Error monitoring in speech production: a computational test of the perceptual loop theory.

A theory of speech monitoring, proposed by Levelt (1983), assumes that the quality of one's speech is checked by the speech comprehension system. This system inspects one's own overt speech but would also inspect an inner speech plan ("the inner loop"). We have elaborated and tested this theory by way of formalizing it as a computational model. This model includes a new proposal concerning the timing relation between planning the interruption and the repair: the proposal that these two processes are performed in parallel. We attempted to simulate empirical data about the distribution of error-to-cutoff and cutoff-to-repair intervals and the effect of speech rate on these intervals (these intervals are shorter with faster speech). The main questions were (1) Is an inner monitor that utilizes the speech perception system fast enough to simulate the timing data? (2) Can the model account for the effects of speech rate on these intervals? We conclude that including an inner loop through the speech comprehension system generates predictions that fit the empirical data. The effects of speed can be accounted for, given our proposal about the time course of planning interruption and repair. A novel prediction is that the error-to-cutoff interval decreases with increasing position in the phrase.

Word order priming in written and spoken sentence production.

An experiment is reported that showed priming of the word order of auxiliary verb and past participle in Dutch subordinate clauses, both in speaking and in writing. Participants completed sentence fragments to full sentences. Prime sentence fragments were constrained so as to be completed with only one possible word order. Target sentence fragments, presented immediately after the prime fragments, could be completed with the same word order as that of the prime and an alternative order with the two words exchanged. Significant priming effects were obtained, so that the same word orders tended to be used in prime and target. We interpret this as evidence for a distinct process of constituent structure linearization during sentence production, which serves to ensure the fluency of speech and writing.

Agrammatic production of subject-verb agreement: the effect of conceptual number.

Three experiments tested the hypothesis that the deficit underlying agrammatic sentence production difficulties can be characterized as a limitation of computational resources and that these resources are not restricted to syntactic processing. This hypothesis was tested by eliciting subject-verb agreement errors in a sentence fragment completion paradigm. Sentence fragments were complex noun phrases, containing a subject (head) noun and a modifying prepositional phrase, containing a "local" noun. We varied the number of "tokens" a singular head noun referred to. Therefore, in one condition, grammatical and conceptual number of the head noun mismatched, whereas these numbers were the same in another condition. In Experiments 1 and 2, we observed an effect of this variable (i.e., more agreement errors when conceptual number was plural and grammatical number singular) in normal controls. Broca's aphasics, on the other hand, showed no effect. Experiment 3 consisted of a sentence/picture matching test. This test showed that the lack of effect with Broca's aphasics cannot be attributed to a comprehension deficit. We argue that these results are incompatible with the notion of a limitation in resources specific for syntactic processing. Instead, we interpret this as the result of a trade-off: Broca's aphasics lack computational resources to take into account both grammatical and conceptual information in morphosyntactic processing and rely on grammatical information only.

Syntactic facilitation in agrammatic sentence production.

Recently, proposals have been made to relate processing difficulties in aphasic language performance to limitations in resources for grammatical processing (Carpenter et al., 1994; Hagiwara, 1995; Kolk, 1995; Martin & Romani, 1994). Such proposals may account for a defining characteristic of agrammatic sentence production: reduced syntactic complexity. Syntactic structures that require deep hierarchical processing or reversals of canonical word order make demands exceeding limited resources. In the present study, we investigate the possibility of counteracting hypothesized resource limitations by increasing the availability of relatively complex sentences (i.e., datives and passives). The phenomenon of "syntactic priming" has been observed in a number of studies with healthy adults (e.g., Bock, 1986). With respect to Broca's aphasia, we hypothesized that increased availability of a syntactic structure, due to syntactic priming, results in a lesser demand on (limited) resources for sentence production. We elicited speech from 12 Broca's aphasics and 12 control subjects in three different conditions: spontaneous speech, picture description without priming, and picture description with priming. In addition, we varied instructions, in order to determine the role of strategies. The main findings were that (a) Brocas show stronger syntactic priming effects than controls; (b) the effects are automatic rather than strategic; and (c) in conditions with priming, Brocas produce relatively complex sentences (e.g., passives). We discuss these results in relation to capacity theories.

Syntactic persistence in Dutch.

Three experiments are reported that showed effects of "structure priming," the tendency to repeat syntactic structure across successive sentences. These effects were demonstrated in a previously untested language, Dutch. All experiments studied spoken sentence production. Importantly, pre-experimental baselines were measured for all target structures in order to assess possible effects of frequency on the magnitude of priming effect. We obtained priming with dative sentences, including datives with medially placed prepositional phrases, a sentence type not tested before. In one experiment we obtained priming effects with passives, including passives with sentence-final verbs, which also have never been tested before. However, we failed to obtain priming effects with active transitives. A comparison with the baseline data suggested that priming is not related to baseline frequency. Further, the results allowed us to draw an empirical generalization: Structure priming is a relatively long-term event, lasting at least several trials. The implications of these findings are discussed.