Story of Psychology - Morton Hunt [309]
The neural approach tells us much about the workings of visual perception at the micro level but little at the macro level, much about the machinery of vision but little about its owner and operator, much about neuronal responses but little about the experience of perception. As one cognitive theorist put it, “Trying to understand perception by studying only neurons is like trying to understand bird flight by studying only feathers.”76
The cognitive approach deals with the mental processes at work in such perceptual phenomena as shape constancy, feature identification, form recognition, cue-derived depth perception, recognition of figures when much of the information is missing, and so on.
The mental processes that yield these results are made up of billions of neuronal events, but cognitive theorists say that it takes macrotheories, not microtheories, to explain these processes. A physicist studying how and when a wave changes form and breaks as it nears the shore cannot derive the laws of wave mechanics from the interactions of trillions of water molecules, not even with a number-crunching mainframe computer. Those laws express mass effects that exist at a wholly different level of organization. The sounds made by a person talking to us are made up of vibrations of the molecules of atmospheric gases, but the meaning of the words cannot be explained in those terms.
So too with the mental processes of visual perception; they are organized mass effects of neural phenomena expressed by mental, not neurophysiological, laws. We have already seen evidence of this, but there is one particularly intriguing and historic example worth discussing. What happens, and at what level, when we call up an image from memory and see it in the mind’s eye? Experiments by cognitive theorists show that this can be explained only in high-level cognitive terms. The most elegant and impressive of such experiments are those of Roger Shepard (now emeritus) of Stanford University on “mental rotation.” Shepard asked subjects to say whether the objects in each of these three pairs are identical:
FIGURE 36
Mental rotation: Which pairs are identical?
Most people recognize, after studying them for a little while, that the objects in A are identical as are those in B. Those in C are not. When asked how they reached their conclusions, they say that they rotated the objects in their minds much as if they were rotating real objects in the real world. Shepard demonstrated how closely this procedure mirrors real rotation by another experiment, in which viewers saw a given shape in degrees of angular difference. This set, for example, shows a single shape in a series of positions:
FIGURE 37
Mental rotation: The greater the distance, the longer it takes.
When subjects were shown pairs of these figures, the time it took them to identify them as the same was proportional to the angular difference in the positions of the figures; that is, the more one figure had to be rotated to match the other, the longer it took for identification.77
This is only one of many perceptual phenomena that involve higher mental processes operating on internalized symbols of the external world. For some years a number of perception researchers have been trying to formulate a comprehensive cognitive theory of what those processes are and how they produce those perceptions.
There are two schools of thought about how to do this. One uses concepts and procedures drawn from artificial intelligence (AI), a branch of computer science. The basic assumption of AI is that human mental activities can be simulated by