Story of Psychology - Morton Hunt [366]
Almost all knowledge is implicit in the structure of the device that carries out the task…It is built into the processor itself and directly determines the course of processing. It is acquired through the tuning of connections as these are used in processing, rather than formulated and stored as declarative facts.105
The new theory, accordingly, came to be known as “connectionism,” and has remained the number one buzz word of current cognitive theory.106 In an interesting reversal, cognitive psychologists no longer think of mental processes as taking place in computer-like serial fashion and their connectionist model of mental processes—based on neurological evidence—has become the guiding standard for computer design.
Rumelhart and his collaborators were not the only psychologists to imagine how connectionism in the human mind might work; a number of other connectionist models have been drawn up in recent years. But the basic concept underlies them all, namely, that the brain functions by means of exceedingly intricate and complex networks of multiple interconnections among its neurons, enabling the mind, among other things, to work both consciously and unconsciously at the same time, make decisions involving multiple variables, recognize meanings of spoken or written words, and on and on.
For our purposes, the Rumelhart et al. model can serve to exemplify the whole genre. A diagram that Rumelhart and two of his collaborators drew up for their book on PDP will make the PDP idea clear if you are willing to take a minute or two to trace it through. It is a portrait not of a bit of brain tissue but of a bit of a theoretical connectionist network:
FIGURE 43
The mind’s wiring? A hypothetical example of a connectionist network
Units 1 to 4 receive inputs from the outside world (or some other part of the network), plus feedback from the output units, 5 to 8. The connections among the units are indicated symbolically by the unnumbered disks: The bigger an open disk, the stronger the connection, the bigger a filled disk, the stronger the inhibition or interference with transmission. Thus, unit 1 does not influence unit 8, but does influence 5, 6, and 7 to varying degrees. Units 2, 3, and 4 all influence 8 in widely varying degrees, and 8 in turn feeds back to the input units, affecting 1 almost not at all, 3 and 4 rather little, and 2 strongly. All this goes on at the same time and results in an array of outputs, in contrast to the single process and single output of serial design.
Although Rumelhart and his collaborators said that “the appeal of PDP models is definitely enhanced by their physiological plausibility and neural inspiration,” the units in the diagram are not neurons and the connections are not synapses.107 The diagram represents not a physical entity but what happens; the brain’s synapses and the model’s connections operate in different ways to inhibit some connections and strengthen others. In both cases, the connections are what the system knows and how it will respond to any input.108
A simple demonstration: What word is partly obscured by inkblots in this diagram?
FIGURE 44
How can you tell what the partly obscured word is?
You probably recognized instantly that the word is pen. But how did you know that? Each partly obscured letter could have been other than the one you took it to be.
The explanation (based on a similar example by Rumelhart and Jay McClelland): The vertical line in the first letter is an input into your recognition system that strongly connects to units in which P, R, and B are stored; the curved line connects to all three. On the other hand, the sight of the straight line does not connect—or, one can say, is strongly inhibited from connecting—to any unit representing rounded letters like C or O. Simultaneously,