Dynamical quantification of schizophrenic speech

Abstract

Schizophrenic speech has been studied both at the clinical and linguistic level. Nevertheless, the statistical methods used in these studies do not specifically take into account the dynamical aspects of language. In the present study, we quantify the dynamical properties of linguistic production in schizophrenic and control subjects. Subjects' recall of a short story was encoded according to the succession of macro- and micro-propositions, and symbolic dynamical methods were used to analyze these data. Our results show the presence of a significant temporal organization in subjects' speech. Taking this structure into account, we show that schizophrenics connect micro-propositions significantly more often than controls. This impairment in accessing language at the highest level supports the hypothesis of a deficit in maintaining a discourse plan in schizophrenia.

Introduction

The discourse of psychotic patients such as schizophrenic (positive or undifferentiated symptomatology) or manic patients is associated with neologism, fading, obstruction, incoherence, hyperphrasia and associative loosening (Andreasen, 1979a, Andreasen, 1979b). Specific linguistic analysis of the discourse of schizophreni patients demonstrates that syntax is globally spared within sentences (Rochester and Martin, 1979), but that cohesion is impaired between sentences (King et al., 1990, Thomas et al., 1990). Patients thus display a disorganization of semantic systems rather than a lack of semantic knowledge (Goldberg et al., 1998, Paulsen et al., 1996). These language impairments in schizophrenia have been related to cognitive deficits such as working memory and attention impairments (Docherty et al., 1996, Sullivan et al., 1997), inability to structure discourse (Hoffman et al., 1986) or lack of conceptual sequencing (Docherty et al., 2000). Such deficits would thus lead to a specific organization of schizophrenic discourse characterized by thematic shifts to idiosyncratic ideas (Harrow et al., 1983) and high semantic priming effects (Minzenberg et al., 2002, Kwapil et al., 1990). Moreover, symptoms such as poverty of speech, perseveration and inappropriate responses have been interpreted within the general model of action self-monitoring (Frith, 1992, Frith et al., 2000). These characteristics thus argue for an impairment of the overall organization of discourse rather than an elementary linguistic problem.

Two important interpretations have been proposed to explain the macroscopic impairment of schizophrenic speech:

• Context-processing deficits are defined as an impaired ability to internally represent, maintain and update context information (Cohen and Servan-Schreiber, 1992). Schizophrenic patients depict such a deficit in the pragmatic use of contextual indices (Titone et al., 2000, Mesure et al., 1998, Cohen et al., 1999), in lexical ambiguity tasks (Copland et al., 2002) and in sentence-completion tasks (Bazin et al., 2000). It has been correlated with structural cerebral abnormalities (McCarley et al., 1999). This has been confirmed by studies of deficits in the integration of contextual data, at a neuropsychological level (Cohen and Servan-Schreiber, 1992, Chapman et al., 1976, Plagnol et al., 1996), as well as at a neurological level (Sitnikova et al., 2002, Salisbury et al., 2002). In this framework, a unifying hypothesis has been proposed suggesting an inability to maintain contextual information, and to use this information to inhibit inappropriate responses (Servan-Schreiber et al., 1996, Braver et al., 1999).

• Language-production models explain a message generation with the creation of a discourse plan that includes the topic of the discourse and the information to be conveyed (Levelt, 1989). The interaction between language-production processes and thought disorders could explain speech disorders in schizophrenia (Barch and Berenbaum, 1996). In this framework, negative thought disorders, such as reduced verbosity or increased pausing, reflect a deficit in generating a discourse plan, and discourse-coherence disturbances, such as tangential responses or distractible speech, reflect a deficit in maintaining a discourse plan and in monitoring the ongoing content of speech (Barch and Berenbaum, 1997).

These two models allow us to hypothesize that the temporal organization of schizophrenic speech should be impaired. Our study is thus an attempt to characterize the temporal organization of schizophrenic speech using specific dynamical methods.

Quantitative studies of linguistic data, based on free speech samples and oral interviews, found low complexity (frequency, depth and locus of embedded propositions), low integrity (syntactic and semantic errors) and dysfluency (number of pause fillers, false starts and repeated words) in schizophrenic speech (Morice and Ingram, 1982, Thomas et al., 1990). Nevertheless, these studies have usually used computer-assisted grammatical analysis and statistical methods based on counting the occurrence of specific items. Such a procedure neglects the temporal dimension of speech. Because of its inherent temporal dimension, speech can be considered as a dynamical process (Zellner Keller and Keller, 2000, Elman, 1995, Port and Van Gelder, 1995) and can be studied using concepts and numerical methods provided by nonlinear dynamics.

Nonlinear dynamics permit the study of complex phenomena evolving with time (Kaplan and Glass, 1995) and has been considered as a promising framework in psychiatry (Globus and Arpaia, 1994, Ehlers, 1995, Nandrino et al., in press). In the case of schizophrenia, the time evolution of biological, clinical and behavioral indices has been characterized: electroencephalographic (EEG) studies have shown modifications in brain dynamics in schizophrenic patients in resting condition (Kim et al., 2000, Jeong et al., 1998) and during sleep stages (Röschke et al., 1995, Röschke et al., 1994, Röschke and Aldenhoff, 1993, Elbert et al., 1992); the time course of psychotic derealization has been associated with a nonlinear dynamical system (Tschacher et al., 1997) and a loss of consistency in response selection and ordering has been observed in a simple choice task (Paulus et al., 1996). Nevertheless, few studies have been devoted to the analysis of temporal and dynamical aspects of speech in schizophrenia.

Nonetheless, linguistic processes in psychopathology have been studied as dynamical phenomena in patient–therapist interactions. These studies showed that the patient–therapist interactions can be quantified using symbolic encoding (Rapp et al., 1991), and that critical transitions occur between periods of stability during the therapeutic process (Schiepek et al., 1997, Kowalik et al., 1997).

In the present article, schizophrenic speech was compared with control speech in a simple immediate-recall task. Speech production was encoded into sequences of discrete symbols according to the linguistic level of each proposition (Kintsch and Van Dijk, 1978). Symbolic sequences were then studied using both classical (i.e., counting) and dynamical methods. The dynamical methods allow both the quantification of the amount of “disorder” in the sequences and a description of transitions between symbols. According to the speech deficiencies and to the memory deficits observed in schizophrenia (Condray et al., 1996, Salamé et al., 1998), we hypothesized that recall complexity should be diminished in patients compared with controls. Moreover, on the basis of context-processing deficits and language-production models, we expected a specific temporal organization in patients' recall.