Valency and the Lexicon

(12) a. The squirrel was frightened.

b. Chatterer saw the bear.

c. Chatterer thought Buster was angry.

d. Joe put the fish on the log.

These possibilities correspond to the productions in Table 8-3. Table 8-3. VP productions and their lexical heads Production Lexical head

VP -> V Adj was VP -> V NP saw VP -> V S thought

That is, was can occur with a following Adj, saw can occur with a following NP, thought can occur with a following S, and put can occur with a following NP and PP. The dependents Adj, NP, S, and PP are often called complements of the respective verbs, and there are strong constraints on what verbs can occur with what complements. By contrast with (12), the word sequences in (13) are ill-formed:

(13) a. *The squirrel was Buster was angry.

b. *Chatterer saw frightened.

c. *Chatterer thought the bear.

With a little imagination, it is possible to invent contexts in which unusual combinations of verbs and complements are interpretable. However, we assume that the examples in (13) are to be interpreted in neutral contexts.

In the tradition of dependency grammar, the verbs in Table 8-3 are said to have different valencies. Valency restrictions are not just applicable to verbs, but also to the other classes of heads.

Within frameworks based on phrase structure grammar, various techniques have been proposed for excluding the ungrammatical examples in (13). In a CFG, we need some way of constraining grammar productions which expand VP so that verbs co-occur only with their correct complements. We can do this by dividing the class of verbs into "subcategories," each of which is associated with a different set of complements. For example, transitive verbs such as chased and saw require a following NP object complement; that is, they are subcategorized for NP direct objects. If we introduce a new category label for transitive verbs, namely TV (for transitive verb), then we can use it in the following productions:

Now *Joe thought the bear is excluded since we haven't listed thought as a TV, but Chatterer saw the bear is still allowed. Table 8-4 provides more examples of labels for verb subcategories.

Table 8-4.

Verb subcatego

ries

Symbol

Meaning

Example

IV

Intransitive verb

barked

TV

Transitive verb

saw a man

DatV

Dative verb

gave a dog to a man

SV

Sentential verb

said that a dog barked

Valency is a property of lexical items, and we will discuss it further in Chapter 9.

Complements are often contrasted with modifiers (or adjuncts), although both are kinds of dependents. Prepositional phrases, adjectives, and adverbs typically function as modifiers. Unlike complements, modifiers are optional, can often be iterated, and are not selected for by heads in the same way as complements. For example, the adverb really can be added as a modifier to all the sentences in (14):

(14) a. The squirrel really was frightened.

b. Chatterer really saw the bear.

c. Chatterer really thought Buster was angry.

d. Joe really put the fish on the log.

The structural ambiguity of PP attachment, which we have illustrated in both phrase structure and dependency grammars, corresponds semantically to an ambiguity in the scope of the modifier.

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