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

1.2 Relative Clauses and Complexity
as found in English SRCs center-embedding recursion exceeds finite-state expressibility.
Mirror recursion is one of three basic recursion types defined by Chomsky (1957) as
relevant for natural language. The second type is identity recursion, which produces
cross-dependencies of the form abab as for example in Swiss-German or Dutch relatives
clauses. Cross-dependencies are very rare but still problematic because they suggest that
language is not context-free. A type even harder to find, if at all, is counting recursion.
It is characterized in terms of an artificial language like a n b n , where the occurrences
of b depend on the number of occurrences of a. It is debatable whether this type of
recursion exists in natural language (cf. Christiansen and Chater, 1999) but this shall
not be relevant in this thesis.
Interestingly human language performance seems to be unable to handle recursion beyond
double-embedding. Double center-embedding already poses severe comprehension
difficulties and is often rated as ungrammatical (Blumenthal, 1966). In addition some
studies of center-embedding show a grammaticality illusion of ungrammatical structures
(Frazier, 1985; Gibson and Thomas, 1999; Vasishth et al., 2008; Christiansen and Chater,
1999). In spite of the fact that cross-dependency is formally most complex it is centerembedding
which is the hardest recursion type for the human comprehender (Bach et al.,
1986). The highly demanding dependencies involved in center-embedding and the potential
ambiguities produced by multiple gap positions yield processing difficulties that
have been extensively investigated in psycholinguistic studies. The human difficulties
in processing recursive structures are also evident language-independently. For example
center-embedding dependencies between digits and letters produce similar difficulties as
in language processing (Larkin and Burns, 1977). Current symbolic psycholinguistic theories
close the obvious gap between grammatical competence and empirical performance
by the involvement of memory limitations, decay, and attention span or by explicitly
defining a limit to the number of recursion levels. However, a question that arises is
whether recursion should be assumed in the human processor at all. Since competence
cannot be directly assessed, it can be empirically accessed only through the link to performance.
But the link in turn is dependent on the underlying competence theory. This
results in a non-falsifiability of an infinite competence, which Christiansen (1992) calls
the Chomskian paradox.
“In particular, I suggest that recursion is a conceptual artifact of the competence/performance
distinction [...], instead of a necessary characteristic of
the underlying computational mechanism.” (Christiansen, 1992; p. 1)
As will be shown in chapter 3 connectionist models are performance models, that
account for memory limitations, recursion limits, and the characteristics of different
recursion types concerning human comprehension performance. The following section
introduces four aspects of psycholinguistics relevant for this thesis.
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