Story of Psychology - Morton Hunt [75]
The distinction was central to Helmholtz’s epistemology. He agreed with Kant that sensations are interpreted and given meaning by the mind, but disagreed that the mind innately possesses “categories” and “intuitions” that supply those meanings. Rather, he said, the mind learns to interpret sensations by means of trial and error—by learning which reactions to a visual sensation produce an expected result and which do not.
Space perception is a case in point. Kant said that the mind innately intuits spatial relationships; Helmholtz argued that we learn about space by means of unconscious inference. As infants, we learn little by little that such visual clues as size, direction, and intensity of hue are related to whether objects are closer or farther, to one side or the other of us, above or below us; through experience we gradually come to make correct judgments about spatial relations. (Every parent who has watched a three-month-old trying to grasp an object knows the process intimately.)
The British empiricist-associationists had said much the same thing but lacked experimental evidence to back it up; Helmholtz, an experimentalist through and through, supported his theory with research findings.
It occurred to him that if he could distort the spatial sensations reaching a subject’s brain—and if his theory was correct—the subject should adapt to the distorted vision and learn to interpret it correctly. He therefore constructed eyeglasses with prismatic lenses that shifted the apparent position of objects to the right of where they actually were. When subjects wearing the glasses tried to touch objects in front of them, they missed—they reached toward the apparent rather than the real position of the objects.
Next, for some minutes he had them reach for and handle the objects while wearing the lenses; at first they had to consciously reach to the left of where they saw the object, but soon they began to reach for objects where they actually were without having to think about it. They had made a perceptual adaptation; their minds had reinterpreted the messages arriving from the optic nerves and they now saw the objects in the context of reality.
Finally, when they took off the spectacles and reached for the objects, they missed again, this time erring to the left of the real position; it took a little while for their normal space orientation to reassert itself.
Helmholtz did agree with Kant about one innate capacity, the ability to interpret cause-and-effect relationships. For the rest, he maintained that virtually all knowledge and ideas are the result of the mind’s interpretation of sensory experience, and that these interpretations, particularly those having to do with spatial perception, are largely the product of unconscious inference.
This view was strongly opposed by psychologists who held that the mind is innately equipped to interpret its perceptions. A key function they explained in innate terms was the combining of the two images coming from the eyes to form a single three-dimensional image. Some said that each point on the retina receives exactly the same bit of information as the corresponding point on the other retina and that the two optic nerves thus combine their images into one. One opponent of Helmholtz’s ideas said that each retina is endowed with innate “signs” that distinguish height, right-left orientation, and depth and that enable the nervous system to fuse the images before they reach the brain.
Helmholtz brusquely dismissed these notions. Nativist theory, he wrote, was “an unnecessary hypothesis”; it relied on unprovable assumptions and added nothing to the demonstrable facts of empiricist theory.33 His strongest evidence that experience is what enables us to perceive paired images as a single one came from the stereoscope. Through this instrument, invented by Charles Wheatstone in 1833, the viewer sees not two identical images but two slightly different ones taken from slightly different angles. The images cast on the retinas