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Chapter 1: Experiments in Perception

A basic issue for psychology, as well as for philosophers down through the ages, is how we come to gain knowledge of the world. Knowledge comes through experience with the world, and that experience is what the study of perception seeks to understand. What we know about the world and the things in it is learned through looking, hearing, touching, smelling, and even tasting the world. Babies seem so inordinately fond of tasting the world because their sense of taste and sense of touch in their mouths are better developed than their other senses. The act of perception begins with our sense organs such as the eye, the ear, and the vestibular system that helps us balance. Those organs are sensitive to different aspects of the external world (and our own bodies), and translate experience into signals to the brain to indicate what is currently being sensed.

But perception is more than just a passive receiving and interpreting of sensory signals. Perception is also an activity that we engage in. Not only do we actively move our eyes to gain new information, be we also move our bodies in order to use our senses most effectively. We move so that we can look around things when we want to know what is behind them, and climb hills to see what is on the other side. Perception is thus an activity that we control (though imperfectly). We can select what parts of the world we perceive by moving in the world, but we can also restrict attention to only part of what we could currently perceive (for example, paying the most attention to a single instrument while listening to an orchestra). And perceiving things also involves knowing what they are�in short, recognizing them. So memory plays a role in perception, as well.

Various aspects of perception, how we actively use perception, and how memory and perception are related are explored in the experiments and exercises that follow.

Psychology had an early concern for mental imagery, but problems with that research led to the virtual abandonment of the study of imagery. More recently, researchers have found ways to measure certain aspects of imagery. One successful technique uses "rotation" of mental images. A person is shown two shapes, and must decide if they are the same or not (regardless of rotation). One shape is sometimes rotated with respect to the other, and people report that they make the judgment by rotating their image of one shape to see if it matches the other. If peoples' reports are accurate, it should take longer to rotate the image if the stimulus is rotated farther from straight-up. Shepard and his colleagues have done a number of experiments of this type, and find that images do seem to be rotated, and at a steady, measurable speed. This exercise has students do two versions of this task to measure the speed of rotation of mental images.

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How many windows are in your family home? Take a moment to answer that question before you continue. In order to answer that question, you probably constructed an image of each room in your house, or of the outside of it viewed from various directions, and counted the number of windows you "saw." That's an example of a mental image. Other examples might be imagining a friend's face, or picturing to yourself which buildings you have to walk by to go to the library.

Research on mental imagery goes back to the very beginnings of psychology and the use of trained introspection as a way to explore the contents of consciousness. The introspective method fell on hard times early in the twentieth century, partly due to the rise of Behaviorism as the principal point of view in psychology (which in turn was partly due to problems with the method of introspection itself). For the behaviorists, psychology was to be restricted to studying the relations between stimuli and responses, defined as overt events in the physical world that could be measured in a physical way. Behaviorists banished the study of mental processes from their psychology, because they believed that such events are fundamentally private--they cannot be observed directly by anyone else, and so cannot be measured by independent observers. With the rejection of the study of the contents of consciousness came a rejection of the study of mental imagery.

The revival of interest in cognition, and the rise of cognitive psychology to a prominent place in experimental psychology, reflected a rejection of the strong claims of the behaviorists. However, the reliance on public events (stimuli and responses that can be observed by more than one person) and the rejection of introspection as a formal research tool remain. Cognitivists are willing to use those stimulus-response measurements to try to infer how cognitive processing works, including how mental images are manipulated. Many thorny problems remain in regard to mental images, but one approach to studying them has met with considerable success, that is the study of how people manipulate mental images. Specifically, these studies are concerned with the rotation of mental images.

If mental images are in some ways analogous to direct perceptions of objects, then images should follow some of the same rules that govern perception of physical objects. For example, if I showed you a picture of someone and asked you who it was, but I handed you the picture upside-down, you would probably have to turn the picture right-side up in order to recognize the face. That operation (rotating the picture) would require some time. Do mental images operate in a similar fashion?

A series of studies suggests that they do. Cooper and Shepard (1973) briefly presented subjects with pictures of letters that were rotated left or right in varying degrees between right-side up and up-side down. The letters also varied in that some were mirror images of actual letters. The subject's task was to report whether the letter was normal or mirror-image. The main question was whether subjects would take longer to make that judgment when the letters were rotated further from vertical. The result was that the farther the letter was rotated, the longer it took to make the judgment about whether it was normal or mirror-image.

Shepard and Metzler (1971) did a similar experiment, except that they presented two figures at once, and subjects had to decide whether the two figures were the same or not.

The figures varied in that one was rotated with respect to the other. Again, the greater the rotation, the longer it took to identify two figures as the same or different. The experiment in which you will participate in this exercise is similar to the Shepard and Metzler (1971) experiment, except 2-dimensional geometric forms are used (see Figure 1).

An issue of considerable interest to both cognitive scientists and neuroscientists is whether the production of mental images uses any of the brain mechanisms used during actual perception. That is, are some of the same parts of the brain used when we actually see an object also used when we

imagine that same object? Some evidence suggests that this is true. Kosslyn, Alpert, Thompson, Maljkovic, Weise,, Chabris, Hamilton, Rauch, and Buonanno (1993) showed similar patterns of activation of cerebral cortex during both a perceptual and an imaginal version of the same task, using PET scans (see Figure 2).

A similar result was reported by Le Bihan, Turner, Zeffiro, Cuenod, Jezzard, and Bonnerot (1993), using the functional Magnetic Resonance Imaging (fMRI). They showed an increase in activation of visual cortex but not of non-visual cortex during both perception and imagery, though the activation was slightly less with imagery. Kosslyn and Sussman (1995) review other evidence for the intimate role of imagery in everyday perception.

Georgopulous, Lurito, Petrides, Schwartz, and Massey (1989) studied activation of single neurons in the motor cortex of a monkey trained to perform a version of a mental rotation task. As the monkey prepared to moves its arm in a certain way, there were changes in activation of neurons along a trajectory, as would be expected if there was an anticipation of moving the arm through a series of locations.

Sex differences in the mental rotation task are another fascinating development. Studies consistently find rather large differences in reaction time for mental rotation tasks, favoring males. For the version of the task used in the experiment accompanying this chapter, the difference is usually about 200 msec. This difference in mental rotation is one of the largest sex differences found in cognitive tasks (Hyde & McKinley, 1997). The cause of the sex difference in mental rotation is not known, though there has been a lot of speculation about both biological and psychological causes (Halpern, 2000, Caplan, Crawford, Hyde, & Richardson, 1996). Research on an analogous task suggests that the sex difference may be reduced or eliminated by practice (Kass, Ahlers, & Dugger, 1998).

There have been two general types of presentations used in studies of mental rotation. Cooper and Shepard (1973) presented single pictures, with subjects comparing them to a stored representation of an upright object, such as a letter. Shepard and Metzler (1971) presented two stimuli at a time, with one rotated relative to the other, as in the experiment accompanying this chapter. Note that this permits the use of arbitrary, abstract stimuli, for which there is no defined �right-side up.� This has the advantage of not relying on subjects� memories, but also has the disadvantage that it is harder to be sure which figure is being rotated�perhaps the subject rotates both at once.

Expected Running Time = 20 minutes

Questions

1. What are the dependent variable and independent variables in this experiment? What are some important controls?

2. Can we rotate images in more than the two dimensions used in this experiment? Shepard and Metzler (1971) performed such an experiment, and you should refer to their article.

3. What is the speed of rotation of the shapes? Calculate this in degrees per second. Was the speed constant? Compare your results to Shepard and Metzler's (1971) findings with "picture-plane" pairs. If there is a difference, how might it have been influenced by the nature of the shapes used? Cooper (1975) also compared a number of different images. This experiment used two of her shapes.

4. How clear are your mental images? Discuss this with your classmates. You may find that some people claim that they "see" their images almost as clearly as if they were actually looking at the object. Others will report that they have only rather vague images. These reports are like the introspection used by early psychologists. What sorts of problems are there in interpreting these reports?

Extension Experiments

Because this experiment involves the presentation of bitmapped image files, it is possible for you to make many modifications to the stimulus materials, using the existing program. In order to do this, you will need to have your stimuli stored as .BMP files. If you use a graphics program to make the stimulus files, simply save them as .BMP files. If you need to draw your stimuli freehand, or copy them from another source, use a scanner to read them into a graphics program, where you can adjust the size and save the result as a .BMP file. Note that you will need to make copies of the stimuli in all necessary rotations. Most graphics programs will easily permit you to rotate an image by a specified amount.

1. Is the speed of rotation dependent on the specific shapes used? (Note that in this experiment, the data were averaged across shapes but the data as to what shape was used on each trial was stored by the computer, and could be subject to analysis.) Cooper (1975) compared a number of different shapes. The two used in this experiment were her Forms B and D.

2. What are the results for non-visual presentation? Specifically, what happens if the shapes are presented haptically? See Carpenter and Eisenberg (1978).

3. Another task that has been used to study the manipulation of mental images has been mental paper-folding, in which a subject is shown a diagram of a box that has been unfolded into a flat surface. Two edges are marked, and the subject must decide whether those edges would meet if the object were folded into a box again. For this task, it seems that you must make a series of discrete manipulations of the mental image, rather than a continuous change such as rotation. Shepard and Feng (1972) did an experiment using this task, and found that there was an increase in reaction time as the number of �folds� required was increased. Stimuli such as they used can easily be drawn on paper. Because of the time required to do this task, a stop-watch can record the reaction times accurately enough.

Caplan, P. A., Crawford, M., Hyde, J. S., & Richardson J. T. E. (Eds.). (1997). Gender differences in human

Carpenter, P. A. & Eisenberg, P. (1978). Mental rotation and the frame of reference in blind and sighted individuals.

Cooper, L. A. (1975). Mental rotation of random two-dimensional shapes. Cognitive Psychology, 7, 20-43.

Cooper, L. A. & Shepard, R. N. (1973). Chronometric studies of the rotation of mental images. In W. G. Chase (Ed.),

Halpern, D. F. (2000). Sex differences in cognitive abilities (3^rd ed.). Hillsdale, NJ: Lawrence Erlbaum Associates.

Hyde, J. S. & McKinley, N. M. (1997). Gender differences in cognition: Resultsfrom meta-analysis. In P. A.

Caplan, M. Crawford, J. S. Hyde, & J. T. E. Richardson (Eds.), Gender differences in human cognition (pp. 30-51).

Kass, S. J., Ahlers, R. H., & Dugger, M. (1998). Eliminating gender differences through practice in an applied visual

Kosslyn, S. & Sussman, A. (1995). Roles of imagery in perception: Or, there is no such thing as immaculate perception. In M. S. Gazzaniga (Ed.), The cognitive neurosciences (pp. 1035-1042). Cambridge, MA: MIT Press.

Kosslyn, S., Alpert, N., Thompson, W., Maljkovic, V., Weise, S., Chabris, C., Hamilton, S., Rauch, S., & Buonanno,

S. (1993). Visual mental imagery activates topographically organized visual cortex: PET investigations. Journal of

Le Bihan, D., Turner, R., Zeffiro, T. A., Cuenod, C. A., Jezzard, P., & Bonnerot, V. (1993). Activation of human primary visual cortex during visual recall : A magnetic resonance imaging study. Proceedings of the National Academy of Sciences USA, 90, 11802-11805.

Paivio, A. (1971). Imagery and verbal processes. New York: Holt, Rinehart & Winston.

Pylyshyn, Z. W. (1973). What the mind's eye tells the mind's brain: A critique of mental imagery. Psychological Bulletin, 80, 1-24.

Shepard, R. N. & Feng, C. A. (1972). A chronometric study of mental paper folding. Cognitive Psychology, 3, 228-243.

James D. St. James	Walter Schneider	Amy Eschman
Millikin University	University of Pittsburgh	Psychology Software Tools

Chapter 1: Experiments in Perception

Examination Version: DO NOT REPRODUCE