Papers

Disturbances of thought and language are fundamental to schizophrenia. Cognitive behavioral and electrophysiological research has implicated problems in two different neurocognitive mechanisms: abnormalities in the structure and function of semantic memory, and abnormalities in combining and integrating words together to build up sentence and discourse context. This review discusses recent electrophysiological evidence suggesting that these two deficits are not completely distinct, but rather that language impairment in schizophrenia results from a dysfunctional interaction between these systems in an effort to build up higher-order meaning. Moreover, although language abnormalities are more pronounced in patients with positive thought disorder, they manifest themselves in all patients when increased demands are placed on the comprehension system. Further investigation of language dysfunction may also provide insights into other aspects of psychotic thought.

This overview outlines findings of cognitive and neurocognitive studies on comprehension of verbal, pictorial, and video stimuli in healthy participants and patients with schizophrenia. We present evidence for a distinction between two complementary neurocognitive streams of conceptual analysis during comprehension. In familiar situations, adequate understanding of events may be achieved by mapping the perceived information on the associative and similarity-based connections between concepts in semantic memory - a process reflected by an N400 waveform of event-related electrophysiological potentials (ERPs). However, in less conventional contexts, a more flexible mechanism may be needed. We suggest that this alternative processing stream, reflected by a P600 ERP waveform, may use discrete, rule-like goal-related requirements of real-world actions to comprehend relationships between perceived people, objects, and actions. This neurocognitive model of comprehension is used as a basis in discussing studies in schizophrenia. These studies suggest an imbalanced engagement of the two conceptual streams in schizophrenia, whereby patients may rely on the associative and similarity-based networks in semantic memory even when it would be more adaptive to recruit mechanisms that draw upon goal-related requirements. Finally, we consider the roles that these conceptual mechanisms may play in real-life behavior, and the consequences that their dysfunction may have for disorganized behavior and inability to plan actions to achieve behavioral goals in schizophrenia.

In two event-related potential (ERP) experiments, we determined to what extent Grice’s maxim of informativeness as well as pragmatic ability contributes to the incremental build-up of sentence meaning, by examining the impact of underinformative versus informative scalar statements (e.g. "Some people have lungs/pets, and…") on the N400 event-related potential (ERP), an electrophysiological index of semantic processing. In Experiment 1, only pragmatically skilled participants (as indexed by the Autism Quotient Communication subscale) showed a larger N400 to underinformative statements. In Experiment 2, this effect disappeared when the critical words were unfocused so that the local underinformativeness went unnoticed (e.g., "Some people have lungs that…"). Our results suggest that, while pragmatic scalar meaning can incrementally contribute to sentence comprehension, this contribution is dependent on contextual factors, whether these are derived from individual pragmatic abilities or the overall experimental context.

This is the first of two articles that discuss higher-order language and semantic processing in schizophrenia. This article reviews clinical characterizations of language output and the phenomenon of positive thought disorder, as well as more principled characterizations of language output in schizophrenia. It also gives an overview of evidence for the predominant theory of language dysfunction in schizophrenia: that it arises from abnormalities in (a) semantic memory and/or (b) working memory and executive function. The companion article (Part 2) focuses on the study of language in schizophrenia using online psycholinguistic methods and considers how the study of schizophrenia may inform our understanding of normal language processing.

This is the second of two articles that discuss higher-order language and semantic processing in schizophrenia. The companion article (Part 1) gives an introduction to language dysfunction in schizophrenia patients. This article reviews a selection of psycholinguistic studies which suggest that sentence-level abnormalities in schizophrenia may stem from a relative overdependence on semantic associative relationships at the expense of building higher-order meaning. Language disturbances in schizophrenia may be best conceptualized as arising from an imbalance of activity across two streams of processing, one drawing upon semantic relationships within semantic memory and the other involving the use of combinatorial mechanisms to build propositional meaning. I will also discuss some of the ways in which the study of schizophrenia may offer new insights into the cognitive and neural architecture of the normal language system.

Clinically, patients with schizophrenia show prominent abnormalities at the discourse level, with production characterized by tangential and illogical relationships between ideas and unclear references. Despite these clinical manifestations, most studies of language in schizophrenia have focused on semantic relationships between single words and the build-up of meaning within single-clause sentences. The present paper discusses the few studies that have gone beyond clause boundaries to fully understand language impairments in schizophrenia. We also give an overview of a relevant literature that considers the neurocognitive mechanisms by which coherence links are established across clauses in healthy adults, providing a framework that may guide future research in this area.

This study examined the electrophysiological correlates of complement coercion. ERPs were measured as participants read and made acceptability judgments about plausible coerced sentences, plausible noncoerced sentences, and highly implausible animacy-violated sentences ("The journalist began/wrote/astonished the article before his coffee break"). Relative to noncoerced complement nouns, the coerced nouns evoked an N400 effect. This effect was not modulated by the number of possible activities implied by the coerced nouns (e.g., began reading the article; began writing the article) and did not differ either in magnitude or scalp distribution from the N400 effect evoked by the animacy-violated complement nouns. We suggest that the N400 modulation to both coerced and animacy-violated complement nouns reflected different types of mismatches between the semantic restrictions of the verb and the semantic properties of the incoming complement noun. This is consistent with models holding that a verb’s semantic argument structure is represented and stored at a distinct level from its syntactic argument structure. Unlike the coerced complement noun, the animacy-violated nouns also evoked a robust P600 effect, which may have been triggered by the judgments of the highly implausible (syntactically determined) meanings of the animacy-violated propositions. No additional ERP effects were seen in the coerced sentences until the sentence-final word that, relative to sentence-final words in the noncoerced sentences, evoked a sustained anteriorly distributed positivity. We suggest that this effect reflected delayed attempts to retrieve the specific event(s) implied by coerced complement nouns.

We used event-related potentials (ERPs) to examine the time-course of processing metaphorical and literal sentences in the brain. ERPs were measured to sentence-final (Experiment 1) and mid-sentence (Experiment 2) critical words (CWs) as participants read and made plausibility judgments about familiar nominal metaphors ("A is a B") as well as literal and semantically anomalous sentences of the same form. Unlike the anomalous words, which evoked a robust N400 effect (on the CW in experiments 1 and 2 as well as on the sentence-final word in experiment 2), CWs in the metaphorical, relative to the literal, sentences only evoked an early, localized N400 effect that was over by 400ms after CW onset, suggesting that, by this time, their metaphorical meaning had been accessed. CWs in the metaphorical sentences also evoked a significantly larger LPC (Late Positive Component) than in the literal sentences. We suggest that this LPC reflected additional analysis that resolved a conflict between the implausibility of the literal sentence interpretation and the match between the metaphorical meaning of the CW, the context and stored information within semantic memory, resulting from early access to both literal and figurative meanings of the CWs.

Origins of impaired adaptive functioning in schizophrenia remain poorly understood. Behavioral disorganization may arise from an abnormal reliance on common combinations between concepts stored in semantic memory. Avolition-apathy may be related to deficits in using goal-related requirements to flexibly plan behavior. The authors recorded event-related potentials (ERPs) in 16 patients with medicated schizophrenia and 16 healthy controls in a novel video paradigm presenting congruous or incongruous objects in real-world activities. All incongruous objects were contextually inappropriate, but the incongruous scenes varied in comprehensibility. Psychopathology was assessed with the Scales for the Assessment of Positive and Negative Symptoms (SAPS/SANS) and the Brief Psychiatric Rating Scale. In patients, an N400 ERP, thought to index activity in semantic memory, was abnormally enhanced to less comprehensible incongruous scenes, and larger N400 priming was associated with disorganization severity. A P600 ERP, which may index flexible object-action integration based on goal-related requirements, was abnormally attenuated in patients, and its smaller magnitude was associated with the SANS rating of impersistence at work or school (goal-directed behavior). Thus, distinct neurocognitive abnormalities may underlie disorganization and goal-directed behavior deficits in schizophrenia.

The schizophrenia research literature contains many differing accounts of semantic memory function in schizophrenia as assessed through the semantic priming paradigm. Most recently, Event-Related Potentials (ERPs) have been used to demonstrate both increased and decreased semantic priming at a neural level in schizophrenia patients, relative to healthy controls. The present study used ERPs to investigate the role of behavioral task in determining neural semantic priming effects in schizophrenia. The same schizophrenia patients and healthy controls completed two experiments in which word stimuli were identical, and the time between the onset of prime and target remained constant at 350 ms: in the first, participants monitored for words within a particular semantic category that appeared only in filler items (implicit task); in the second, participants explicitly rated the relatedness of word-pairs (explicit task). In the explicit task, schizophrenia patients showed reduced direct and indirect semantic priming in comparison with healthy controls. In contrast, in the implicit task, schizophrenia patients showed normal or, in positively thought-disordered patients, increased direct and indirect N400 priming effects compared with healthy controls. These data confirm that, although schizophrenia patients with positive thought disorder may show an abnormally increased automatic spreading activation, the introduction of semantic decision-making can result in abnormally reduced semantic priming in schizophrenia, even when other experimental conditions bias toward automatic processing.