Read The Story of Psychology Online
Authors: Morton Hunt
The Story of Psychology (88 page)
FIGURE 28
Bottom-up processing alone would leave us unsure what we were seeing; context—top-down processing—leads us to see the first one as an H, the second one as an A, although they are identical. Similarly, in the ambiguous images we saw above in
FIGURE 24
, it is whatever top-down influence we choose to exert that yields what we finally see.)
After the New Look petered out, perception research revived with the advent of a new and powerful theory, information processing, which in the 1960s and 1970s was beginning to transform cognitive psychology with its vision of an orderly series of processes by which sensations are transformed into thought, and thought to action. This theory postulates (and provides experimental evidence of) the metamorphosis of sensory input in a sequence of steps, including very brief storage in the sense organ, encoding into nerve impulses, short term memory storage in the mind, rehearsal or linkage with known material, long-term memory storage, retrieval, and so on. The theory made it possible for psychologists to be specific about how the mind handles incoming sensory material, and it revived interest in the cognitive approach to perception. By the 1970s
research in the field of cognition was proliferating, as we will see in a later chapter.
But by then many significant discoveries had been made about the physiology of perception. Ever since, the two styles of looking at looking, the physiological and the cognitive, have existed side by side, seemingly opposed to each other but in reality focused on different aspects of the same phenomena, as discussed from here on.
How do we see the shapes of things? The question may seem absurd— how could we
not
see them? But the perception of form is neither automatic nor foolproof. We see a shadowy figure in the park at night and cannot tell whether it is a bush or a lurking person; we read a carelessly scrawled signature and do not know whether it starts with C, G, or O; we arrive home exhausted after a long flight, spot our car in the vast airport parking lot, and trudge toward it, only to find as we draw near that it is a lookalike of another make; we enjoy a jigsaw puzzle precisely because we find it challenging and rewarding to locate the piece that will fit into the edge we have just created.
Research on form perception seeks to identify the mechanisms, both neural and cognitive, that enable us to recognize shapes—and that sometimes fail us. Much of that research in the past half century has taken the cognitive approach. The Gestaltists and their followers explored the mind’s tendency to group related elements into coherent forms, fill in gaps in what we see, distinguish figures from background, and so on. They and others also said it was inborn higher mental processes that account for the “constancy phenomenon”—our seeing things as unchanged despite distortions of the retinal image, as when we perceive a book lying at an angle to us as having square corners although on the retina, as in a photograph, the book is a rhomboid with two acute and two obtuse angles.
But such perceptions are results, not processes. By what steps does the mind achieve them? It is one thing to say that we fill in gaps in familiar but incomplete forms that we see; it is quite another to determine by what specific means we achieve this. Many studies exploring cognitive processing of visual information in fine detail have identified some of them. A few examples of the findings:
—Research on the subjective-contour phenomenon (as in the illusory triangle in
FIGURE 25
above) indicates that we create the imagined contours partly through association (the three angles remind us of previously seen triangles) and partly through clues that experience has taught us signify interposition (an object’s obstructing our view of another one). As the perception researcher Stanley Coren pointed out, the gaps in the circles and in the existing triangle suggest that something else—the illusory triangle—is in the way, partially obscuring them. Because of the apparent interposition, the mind “sees” the imaginary triangle.
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—Some experiments have explored how we recognize a shape we are searching for, particularly when it is lost in a jumble of other shapes. One important process is “feature detection”—conscious searching for known and recognizable elements of a particular figure so as to distinguish it from similar shapes. In each of the following columns there is a single X. If you time your own search for it with a sweep-second hand, you will find that you locate it far faster in the first list than in the second:
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| ORDQCG | WEFIMZ |
| CRUDOQ | EVLMZW |
| QUORDC | VIMWZE |
| CUORCD | ZIVFEW |
| DROCUD | VIZELM |
| DOCURD | MFWIVZ |
| DRGCOD | ZVXIEW |
| ORCDUQ | WVLZIE |
| ODQRUC | EWMZFI |
| DRXOQU | MEZFIV |
| DUGQOR | IWEMVZ |
| RGODUC | WEZMFV |
| GCUDOC | EFLMIV |
| DGOCDR | WZIEFV |
The task of matching the pattern of the X retrieved from memory to what we are looking at is far easier and quicker when the hidden X is among rounded letters than among letters made up of straight lines and angles, like the X itself, in which case we must pay close attention to minor features. Or, as another explanation has it, we
often identify visual images by “preattentive” processes—automatic ones concerned with overall image—but when that does not suffice, we shift to “focused attention” and consciously search for minor distinguishing features of the sought object.
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—In 1954 Fred Attneave, of the University of Oregon, asked subjects to represent certain figures by a series of ten dots; they tended to place the dots on those points where the direction of the outline changed most sharply. Attneave’s conclusion was that one way we recognize patterns is by means of analysis of its “points of change.”
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He also created some figures, greatly simplified from reality, by drawing straight lines from one point of change to another. Although this reduces curves to straight lines, the figures are still immediately recognizable, as in this example:
FIGURE 29
No curve exists, yet one sees a curved object.
—Skilled readers see words as wholes, without identifying them letter by letter, as beginning readers do. But even in rapid reading, a great deal of high-speed feature detection goes on, as shown by experiments conducted by Eleanor J. Gibson (the wife of James Gibson, mentioned above) and colleagues at Cornell in the 1960s. They made up a batch of nonsense monosyllables, some of which obey English rules of spelling and therefore are pronounceable (“glurck,” “clerft”) and then switched the consonant groups around to make others, with the same letters, that violate the rules and are not pronounceable (“rckugl,” “ftercl”). When skilled readers saw the words in tachistoscopic flashes, they could read the legal combinations far more easily than the illegal ones, even though none of the letter groups was a known word. One possible explanation was that they pronounced the words to themselves and
were better able to hold pronounceable ones in short-term memory than unpronounceable ones. But Gibson repeated the experiment at Gallaudet College with deaf students who had never heard words pronounced, and she got the same results. This could only mean that in perceiving each pseudo-word, readers distinguished all the letters and instantly recognized which groups of them obeyed the rules of legitimate patterns of spelling in English and which did not.
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—Researchers working with visual illusions found that if subjects were instructed to look long at an illusion, and in some cases to let their eyes wander back and forth over it, the force of the illusion would wane. Even though the cues in the illusion mislead the mind, attentive looking enables the mind to extract much of the reality from the cues.
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—In the late 1950s and early 1960s, Irvin Rock, a young psychologist who would become a leading figure in perception research, showed subjects a square tilted at 45 degrees and asked them what it looked like; they said a diamond. He then tilted
them
by 45 degrees, causing the figure to be projected as a square on their retinas. But they saw it in a room with respect to which it was tilted and could feel themselves tilted with respect to that room; these two sources of information, processed by the mind, caused them still to see the square as a diamond. This simple experiment profoundly influenced Rock’s thinking about perception and led him to conclude that until perceptual phenomena have been analyzed from a psychological viewpoint, it is premature to do so on a neurophysiological level.
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These findings, and many more from studies made in the succeeding decades, made it clear that form is the most important cue for object recognition. Early in life toddlers learn to identify objects by their shape; they quickly gain the ability to distinguish between a dog and a cat, and having learned what an apple is, they recognize green ones, yellow ones and red ones as apples. Not long ago the psychologist Barbara Landau showed three-year-olds a meaningless shape and told them it was a “dax”; then she showed them other objects with the same shape but made of different materials, sizes, and colors, but the children identified each of them as a “dax.”
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And yet to this day, say Michael Gazzaniga and Todd Heatherton,
“how we are able to extract an object’s form from the image on our retina is still somewhat mysterious.”
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They cite such commonplace mysteries as our ability to recognize objects from different perspectives and in unusual orientations, and to tell where one object ends and another begins, as in the case of a horse and rider. Hypotheses about how we do it are plentiful; proven theories are nonexistent.
From the 1940s on, neurophysiologists were making discoveries about visual perception that were as significant as those of the cognitivists. As early as the 1930s, they were able to record the electrical activity of small groups of nerve cells, and by the 1940s laboratory researchers had perfected glass probes containing electrodes so fine—the hairlike tip might be a thousandth of a centimer in diameter—that they could be inserted into a single cell of the retina, geniculate body, or visual cortex of a cat or a monkey that had been given local anesthetic. With this kind of apparatus, researchers could observe the individual cell’s electrical discharges when the animal was shown a light or some other display.
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This technique produced a historic discovery about form perception. In the late 1950s, David Hubel and Torsten Wiesel, two brilliant neurophysiologists at Harvard Medical School, were testing the responses of visual cortical cells in cats. They would implant a microelectrode in a cell of a cat’s visual cortex; although they could not pick a particular cell, by inserting the probe at about the right spot and right angle they knew what area they were reaching. Wiesel once likened the process to spearing cherries from a bowl with a toothpick; you may not be able to see which cherry you’re spearing, but you’re sure to hit something. The cat would be restrained in a harness while the researchers shone spots or bars of light and other figures on a screen. By securely fixing the position of the cat’s head, the researchers could know which part of the retina the image fell on and link this with the cortical area being probed. Through an amplifier and loudspeaker, they could hear the cell fire; at rest, it might produce a few “pops” per second, but chatter away at fifty or a hundred pops per second when stimulated.
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Since both the retina and the cortex are complicated structures, it took great patience to discover which cells, at what location and in which layer of the cortex, respond to messages from different areas of the retina.
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One day in 1958, this excruciatingly fine-detailed work yielded an astonishing and half-accidental finding. Hubel and Wiesel had positioned an electrode tip in a cell but for hours couldn’t induce rapid firing. As Hubel recalled, a few years ago:
We tried everything short of standing on our heads to get it to fire. (It did fire spontaneously from time to time, as most cortical cells do, but we had a hard time convincing ourselves that our stimuli had caused any of that activity.) To stimulate, we were using mostly white circular spots and black spots. After about five hours of struggle, we suddenly had the impression that the glass with the [black] dot was occasionally producing a response, but the response seemed to have little to do with the dot. Eventually we caught on: it was the sharp but faint shadow cast by the edge of the glass [slide] as we slid it into the slot that was doing the trick. We soon convinced ourselves that the edge worked only when its shadow was swept across one small part of the retina and that the sweeping had to be done with the edge in one particular orientation.
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