So far, we have mainly addressed the conceptual entities contained in a Conceptual Structure representation and have dedicated very little attention to the semantic relations assigned to hold between those entities. To build propositional seman-tic representations of more complex utterances we need to relate projected concept instances by semantic relations in Conceptual Structure. For verb-centred represen-tations these semantic relations define the participants’ thematic roles and allow us to specify who did what to whom.
Thematic roles as introduced by Gruber (1965) and later by Fillmore (1968) ini-tially were intuitive linguistic abstractions to distinguish and classify the different, semantically unique participant functions in an utterace. While the use of thematic roles is not limited to words from a specific lexical category, thematic roles most frequently are defined for verbs where they mark the different semantic functions of each verbal argument, not just in the form of a syntactically motivated label but with a genuine semantic commitment (Dowty, 1989). Clearly, the ability to gener-alise over verbal argument structures is linguistically desirable.
Historically, however, the extensive and diverse debate of thematic roles in the liter-ature has shown that a precise delineation of thematic roles is extremely difficult, if not impossible, to achieve and invariably depends on the granularity of the approach chosen.1 While some theories only differentiate between the two basic roles proto-agentandproto-patient(e.g., Dowty,1989), other theories such as HPSG (e.g., Pollard and Sag, 1994) adopt a strongly lexicalised view of thematic roles, which results in a set of semantically highly differentiated, but largely verb-specific roles.2 Despite the enormous spectrum of approaches to thematic roles, according toL¨obner (2003) there appears to be a consensus on the definition of a few, central thematic roles such as agent, theme/patient, experiencer, instrument, location, goal, andpath. We define the set of thematic roles used in our work in Section5.3.
Whether or not thematic roles also constitute a psycholinguistic and cognitive reality has been widely debated. Ferretti et al. (2001) conducted four single-word priming studies that provide substantial support for the hypothesis that access to verbs in the mental lexicon immediately makes available typical schema information centred around the verb. In their study Ferretti et al. tested whether the auditory presen-tation of verbs primed other agents, patients, patient features, instruments, or locations that had previously been identified as typical for the stimulus verb.
A priming effect of the verb-centred thematic roles would be expected if the entire situation information associated with the verb is made available immediately upon processing the verb prime.
Ferretti et al. indeed observed that priming occurred for typical fillers of theagent and patient roles. A narrow range of typical instrument fillers was also primed while location fillers were not. Associative relatedness was excluded as possible
1A good introduction into the different approaches to thematic roles can be found inDowty(1989).
2The ability to abstract semantic relations beyond the level of single lexemes, which constituted one of the initial motivations for the introduction of thematic roles, tends to be lost in strongly lexicalised approaches.
cause for the observed priming effects. The authors conclude that thematic role information is indeed closely intertwined with the verb’s definition of meaning in the mental lexicon and that activation of the verb also activates the verb’s thematic roles.
Jackendoff’s treatment of lexical semantics proceeds along the same lines (Jackendoff, 1990, pp. 45): A verb’s entry in the mental lexicon contains argument slots, each of which carries specific requirements about the Conceptual Structure categories from which its filler candidates may be selected. Since human language permits the delib-erate use of such selection criteria, e.g. in metaphoric or ironic usage, we conclude that this selection criteria are not absolute hard rules but simply express degrees of preference. We interpret this as a Conceptual-Structure equivalent of traditional sortal constraints and capture this notion as modelling requirement R23:
Requirement R23
In a model for the interaction between non-linguistic modalities and linguistic understanding based on Conceptual Semantics, a verb’s lexical entry must indicate for each argument slot from which conceptual categories the argument fillers may preferably be selected.
Jackendoff further argues that thematic roles are “relational notions defined struc-turally over Conceptual Structure”. In Jackendoff’s view, a thematic role is nothing more than a specific label on a prominent semantic relation between an argument in-dex in the verb’s lexical Conceptual Structure representation and the corresponding argument slot. Jackendoff does not elaborate on how, mechanistically, the mapping between the semantic and the syntactic representations of the verb’s argument slots is achieved. Since both are encoded on different levels of linguistic representation, we conclude that the mapping needs to be performed by the interface between those two levels of representation. The syntax-semantics interface must hence also contain correspondence rules that are capable of performing this specific mapping.
Kako (2006) reports consistent interpretations of nonsensical verbs and verbal ar-guments used in syntactically well-formed frames. These findings support the view that the mapping between syntactic structure and thematic roles is generic rather than lexically specific. This finding is also in line with our expectation that a sep-arate mapping rule for every lexeme would be representationally highly inefficient.
Irrespective of the actual mechanism via which the mapping is achieved in human language processing, we can add Requirement R24 for lexical representation:
Requirement R24
In a model for the interaction between non-linguistic modalities and linguistic understanding, a verb’s thematic roles must be relateable to its syntactic argument structure via correspondence rules in the syntax-semantics interface.
Combining these verb-centred functional representations with the projections of role filling entities discussed in Section 3.4, we now have a sufficiently expressive in-ventory at hand to represent the semantic structure of propositions in Conceptual
"
event
KISS (hagent humanbennet
i
, htheme
humanlinnea] )
#
Figure 3.4: Conceptual Structure representation for the propositionBennet is kissing Linnea.
Structure: They are complex function-argument representations in which thematic relations hold between an instance of a situation concept as lexicalised by a verb and instances of entity concepts that act as role fillers. An example for a representation of a simple proposition is given in Figure 3.4.
An assumption in Jackendoff (1990) with substantial cognitive and philosophical implication is that every concept instance represented in Conceptual Structure carries information about the conceptual category it instantiates, such assituation, event orhuman. This presupposes that the cognition of an entity has resulted in its categorisation as the member of a certain class prior to its projection into Concep-tual Structure. We hence assume that every projected instance instantiates at least one concept from the concept hierarchy and add this as a modelling requirement for our Conceptual Structure representations:
Requirement R25
In a model for the interaction between non-linguistic modalities and linguistic understanding based on Conceptual Semantics, every concept instance must instantiate at least one concept from the concept hierarchy.
Conceptual Semantics makes another, similarly fundamental assumption about the representation of propositional knowledge in Conceptual Structure, namely that propositional representations in Conceptual Structure are inherently verb-centric.1 Since this view is compatible with our largely syntax-inspired view of semantic verb-argument structure, we include it as an additional modelling requirement for our model based on Conceptual Semantics.
Requirement R26
In a model for the interaction between non-linguistic modalities and linguistic understanding based on Conceptual Semantics, Conceptual Structure represen-tations are verb-centric.
1An in-depth discussion of this assumption is beyond the scope of this thesis. Since Jackendoff’s Conceptual Semantics sets out to be a universal account of cognition, a rigorous, linguistically universal justification for this claim would be required. As far as we are aware, Jackendoff does not provide such an argument. For a more detailed description of the nature of Conceptual Structure representations, refer to Jackendoff(1983, Chapter 4).