Connectionist Modeling of Experience-based Effects in Sentence ...

Chapter 1 Preliminaries
referrential constituents like nouns and main verbs as they refer to objects and events,
respectively. Pronouns, however, do not induce memory cost because they are assumed
to be immediately accessible. The assumption behind Integration Cost is that every
stored item receives an activation which decays depending on the number of newly encoded
discourse referents while it is maintained in memory. Integrating an element, i.e.,
relating it to its head, needs more processing effort when the element has less activation.
Thus the integration cost is a function, monotonously increasing with the number of
intervening discourse referents. The cost accounts only implicitly for decay over time
since time is only represented discretely by successive discourse referents. The unit of
Integration Cost is energy units (EUs).
The memory capacity limit is accounted for by the second principle of DLT: Storage
Cost. It rests on the assumption that the parser constantly predicts the most probable
complete sentence structure given the previous material and keeps it in memory.
Structural complexity is calculated by the number of syntactic heads contained. The
more complex the predicted structure, the more syntactic heads it contains. Every predicted
head uses up memory resources, so-called memory units (MUs). Memory load
also affects processing, because storage and processing use the same resources (Just and
Carpenter, 1992). Consequently, the more heads are predicted the higher the processing
cost. The important difference between the two costs is the location of their effects.
While Integration Cost accounts for processing differences only at the integration site,
Storage Cost for a predicted structure affects processing of every following part of the
sentence. Figure 1.1 shows the Integration Cost C(I) and the Storage Cost C(S) at
each point in an English object relative clause. Seeing the sentence-initial determiner
ORC The reporter whoi the senator attacked ei admitted the error
C(I) 0 0 0 0 0 1+2 3 0 0+1
C(S) 2 1 3 4 3 1 1 1 0
Total 2 1 3 4 3 3 4 1 1
Figure 1.1: DLT cost metrics for an English ORC according to Gibson (1998).
induces the prediction of a main clause. Hence predictions for an NP and a main verb
have to be stored. Note that DLT considers the prediction of the main verb as cost-free,
but in literature, it is mostly assigned a cost. For simplicity, in this work Storage Cost
will be consistently assumed for the main verb. Having completed the NP only the verb
is predicted. At the relative pronoun who a Storage Cost of 3 is assigned because an
embedded SRC is predicted, containing two heads: the embedded verb and a subject
gap. Seeing another determiner changes the prediction into an ORC, which contains
one more head, namely the embedded subject. On senator only the embedded verb,
the object gap, and the main verb stay predicted. On the embedded verb attacked then
two integrations take place. The subject integration of attacked costs 1 EU because the
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