The Story of Psychology (89 page)

In short, the cell responded strongly to a horizontal line or edge but only weakly or not at all to a dot, a tilted line, or a vertical line.

Hubel and Wiesel (and other researchers) went on to show that some other cells are specifically responsive to lines at an angle or to vertical lines or to right angles or to distinct edges (where there is a contrast between an object and what surrounds it). It became clear that the cells of the visual cortex are so specialized that they respond only to particular details of images on the retina. Hubel and Wiesel won a Nobel Prize in 1981 for this and related brain research.

One bizarre offshoot of the Hubel and Wiesel work was the notion of the “grandmother cell”—a parody, by J. Y. Lettvin, at the time of Hubel and Wiesel’s work, of what he considered the simplistic notion that single neurons in the brain might detect and represent every object, including one’s grandmother. The parody had enough appeal to be seriously considered by some perception specialists but actually became shorthand for all the overwhelming practical arguments against a one-to-one object coding scheme.
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In any case, Hubel and Wiesel’s line-detector cells are a proven reality. Interestingly, this response is partly acquired, even though it is neurological. In a 1970 experiment kittens were reared in a vertical cylinder lined with vertical stripes and never saw horizontal lines. When they were tested for vision, at five months, they were blind to horizontal lines or objects. The neural explanation is that the cortical cells that respond to horizontal lines had failed to develop during the early stages of the kittens’ lives.
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Similarly, people reared in cities have more exposure to
vertical and horizontal lines during early childhood than to lines oriented otherwise, and develop a greater sensitivity to the former. A research team tested a group of city-reared college students and a group of Cree Indians who grew up in traditional tents and lodges with few verticals and horizontals. The city-reared students exhibited the oblique effect; the Crees did not.
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You can also experience the specificity of the vertical, horizontal, and oblique detector cells of your retina by staring fixedly at the center of this pattern:

FIGURE 30
A pattern that confuses the retina’s line-detector cells

The whirling and vibrating you see are probably due to the fact that when you look at the center, where rays of varied angles are close together, the eye’s continual movements cause the image on the retina to shift from one kind of angled line to another, sending a jumble of signals that confuses the cortical receptors of specialized directional sensitivity.

The line detection ability of specific neurons is also exemplified by the following two displays, in each of which one object “pops out” because its lines have a unique stimulus property for those neurons:

FIGURE 31
Line Detection: The one inconsistent figure in each set “pops out.”

The microelectrode technique enabled neurophysiologists to decipher the architecture of the visual cortex—the neurons are arranged vertically, about a hundred in a column, and in layers that run through the columns—and to measure the responses of neurons in every part of the visual cortex to a broad variety of stimuli. The result was a detailed picture of how different cells in different parts of the visual cortex distinguish among all sorts of shapes, contrasts in brightness, colors, movements, and depth cues. A neuron-to-neuron and column-to-column synaptic hook-up of immense complexity links the responses of all these cells, presenting the brain with a composite message of the coded information of what had been a retinal image.
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Where and how that assembled message is “seen” by the mind was not apparent, although it was clear, from much of the cognitive perception research, that the specialized responses of the visual cortical cells are not the final product, at least not in human beings. In simple animals the neuronal responses may be enough to produce appropriate action (either flight or attack). In human beings, the neural messages are often meaningless until they are interpreted by cognitive processes. In the case of the illusory triangle, the viewer’s mind, not cortical cells, supplies the missing parts of the figure. The same is true of many other incomplete or degraded images, where the viewer, consciously invoking higher mental processes, fills in the missing parts and sees what is not there. A case in point:

FIGURE 32
A degraded image. What is it?

At first, most people see this figure (by Irvin Rock) as a meaningless array of dark fragments. How the reversal to the white regions and to perception of the hidden word comes about is not known, but once it has been seen, the mind is almost unable to see the figure again as meaningless fragments.

The metaphor of the eye as camera implies that we see the world in snapshots, but our visual experience is one of unbroken movement. Indeed, the perception of our movement through the environment and the movement of things in the environment is one of the most important aspects of seeing. Vision without perception of movement would be almost valueless, perhaps even worse than no vision, to judge from a rare case reported in the journal
Brain
in 1983.

The patient was a woman who was admitted to a hospital after experiencing severe headaches, vertigo, nausea, and, worst of all, a disabling loss of the perception of movement. A brain scan and other tests showed that she had suffered damage to a part of the cerebral cortex outside the primary visual receiving area that is known to be crucial to movement perception.
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From the report:

Even without such evidence, we can tell that movement perception is of paramount importance. Perception of our own movement guides us in making our way through our environment; perception of objects coming toward us enables us to escape harm; perception of the movement of our hands provides data vital to control when we are reaching for an object or doing fine manual work; perception of our minute bodily movements when standing keeps us from weaving or losing balance. (If you stand with your feet close together and shut your eyes, you will find it difficult to remain perfectly steady.)

Much research on movement perception for the past half-century has dealt with external variables: how the size, speed, location, and other characteristics of moving objects affect the way they appear to us. Such research is akin to psychophysics: it gathers objective data but says nothing of the internal processes responsible for the experiences. Still, it has provided important clues to those processes, both of the innate neural and the acquired cognitive kinds.

A typical finding about an innate low-level process: Researchers projected a shadow or boxlike figure on a screen in front of infants, then made the shadow or figure rapidly expand. When it did, the infants reared back as if to avoid being hit. The reaction is not a result of experience; a newborn who has never been hit by an approaching object will react in this fashion, as will many young and inexperienced animals. The avoidance response to a “looming” figure is evidently a protective reflex built into us by evolution; the visual impression of an object coming at us triggers escape behavior without involving higher mental processes.
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A typical finding about an acquired high-level process: In 1974 the psychologists David Lee and Eric Aronson built a floorless little room that could be slid one way or another across an unmoving floor. When they placed in it a toddler of anywhere from thirteen to sixteen months and slid the room in the direction he or she was facing—that is, away from the child’s face—the child would lean forward or fall; if they slid it in the other direction, the child would lean backward or fall. The explanation is that when the walls moved away, the child felt as if he or she were falling backward and automatically tried to compensate by leaning
forward, and vice versa. This seems to be acquired behavior. The child learns to use “optic flow” information when beginning to walk. (Optic flow is the movement of everything within our visual field when we move. As we walk toward some point, for instance, everything around it expands outward toward the limits of our vision.)
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These and other fruitful studies of movement perception revealed additional defects of the long-held notion that the eye is a kind of camera. One such defect is that although the eye has no shutter, moving objects do not cause a blur, nor do we see a blur when we move our eyes as we do on a photograph if the camera is moved during exposure. Accordingly, much research on motion perception has sought to discover why there is no blurring. One hypothesis that gained favor was based on the finding by Ulric Neisser and various others that when we view an image flashed on a screen by a tachistoscope for even a tiny fraction of a second, we can briefly see it afterward in the mind’s eye. In 1967 Neisser used the term “icon” for this very brief visual memory, measured its duration as about half a second (later research reported as little as a quarter of a second) to two seconds, and found that it is erased if a new pattern is presented before it has faded.
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Other vision researchers then suggested that since the eye sweeps across the field of vision or follows moving objects in a series of jumps known as “saccades,” it sees nothing while moving but at every momentary stop sends an iconic snapshot to the brain. The snapshots are assembled there into a perception of motion, somewhat as if one were watching a movie.
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This hypothesis was widely accepted in the 1970s and the early 1980s. But some leading investigators began to doubt that the icon, observed only under unnatural laboratory conditions, exists in normal perception; if it does not, the saccadic-iconic hypothesis of movement perception collapses. As Ralph Haber saw it: