In linguistic theory, thematic roles have traditionally been regarded as determinant in expressing generalizations about the syntactic realization of a predicate's arguments (see [EAG96]).
Most characterizations of thematic roles have been carried out in
terms of primitive semantic properties of predicates. For example,
[Jac72] suggested that thematic relations should be defined
in terms of the three semantic subfunctions CAUSE, CHANGE
and BE which constitute some of the primitive building blocks of
lexical conceptual representations. According to this treatment, the
lexical-conceptual representation of a transitive verb like open
would be as shown below where NP1 is interpreted as agent and
NP2 as theme.
In addition to [Dow79], other model-theoretic formalizations are presented in [Car84] and [Dow89]. In [Dow89], Dowty defines thematic role types as abstractions above individual thematic roles of specific verbs. The individual thematic role of a verb is defined as the set of all properties which the verb entails for a given argument position. For example the individual role for the subject argument of the verb love would correspond to the set of properties which can be attributed to the first argument of the predicate love through semantic entailment, e.g. the properties which characterize the lover participant. A thematic role type can then be defined as the intersection of some set of individual thematic roles. For example, the role type ``recipient'' would be the set of all entailed properties shared by a particular individual role of verbs such as give, sell, buy, receive and tell.
As Dowty himself hastens to point out, this method is not guaranteed to yield useful results. Even assuming that each individual role will effectively intersect with at least another individual role, the number of resulting role types might just be too big to be useful at all. More generally, the identification of an appropriate set of semantic roles [Pal94] is problematic; in practice this means the number of roles varies significantly across different proposals.
The problems just pointed out have led several scholars -- e.g. [Som87], [Roz89], [Dow91] -- to put forward alternative conceptions of semantic roles. Although from different angles, these all criticize the use of necessary and sufficient conditions for the identification of roles, and advocate more flexible approaches. These approaches appear particularly suitable for the construction of large scale lexicons since they overcome many problems of role identification inherent to traditional approaches, i.e. the difficulty in enumerating precise criteria which qualify the conceptual makeup of a given semantic role.
[Dow91] proposes to abandon the use of discrete role types to
provide a total indexing of verbal arguments in favour of a weaker
method where the relation between role types and clusters of
entailments of verb meanings need not be unique. Dowty assumes that
there are only two ``thematic-role-like concepts'' for verbal
predicates: the proto-agent and proto-patient role.
Proto-roles are conceived of as ``cluster-concepts'' which are
determined for each choice of predicate with respect to a given set of
semantic properties. The properties which contribute to the definition
of the proto-agent and proto-patient roles are listed below.
The work of [Dow91] has been taken as the starting point of the EAGLES recommendations on the encoding of thematic roles [EAG96]).
[San92a,San92b,San93a,San93b] propose to
extend the functionality of Dowty's prototype roles by including in
the defining clusters properties which are instrumental for the
identification of semantic verb (sub)classes. For example, it is well
known that at least six subtypes of psychological verbs can be
distinguished according to semantic properties of the stimulus and
experiencer arguments (see [Jac90] and references therein), as
shown in Table 2.1.
[Ash95] assume that both causation and change can be specified along the following dimensions so as to yield a thematic hierarchy such as the one described in the lattice structure in Fig 2.2.
[SanFCa] proposes to enrich this characterization by
|verb class||REPRESENTATIVE PREDICATES|
|assertive||hypothesize, insists, suggest|
|directive||beg, order, persuade|
|expressive||congratulate, apologize, welcome|
|declarative||declare, fire, resign|
|perceptive||hear, see, touch|
|emotive||fear, like, please|
|formal||build, eat, demolish|
|matter||paint, knead, carve|
|locative||send, swim, sit|
|locative verb class||REPRESENTATIVE PREDICATES|
|+motion, +manner||swim, walk|
|+motion, +direction||go, reach|
|+motion, +direction, + manner||swim/wal across|
Semantic roles are assumed to be the source of grammatical relations in many linguistic theories. Grammar frameworks such as Government and Binding (GB), Lexical Functional Grammar (LFG) and Functional Grammar (FG) all posit a level of semantic, or thematic, relations to which grammatical relations systematically relate. In particular, semantic roles are the standard devices used for organising predicate argument structures within the lexicon, where arguments are identified on the basis of semantic roles. GB, LFG and FG follow a lexicalist approach to grammar which makes the lexicon the source of syntactic representations; this implies that grammatical relations are, one way or another, projected from predicate argument structures specified within the lexicon.
The principles guiding the mapping of lexical representations onto syntactic structures vary across the different theories. A first distinction can be made between multi-stratal frameworks such as Government and Binding and mono-stratal ones such as Lexical Functional Grammar and Functional Grammar: whereas in the former the mapping is onto D- structure representations, in the latter the projection is directly onto surface representations. Hence, in GB the attention is focused on the way in which thematic roles are mapped onto structural positions at D-structure; the actual syntactic realization of these roles in the surface of the sentence is then accounted for at the level of the mapping between D- and S-structure. By contrast, LFG and FG link semantic relations directly to their surface syntactic realization. From this it follows that the mapping conditions in the two cases are different. In multi-stratal frameworks, D-structure appears to be a pure structural representation of thematic relations, regardless of their syntactic expressions; this could explain why GB lexical representations do not systematically have to specify the syntactic expression of arguments. In mono-stratal frameworks, such mapping conditions have to account for the variation in the syntactic realization of the same semantic relation in the surface of the sentence.
In spite of these different conceptions of the mapping between semantic and syntactic relations, all frameworks considered here share the general assumption that the relationship between semantic and syntactic relations is constrained by some sort of hierarchy of semantic roles. This idea dates back to [Fil68] who first formulated the view that subject selection is in some way sensitive to a hierarchy of ``cases'', i.e. semantic relations. Following Fillmore, most theories invoke (though to a different extent) a mapping between an ordered list of semantic (i.e. a hierarchy) and an ordered list of grammatical relations (either expressed as different positions within phrase markers, or explicitly or implicitly organized in hierarchical terms). Given a thematic/semantic hierarchy (agent > theme ...) and a syntactic hierarchy (subject > object ...), the general form of the mapping is as follows: map the semantic roles of a given argument structure, which have been ordered according to the hierarchy, into the syntactic hierarchy from left to right. Under this view, the mapping is controlled by hierarchical, i.e. relative, strategies (that is, ``higher'' semantic roles are mapped onto ``higher'' syntactic relations), rather than invariable correspondence relations (such as a given semantic role always maps onto a given grammatical relation).
However, this mapping between hierarchies is not always sufficient to predict the syntactic realization of an argument. Whenever this is the case, the mapping is constrained through additional information on the syntactic realization of arguments; this practice is adopted, though to a different extent, within the GB and LFG frameworks. In GB, when the syntactic expression of an arguments cannot be predicted on the basis of general rules, as in the case of psychological verbs, this information is specified at the level of lexical representations in the form of Case-grid [Bel88]. By contrast, LFG lexical representations systematically include the ``syntactic function assignment'', i.e. the explicit stipulation of the syntactic realization of verb's arguments; according to latest developments [Bre89], this specification is made in underspecified form. Due to its peculiar conception of grammatical relations, FG never contains specifications of this kind: since subject and object selection is made on the basis of pragmatic considerations, the mapping between semantic and syntactic functions only defines the range of possible syntactic realizations, thus stating preferences rather than constraints within the range of possible mappings.
A kind of regular mapping between grammatical relations and semantic roles is also assumed in Dowty's conception of proto-roles and further developments. In fact, proto-roles are related to argument selection through the so-called ``Argument Selection Principle'', according to which the argument for which the predicate entails the greatest number of Proto-Agent properties will be lexicalized as the subject of the predicate and the argument having the greatest number of Proto-Patient properties will, all else being equal, be lexicalized as the direct object. The basic idea underlying this approach to argument selection is that the ranking according to which the arguments of a verb compete with one another with respect to subjecthood and objecthood is provided by the clustering of semantic properties, rather than by the mapping between specific positions (say between Agent and Subject). This is to say that argument selection of subject and object is determined by the total number of Proto-Agent entailments and Proto-Patient entailments shown by each argument of a verb.
To sum up, three different aspects have been taken as defining features of the kind of mapping between lexical and syntactic representations, i.e. whether:
All grammar frameworks considered in this brief survey have been characterised with respect to these features.
Concerning formalization, three treatments can be distinguished. First, approaches which rely on an informal specification such as Jackendoff's lexical conceptual structures [Jac90]. Second, approaches such as those proposed by [Dow79], [Dow89] which are developed within a model-theoretic framework. Third approaches which provide an algebraic specification within a typed feature structure formalism; these tend to be more oriented towards NLP applications (e.g. [San92b], [San93a], [San93b], [SanFCa]).
a theme NP which has cumulative reference induces a durative reading at the sentential level, while with a theme NP which has quantized reference a terminative reading obtains.This pattern is shown in the examples below where a sequence of two question marks indicates incompatibility -- under a single event reading -- between a quantified NP and the durative adverbial all day which forces an atelic (i.e. durative) interpretation on the sentences.
Thematic roles are also used in the EUROTRA MT lexica (§ 3.9.1), DELIS (§3.10.5), the EDR Concept Description Dictionary (§3.6).
Thematic or semantic roles represent another type of grammatical relation - a semantic one - holding between a predicate and its arguments, which can be usefully exploited in the framework of NLP applications. Information on the thematic role borne by arguments with respect to their predicates is useful to abstract away from the issue of their concrete syntactic realization (e.g. in terms of grammatical functions such as subject, object indirect object and the like). In Machine Translation, for instance, this information can be used to map the predicate-argument structures of two translation equivalents such as, for example, English like and Italian piacere, although the syntactic realization of the two verbs is radically different: namely, what is the subject of English like becomes an indirect object in Italian piacere, while what is the object of like is turned into the subject in the Italian translation. A characterization of the argument structure of the two verbs in terms of semantic roles attains the purpose of neutralizing their different syntactic behaviour in the specific language. Thematic roles have been used in Machine Translation (§4.1) to express generalizations about complex translation equivalence (see [Dor90], [Dor93], [San92b], [Cop93]). Another possible use of semantic roles in the framework of NLP applications is based on their semantic content which, in principle, can be used to infer the semantic role borne by a given constituent in a sentence. In a syntactically - either structurally or functionally - ambiguous context, recognition of the semantic role borne by a given constituent can help to resolve the syntactic ambiguity due to the pervasive regularities observed in the mapping between semantic roles and grammatical relations (see §2.4.3). This kind of hypothesis is entertained and corroborated by psycholinguistic studies on language comprehension showing that thematic information is highly instrumental in the resolution of local ambiguities and garden-paths [Car88], [Pri88], [Sto89]. In particular, the results of the experiments carried out in these studies show that there appears to be a persistent default pattern of thematic assignment throughout natural language - animate subjects are Agents and objects are Themes, inanimate subjects are Themes. These results validate the hypothesis that thematic assignments have a bearing on syntactic parsing which is thus performed on the basis of semantic information. Yet, when the exploitation of this hypothesis is considered for disambiguation purposes in the framework of wide coverage NLP systems, the picture emerging from psycholinguistic studies changes radically due to the impossibility of having a coherent characterisation of semantic roles while keeping their being the source of coherent syntactic representations [Mon95].