Theories in physics

Semantic network theory Psychology

Figure 6.1: A taxonomic tree. From Collins and Quillian (1969).

The network is displayed as a taxonomic tree or isa hierarchy (a term we introduced briefly in chapter 3). Each node (labelled by an underscored word) is connected upwards to its superset and downwards to its subset. A canary, in this schema, is a bird and, more generally, an animal. A shark, too, is shown to be an animal, but it is not a bird, as there is no link up from `shark' to `bird'. An ostrich is a bird because, like the canary, it is one of the children of the `bird' node. The links sideways from each node state properties that are true of the node - that birds can fly, for example, or that canaries can sing - and properties of higher nodes are inherited by the lower nodes to which they are connected unless there is a property attached to a lower node that explicitly overrides it. Thus, we may infer from the tree that canaries can fly because birds in general can fly, whereas we are inhibited from making the same inference for ostriches since there is the explicit statement at the ostrich node that it `can't fly'.

Collins and Quillian's experiments consisted in presenting subjects with sets of true and false sentences, and measuring their reaction time in deciding whether the sentences were true or false. Taking the number of links to be traversed between two nodes to be a measure of the semantic distance between concepts, they predicted that a person would require more time to decide, for example, that ``A canary is an animal'' or ``A canary has skin'' than to decide that ``A canary is a bird'' or ``A canary can sing'' since in the former cases the search for the relevant information requires rising through more links in the hierarchy. The experimental results met their predictions. Table 6.1 illustrates the kinds of stimulus sentences, with approximate reaction times, that were used in these experiments. While subsequent research on reaction times to sentences, such as that by Conrad (1972) and Smith, Shoben, and Rips (1974), raised doubts about the soundness of the model in the form it then had, Collins and Quillian's hierarchical nets were an important source of many good ideas for, and the direct forerunners of, more recent networks, particularly in the domain of language understanding.

Table 6.1: Reaction times for sentences with differing semantic distances. From Collins and Quillian (1969).
Sentence True sentences Mean reaction
type time (msec)
P0 A canary can sing. 1.3
P1 A canary can fly. 1.38
P2 A canary has skin. 1.47
S0 A canary is a canary. 1.00
S1 A canary is a bird. 1.17
S2 A canary is an animal. 1.23
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