CONTEXT: Loosening of associations has long been considered a core feature of schizophrenia, but its neural correlate remains poorly understood. OBJECTIVE: To test the hypothesis that, in comparison with healthy control subjects, patients with schizophrenia show increased neural activity within inferior prefrontal and temporal cortices in response to directly and indirectly semantically related (relative to unrelated) words. DESIGN: A functional neuroimaging study using a semantic priming paradigm. SETTING: Lindemann Mental Health Center, Boston, Mass. PARTICIPANTS: Seventeen right-handed medicated outpatients with chronic schizophrenia and 15 healthy volunteers, matched for age and parental socioeconomic status. INTERVENTIONS: Functional magnetic resonance imaging as participants viewed directly related, indirectly related, and unrelated word pairs and performed a lexical decision task. MAIN OUTCOME MEASURES: Event-related functional magnetic resonance imaging measures of blood oxygenation level-dependent activity (1) within a priori temporal and prefrontal anatomic regions of interest and (2) at all voxels across the cortex. RESULTS: Patients and controls showed no behavioral differences in priming but opposite patterns of hemodynamic modulation in response to directly related (relative to unrelated) word pairs primarily within inferior prefrontal cortices, and to indirectly related (relative to unrelated) word pairs primarily within temporal cortices. Whereas controls showed the expected decreases in activity in response to semantic relationships (hemodynamic response suppression), patients showed inappropriate increases in response to semantic relationships (hemodynamic response enhancement) in many of the same regions. Moreover, hemodynamic response enhancement within the temporal fusiform cortices to indirectly related (relative to unrelated) word pairs predicted positive thought disorder. CONCLUSION: Medicated patients with chronic schizophrenia, particularly those with positive thought disorder, show inappropriate increases in activity within inferior prefrontal and temporal cortices in response to semantic associations.
Traditionally, event-related brain potential (ERP) studies of language processing have presented words at a fixed rate using rapid serial visual presentation. Recent studies suggest, however, that the processes engaged during sentence comprehension are contingent on word presentation rate. These findings underscore the importance of allowing participants to read at a natural pace. The present study employed simultaneous self-paced reading and ERP methodologies to examine behavioral and neural responses while participants read sentences containing pragmatic or morphosyntactic violations or no violations. ERP and self-paced reading results replicated previous findings. This novel combination of behavioral and ERP methodologies combines the high temporal resolution and direct neural measures offered by ERPs with the more natural reading environment and information about processing load provided by self-paced reading.
In 1980, the N400 event-related potential was described in association with semantic anomalies within sentences. When, in 1992, a second waveform, the P600, was reported in association with syntactic anomalies and ambiguities, the story appeared to be complete: the brain respected a distinction between semantic and syntactic representation and processes. Subsequent studies showed that the P600 to syntactic anomalies and ambiguities was modulated by lexical and discourse factors. Most surprisingly, more than a decade after the P600 was first described, a series of studies reported that semantic verb-argument violations, in the absence of any violations or ambiguities of syntax can evoke robust P600 effects and no N400 effects. These observations have raised fundamental questions about the relationship between semantic and syntactic processing in the brain. This paper provides a comprehensive review of the recent studies that have demonstrated P600s to semantic violations in light of several proposed triggers: semantic-thematic attraction, semantic associative relationships, animacy and semantic-thematic violations, plausibility, task, and context. I then discuss these findings in relation to a unifying theory that attempts to bring some of these factors together and to link the P600 produced by semantic verb-argument violations with the P600 evoked by unambiguous syntactic violations and syntactic ambiguities. I suggest that normal language comprehension proceeds along at least two competing neural processing streams: a semantic memory-based mechanism, and a combinatorial mechanism (or mechanisms) that assigns structure to a sentence primarily on the basis of morphosyntactic rules, but also on the basis of certain semantic-thematic constraints. I suggest that conflicts between the different representations that are output by these distinct but interactive streams lead to a continued combinatorial analysis that is reflected by the P600 effect. I discuss some of the implications of this non-syntactocentric, dynamic model of language processing for understanding individual differences, language processing disorders and the neuroanatomical circuitry engaged during language comprehension. Finally, I suggest that that these two processing streams may generalize beyond the language system to real-world visual event comprehension.
Recent event-related potential studies report a P600 effect to incongruous verbs preceded by semantically associated inanimate noun-phrase (NP) arguments, e.g., "eat" in "At breakfast the eggs would eat...". This P600 effect may reflect the processing cost incurred when semantic-thematic relationships between critical verbs and their preceding NP argument(s) bias towards different interpretations to those dictated by their sentences' syntactic structures. We have termed such violations of alternative thematic roles, 'thematic role violations.' Semantic-thematic relationships are influenced both by semantic associations and by more basic semantic features, such as a noun's animacy. This study determined whether a P600 effect can be evoked by verbs whose thematic structures are violated by their preceding inanimate NP arguments, even in the absence of close semantic-associative relationships with these arguments or their preceding contexts. ERPs were measured to verbs under four conditions: (1) non-violated ("At breakfast the boys would eat..."); (2) preceded by introductory clauses and animate NPs that violated their pragmatic expectations but not their thematic structures ("At breakfast the boys would plant..."); (3) preceded by semantically related contexts but inanimate NPs that violated their thematic structures ("At breakfast the eggs would eat..."); (4) preceded by semantically unrelated contexts and inanimate NPs that also violated their thematic structures ("At breakfast the eggs would plant..."). Pragmatically non-thematic role violated verbs preceded by unrelated contexts and animate NPs evoked robust N400 effects and small P600 effects. Thematically violated verbs preceded by inanimate argument NPs evoked robust P600 effects but no N400 effects, regardless of whether these inanimate arguments or their preceding contexts were semantically related or unrelated to these verbs. These findings suggest that semantic-thematic relations, related to animacy constraints on verbs' arguments, are computed online and can immediately impact verb processing within active, English sentences.
Impairments in the buildup and use of context may lead to disorders of thought and language in schizophrenia. To test this hypothesis, event-related potentials (ERPs) were measured while patients and healthy controls read sentences that were highly causally related, intermediately related, or unrelated to preceding contexts. Although patients were slower than controls, both groups used the discourse context similarly as evidenced by similar reaction time patterns across conditions. Neurally however, different patterns emerged between patients and controls: within the N400 time window, patients failed to modulate their neural responses across conditions. This failure to differentiate between conditions was specifically correlated with positive thought disorder. Results suggest that schizophrenia patients, particularly those with positive thought disorder, fail to make immediate use of discourse context to build up semantic coherence in the brain.