Computational modelling From flowcharts to simulations

In the past, many experimental cognitive psychologists stated their theories in vague verbal statements. This made it hard to decide whether the evidence fitted the theory. In contrast, cognitive scientists produce computer programs to represent cognitive theories with all the details made explicit. In the 1960s and 1970s, cognitive psychologists tended to use flowcharts rather than programs to characterise their theories. Computer scientists use flowcharts as a sort of plan or blue-print for a program, before they write the detailed code for it. Flowcharts are more specific than verbal descriptions, but can still be underspecified if not accompanied by a coded program.

An example of a very inadequate flowchart is shown in Figure 1.1. This is a flowchart of a bad theory about how we understand sentences. It assumes that a sentence is encoded in some form and then stored. After that, a decision process (indicated by a diamond) determines if the sentence is too long. If it is too long, then it is broken up and we return to the encode stage to re-encode the sentence. If it is ambiguous, then its two senses are distinguished, and we return to the encode stage. If it is not ambiguous, then it is stored in long-term memory. After one sentence is stored, we return to the encode stage to consider the next sentence.

In the days when cognitive psychologists only used flowcharts, sarcastic questions abounded, such as, "What happens in the boxes?" or "What goes down the arrows?". Such comments point to genuine criticisms. We need to know what is meant by "encode sentence", how long is "too long", and how sentence ambiguity is tested. For example, after deciding that only a certain length of sentence is acceptable, it may turn out that it is impossible to decide whether the sentence portions are ambiguous without considering the entire sentence. Thus, the boxes may look all right at a superficial glance, but real contradictions may appear when their contents are specified.

In similar fashion, exactly what goes down the arrows is critical. If one examines all the arrows converging on the "encode sentence" box, it is clear that more needs to be specified. There are four different kinds of thing entering this box: an encoded sentence from the environment; a sentence that has been broken up into bits by the "split-sentence" box; a sentence that has been broken up into several senses; and a command to consider the next sentence. Thus, the "encode" box has to perform several specific operations. In addition, it may have to record the fact that an item is either a sentence or a possible meaning of a sentence. Several other complex processes have to be specified within the "encode" box to handle these tasks, but the flowchart sadly fails to addresses these issues. The gaps in the flowchart show some similarities with those in the formula shown in Figure 1.2.

Not all theories expressed as flowcharts possess the deficiencies of the one described here. However, implementing a theory as a program is a good method for checking that it contains no hidden assumptions or vague terms. In the previous example, this would involve specifying the form of the input sentences, the nature of the storage mechanisms, and the various decision processes (e.g., those about sentence length and ambiguity). These computer programs are written in artificial intelligence programming languages, usually LISP (Norvig, 1992) or PROLOG (Shoham, 1993).

There are many issues surrounding the use of computer simulations and the ways in which they do and do not simulate cognitive processes (Cooper, Fox, Farrington, & Shallice, 1996; Costello & Keane, 2000; Palmer & Kimchi, 1986). Palmer and Kimchi (1986) argued that it should be possible to decompose a theory successively through a number of levels (from descriptive statement to flowchart to specific functions in a program) until one reaches a written program. In addition, they argued that it should be possible to draw a line at some level of decomposition, and say that everything above that line is psychologically plausible or meaningful, whereas everything below it is not. This issue of separating psychological aspects of the program from other aspects arises because there will always be parts of the program that have little to do

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