of the Whole

A more recent approach to perception, feature analysis considers how we perceive a shape, pattern, object, or scene by reacting first to the individual elements that make it up. These individual components are then used to understand the overall nature of what we perceive. Feature analysis begins with the evidence that individual neurons in the brain are sensitive to specific spatial configurations, such as angles, curves, shapes, and edges The presence of these neurons suggests that any stimulus can be broken down into a series of component features for example, the letter “R” is a combination of a vertical line, a diagonal line, and a half circle According to feature analysis, when we encounter a stimulus such as a letter our brain’s perceptual processing system initially responds to its component parts. Each of these parts is, compared with information about components that is stored in memory. When the specific components we perceive match up with particular components that we have encountered previously, we are able to identify the stimulus (Spilltnanll & Wt.mer, 1990; Uman, 1996).

In short

In this model, Stimuli are thought of as combination of elemental or primitive features.

  • The features for alphabets may consist of horizontal lines _
  • vertical lines I
  • Lines at approx 45 degree angle /

• And curves ( Like alphabet T has two line one vertical and other is horizontal line both make alphabet T. All letters are consisted on these four patterns. Our mind analyzes letters according to these features.

Example

Many other details are unimportant So,

A A A A A A A A A All of the above can be recognized as the same letter. Even they are in different shapes and different styles. In the feature model we do not need a template for each letter but only for every feature, this would be a great saving. We have 26 letters small and capital. If letters have many features in common, subjects are prone to confuse them. By these four features we can store all 26 letters. English model is also applicable in Urdu letters with the addition of Dot (nukta). Urdu writing is much more complex than English. Urdu reading is also difficult as well. The feature model has a number of advantages over the template models. First, since the features are simpler, it is easier to see how the system might try to correct for the kinds of difficulties caused by templates model. A second advantage of the feature combination scheme is that it is possible to specify those relationships among features that are most critical to the pattern.

Top down and bottom up processing In top-down processing, perception is guided by higher-level knowledge, experience, expectations, and motivations. we are able to figure out the meanings of the sentence with the missing letters because of our prior reading experience, and because written English contains redundancies. Not every letter of each word is necessary to decode its meaning. Moreover, your expectations played a role in your’ being able to read the sentence. You were probably expecting a statement that had something to do with psychology, and not the lyrics to a Grateful Dead song.

Top-down processing cannot occur on its own. Even though top-down processing allows us to fill in the gaps in ambiguous and out-of-context stimuli, we would be unable to perceive, the meaning of such stimuli without bottom-up processing. Bottom-up processing consists of recognizing and processing information about the individual components of the stimuli. We would make no headway in our recognition of the sentence without being able to perceive the individual shapes that make up the letters. Some perception, then, occurs at, the level of the patterns and features of each of the separate letters.

It should be apparent that top-down and bottom-up processing occur simultaneously, and interact with each other, in our perception of the world around us (Kimchi. 1992; Egetll & Yantis, 1997). It is bottom-up processing that permits us to process the fundamental characteristics of stimuli, whereas top-down processing allows us to bring our experience to bear on perception. And as we learn more about the complex processes involved in perception, we are developing a better understanding, of how our brain continually interprets information from our senses and permit us to make responses appropriate to the environment (Rees, Frith, & Lavie. 1997).

Imagine if every time an object changed we had to completely reprocess it. The next time you walk toward a building, you would have to re-evaluate the size of the building with each step, because we all know as we get closer, everything gets bigger. The building which once stood only several inches is now somehow more than 50 feet tall.

Luckily, this doesn’t happen. Due to our ability to maintain constancy in our perceptions, we see that building as the same height no matter what distance it is. Perceptual constancy refers to our ability to see things differently without having to reinterpret the object’s properties. There are typically three constancies discussed, including size, shape, brightness.

Size constancy refers to our ability to see objects as maintaining the same size even when our distance from them makes things appear larger or smaller. This holds true for all of our senses. As we walk away from our radio, the song appears to get softer. We understand, and perceive it as being just as loud as before. The difference being our distance from what we are sensing.

Everybody has seen a plate shaped in the form of a circle. When we see that same plate from an angle, however, it looks more like an ellipse. Shape constancy allows us to perceive that plate as still being a circle even though the angle from which we view it appears to distort the shape.

Brightness constancy refers to our ability to recognize that color remains the same regardless of how it looks under different levels of light. That deep blue shirt you wore to the beach suddenly looks black when you walk indoors. Without color constancy, we would be constantly re-interpreting color and would be amazed at the miraculous conversion our clothes undertake.

Depth Perception: Translating 2-D to 3-D

As sophisticated as the retina is the images projected onto it are flat and two-dimensional. Yet the world around us is three dimensional, and we perceive it that way. How do we make the transformation from 2-D to 3-D?

The ability to view the world in three dimensions and to perceive distance, a skill, known as depth perception, is due largely, to the fact that we havae two eyes. Because there is a certain distance between the eyes, a slightly different image reaches each retina. The brain then integrates these two images into one composite view. But it does not ignore the difference in images, which is known as binocular disparity ‘This disparity allows the brain to estimate the distance of an object from us.

You can get sense of binocular disparity for yourself. Hold a pencil at arm’s length and look at it first with one eye and then with the other. There is little difference between the two views relative to the background. Now bring the pencil just 6′ inches away from your face, and try the same thing. This time you will perceive a greater difference between the two views.

The fact that the discrepancy between the images in the two eyes varies according to the distance of objects that we view provides us with a means of determining distance. If we view two objects, and one is considerably closer to us than another, the retinal disparity will be relatively large and we will have a greater sense of depth between the two. On the other hand, if the two objects are at similar’ distance from us, the retinal disparity will be’ minor, and we wi11 perceive them as being at similar distance from us.

Filmmakers, whose medium compels them to project image in just two dimensions; have Filmmakers,tried to create the illusion of depth perception by using two cameras, spaced slightly apart, to produce slightly different images, each destined for different eye. In a 3-D movie, the two images are, projected simultaneously, this produces a double image, unless special glasses are worn to allow each image to be viewed by, the eye for which it is intended. The special glasses-familiar to moviegoers since the first 3-D movie, Bawana Devil, appeared in 1952-provide a genuine sense ‘of depth. Similar techniques are being developed to show 3-D movies on television.

In some cases certain cues permit us to obtain the sense of depth and distance with just one eye these cues are known as monocular cues. One monocular cue, motion parallax, is the change in the position of an object on retina as the head moves from side to side. The brain is able to calculate the distance of the object by the amount of change in the retinal image.

Movement perception

It is process through which humans and other animals orient themselves to their own or others’ physical movements. Most animals, including humans, move in search of food that itself often moves; they move to avoid predators and to mate. Animals must perceive their own movements to balance themselves and to move effectively; without such perceptual functions the chances for survival would be sharply reduced

Visual cues to movement

The eye is by far the most effective organ for sensing movement. Some animals are especially sensitive to visual stimuli that move in specific ways. For instance, electrical patterns from the eye of a frog show that some elements in the organ respond only when the stimulus is about the size of a fly moving in the insect’s range of speed. Generally the eyes of lower animals seem to respond selectively to what is of importance to survival. In these animals the eye’s retina does much of the visual processing. This is an economical arrangement since the animal tends to respond only to essential stimuli, the brain having little to do but relay signals to the motor system. It is an inflexible mechanism, however; higher animals process visual information in more elaborate ways, the brain being more heavily involved. Thus, some cells in the visual area of the cat’s brain respond only to moving stimuli, sets of movement-detector cells functioning specifically for each direction across the field of view. Features of human visual experience also suggest that movement detectors exist in the human brain.

Perceptual Illusions: The Deceptions of Perceptions

Perceptual illusions and ambiguous figures were of special interest to the Gestaltists. Artists have also been fascinated by these perceptual phenomena. Perceptual illusions and ambiguous figures are of special interest in the investigation of thinking because: Illusions seem to indicate that our mind does not always accurately represent the perceptual input. For the Gestaltist, this suggested that the mind was “actively” involved in interpreting the perceptual input rather than passively recording the input.

Müller-Lyer Illusion The Müller-Lyer illusion is an optical illusion consisting of nothing more than an arrow. When viewers are asked to place a mark on the figure at the mid-point, they invariably place it more towards the “tail” end. Another variation consists of two arrow-like figures, one with both ends pointing in, and the other with both ends pointing out. When asked to judge the lengths of the two lines, which are equal, viewers will typically claim that the line with inward pointing arrows is longer. One possible explanation is that one sees the lines as three-dimensional, such as the outgoing and ingoing corners of a room. Another possible explanation is that the line with arrows pointing inwards may simply appear longer because the arrows themselves extend past the line.

experimental psychology  PERCEPTION:

Ebbinghaus Illusion

experimental psychology  PERCEPTION:

A B You probably perceive the middle circle as smaller in the figure B than the circle in the center of the figure

A. They are actually the same size.

Poggendorff Illusion

This illusion was discovered in 1860 by physicist and scholar JC Poggendorff, editor of Annalen der Physik und Chemie, after receiving a letter from astronomer F. Zöllner. In his letter, Zöllner described an illusion he noticed on a fabric design in which parallel lines intersected by a pattern of short diagonal lines appear to diverge (Zöllner’s illusion). Whilst pondering this illusion, Poggendorff noticed and described another illusion resulting from the apparent misalignment of a diagonal line; an illusion which today bears his name. The Poggendorff Illusion is an optical illusion that involves the brain’s perception of the interaction

experimental psychology  PERCEPTION:

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