Laboratory for Shape Perception
Purpose: This lab experience is intended to illustrate a number of different aspects of shape perception, including a few approaches to understanding shape perception.
First task: Look
at this stimulus.
Click on this link
to the Intraub stimulus and look at it for a brief time and then continue
with the lab. Don't return to look at this image!
1. Gestalt Principles of Organization
Our experience of the world is quite organized. That is, in spite of the fact that our visual experience is based on the building blocks of receptive fields of varying complexity, we don't experience receptive fields. Instead we perceive objects such as people, desks, etc. What happens to the visual input between receptive fields and objects? The answer to that seemingly simple question is quite complex. One piece of the answer may lie in the Gestalt organzational principles.
As your textbook indicates, people have identified quite a few Gestalt principles of organization. Below you will see figures that illustrate a number of these principles.
Common Fate
(We organize together pieces that move together. Thus, we see the red dots as a unit moving together. However, we see three of the green dots as a unit distinct from the other three green dots, because the three dots move together.)
Uniform Connectedness
According to uniform connectedness, we organize as a single unit those parts of the array that appear to be connected. How do you see each of the four examples? Can you see the operation of uniform connectedness? On your lab worksheet, note how uniform connectedness might conflict with other Gestalt principles in the various examples. How does your ultimate experience of the stimuli help you to understand the functioning of the various Gestalt principles? [You can learn more about the principle of uniform connectedness by reading: Palmer, S. & Rock, I. (1994). Rethinking perceptual organization: The role of uniform connectedness. Psychonomic Bulletin & Review, 1, 29-55.]
On your worksheet, indicate whether each of the following images organizes into rows, columns, or neither. Then, indicate which Gestalt principles seem to be operating to provide that perception.
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You can learn more about Gestalt principles at several different sites. One such site was developed by Chandler.
Figure/Ground Separation. As you may recall, the Gestalt psychologists were also interested in the distinction between figure and ground. And the classic Face/Vase illusion (Rubin, 1915) is an example that supports the adage associated with the Gestalt psychologists, "The whole is different from the sum of its parts." Thus, the important point illustrated by multistable images such as the Face/Vase illusion is that the component features remain constant, but the perceptual experience varies. But just what are the principles that allow us to perceive the edges that form either the faces or the vase? How would you interpret the two possible perceptions of this stimulus, using the Gestalt principles? You can learn more about figure-ground separation at this link.
Camouflage. A related issue is the notion of camouflage. That is, for some stimuli, it's difficult to detect the stimulus because of the background against which it's seen. At the Sandlot Science web page, you can find a number of different illustrations of the difficulty of detecting a camouflaged stimulus. Pay particular attention to camouflage in nature and in war. Then, to see the impact of motion on shape perception, look at the Hidden Dog Puzzle. You may also want to check out the video of a camouflaged octopus.
2. Early Stages of Visual Perception: Segmenting the Scene and Feature Integration Theory
In order to determine figure (as distinct from background), very early in perceptual organization our visual system needs to determine where boundaries or edges might be present in the scene. What cues seem to be critical for determination of such boundaries/edges? Determination of such boundaries should strike you as critical for the delineation of objects. Can you see how the Gestalt principles might be involved in determining boundaries?
In some cases, boundaries or edges seem to leap out from the scene. In other cases, we must engage in some effort to determine the boundaries. This effortless/effortful distinction also arises when we conduct searches for particular objects. The exercises below illustrate the distinction.
a. In the scene below, you should be able to determine four distinct areas with irregular boundaries. However, some of the boundaries seem obvious, and simply "pop out." Other boundaries are less obvious unless you do some additional "work." What makes the "O-Slanted T" boundary more readily apparent than the "T-L" boundary? What makes the "Slanted T-Upright T" boundary more readily apparent than the "T-L" boundary? On the accompanying worksheet, sketch out the boundaries. Then describe the characteristics of the features of the scene that lend themselves to more rapid organization.
b. In the scene below, you will see some distinct areas and some boundaries. On your accompanying worksheet, sketch out the boundaries that you see in the figure below. Given what you observed in 2a, do these boundaries surprise you at all? Look carefully at the figure. What do the boundaries you observe tell you about the factors that determine the boundaries? How would you organize the scene if no colors were present (as is the case on your worksheet)? Sketch out the boundaries that would be present in the absence of color information.
[You can learn more about these principles by reading: Treisman, A. (1986). Features and objects in visual processing. Scientific American, 255, 114B-225.]
c. Your experience of these stimuli should suggest to you that some boundaries are perceived effortlessly and rapidly. Others require more effort, which is to say that you have to do a serial search to determine where the boundaries might be. In a similar fashion, Bob Weber, Jr. draws cartoons (Slylock Fox) in which there are differences between two images, but they are not readily apparent. Find the six differences in the images below and note them in the order in which you spot them. Why is it that the differences don't simply "pop out" but require a serial search?
Bob Weber, Jr. 9/28/2007
d. In the figures below, if you had to locate the "different" object, you probably find the task easier in one display than the other. What principle seems to be in operation? Note that in both cases, all of the stimuli are identical except the one target. So, what is the underlying principle that makes one search relatively easy and the other search relatively difficult? [Learn more about this effect from: Treisman, A. & Souther, J. (1985). Search asymmetry: A diagnostic for preattentive processing of separable features. Journal of Experimental Psychology: General, 114, 285-310.]
e. In the array below, locate the red O. What aspects of the display make locating the red O more difficult? [And does the display make you think "ON" or "NO" ? :-) ]
f. To learn more about visual search, check out Christopher Healy's page, especially regarding preattentive processes and the hierarchy of features. Healy demonstrates 18 different features (e.g., orientation, closure, size) that might lead one to perceive a difference in the stimulus array. Which of those features create more obvious differences? Which create less obvious differences?
g. Anne Triesman's work (some illustrated above) resulted in her Feature Integration Theory, in which we identify primitives in an array based on these preattentive processes. We then combine the features through focused attention to arrive at our perception of the object. Read over Healy's summary of Treisman's theory, so that you can briefly describe the theory. Suppose that you were briefly presented with a stimulus such as this:
AB
Why might you think that instead you'd seen this display (an illusory conjunction)?
AB
3. Perceiving Objects
a. Spatial Frequency Analysis
To illustrate the principle of spatial frequency analysis as it applies to object perception, check out the demonstration at Michael Bach's site. How is spatial frequency information affected by movement toward or away from an object? Can you articulate the relationship between receptive field size and spatial frequency analysis?
b. OK, by this point in the course you should have a sense that edges are important for perception. You should also have a sense that we must organize the edges in order to arrive at objects. As you know from your textbook, a number of theories have been proposed to account for how we arrive at the perception of objects. Some of these theories are more bottom-up (data driven) in their orientation. Some of these theories are more top-down (conceptually driven). First, distinguish between bottom-up and top-down processing.
Below you see a simple illustration of an object. Using only what you know at this point in the course, briefly describe the processes that you think allow you to arrive at your perception of the object. (Don't feel compelled to draw on any of the particular theories that you've studied.) Would you describe your proto-theory as more bottom-up or more top-down?
c. You have surely seen impossible objects, such as the impossible objects at Sandlot Science. Now, briefly describe the principles that allow you (or an artist) to portray such impossible objects. A number of artists create impossible images, a topic we will also address in the next lab. Some of these artists create objects that are multistable (as in the Face/Vase illusion above or the Necker cube) or contain hidden images. Some artists that might interest you include:
d. Many images may be seen at more than one level (global vs. local). Of course, some of these images may also be considered multistable. In psychological research, the global/local distinction in object perception is illustrated in the work of Navon. For example, in the images below, do you see the larger letters (global) or the smaller letters (local)?
Some artists whose work may be viewed on different levels (global/local) are listed below:
e. How accurately do you perceive the world? Surely, you'd be able to detect fairly substantial changes in the world, right? Maybe not! You may have seen references to change blindness on Christopher Healy's page. Rensink has developed a number of demonstrations of change blindness (as well as doing a lot of important research on this phenomenon). How might the difficulty of detecting these changes be similar to finding the differences in Bob Weber's cartoons ("Six Differences")?
4. Contextual Effects in Shape Perception
a. Orientation. Our experience of perceiving objects is typically obtained in one canonical orientation. It is certainly true that our experience with faces is largely with upright faces. For that reason, when we experience a face in an unusual orientation, we're at a distinct disadvantage, as seen in the figure below. You may even have a difficult time determining the person pictured. But in all likelihood, you will not detect that either picture is particularly unusual. However, when the pictures are displayed in the normal orientation, the differences between the two pictures are readily apparent.
You can see some other orientation effects here.
Below are some familiar faces in usual (upright) and unusual (upside-down) orientations. In each case, see how quickly you can name the people in the pictures. You should find the task to be more difficult when the faces are in an unusual orientation.
Write the names of these folks on your worksheet. Then do the same for the next set of faces. Which task was more difficult? Why?
b. Subjective Contours. Yet another sort of contextual effect is that of subjective (illusory) contours. What produces such contours? You might gain a better sense of the underlying operations by examining situations in which the illusory figure is more or less prominent. And, in so doing, you may gain a better sense of how we might perceive all objects. [To learn more about the logic underlying the examples shown below, read Hoffman, D. D. (1998). Visual intelligence: How we create what we see. NY: Norton.]
Here's one example of a scenario under which the subjective contours are more (and then less) prominent:
In each of the sets seen below, indicate which example (left or right) yields a more prominent illusory figure. Then, try to determine what factors are producing the different effects. In other words, how do the two examples in each set differ? Why might that difference lead to differences in the resulting illusory figure?
First Set:
Second Set:
Third Set:
c. Generally, context is an aid to perceiving the world. However, occasionally the context within which we view an object will distort our perception of the object. Here are several examples of the distorting effects of context on our perception of shape.
The Hering Illusion and Variants
5. Scene Perception
We must organize the scene into shapes (objects) in the foreground and background. People are still debating the stage in visual processing at which the scene is organized, but we certainly organize the scene. Think back to the beginning of this lab exercise. You saw spaghetti and sauce on a fork. You don't have to be an artist, but you should be able to draw a rough sketch of the image that you saw earlier. Do so in your lab packet right now. When you are finished drawing, click on this link.
Well, the Vanna on the right looks more like the one we typically see turning letters.