Papers

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.

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.

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.

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.

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.

To build up coherence between sentences (comprehend discourse), we must draw inferences, i.e. activate and integrate information that is not actually stated. We used event-related fMRI to determine the localization and extent of brain activity mediating causal inferencing across short, three-sentence scenarios. Participants read and made causal coherence judgments to sentences that were highly causally related, intermediately related or unrelated to their preceding two-sentence contexts. The highly related and intermediately related scenarios were matched in terms of semantic similarities between their individual component words. A pre-rating study established that causal inferences were generated to the intermediately related but not to the highly related or unrelated scenarios. In the scanner, sentences that were intermediately related (relative to highly related or unrelated) to their preceding contexts were associated with longer judgment reaction times and sustained increases in hemodynamic activity within left lateral temporal/inferior parietal/prefrontal cortices, the right inferior prefrontal gyrus and bilateral superior medial prefrontal cortices. In contrast, sentences that were unrelated (relative to highly related) to their preceding contexts were associated with only transient increases in activity (at, but not after, the peak of the hemodynamic response) within the right lateral temporal cortex and the right inferior prefrontal gyrus. These data suggest that, to make sense of discourse, we activate a large bilateral cortical network in response to what is not explicitly stated. We suggest that this network reflects the activation, retrieval and integration of information from long-term semantic memory into incoming discourse structure during causal inferencing.

In two experiments, direct and indirect semantic priming were measured using event-related potentials. In Experiment 1, participants rated the relatedness between prime and target on a seven-point scale. In Experiment 2, participants simply read the primes and targets as they monitored for a semantic category in probe filler items. Significant direct and indirect N400 priming effects were observed in both experiments. In Experiment 1, the indirect N400 priming effect remained significant when indirectly related and unrelated word pairs were matched for participants’ explicit relatedness judgments. In both experiments, the indirect N400 priming effects were preserved when indirectly related and unrelated word pairs were matched on more global and objective measures of semantic similarity. These findings are discussed in the context of current theoretical models of semantic memory and semantic priming.

An impairment in the build-up and use of context has been proposed as a core feature of schizophrenia. The current study tested the hypothesis that schizophrenia patients show impairments in building up context within sentences because of abnormalities in combining semantic with syntactic information. Schizophrenia patients and healthy controls read and made acceptability judgments about sentences containing verbs that were semantically associated with individual preceding words but that violated either the meaning (animacy/semantic constraints) or the syntactic structure (morphosyntactic constraints) of their preceding contexts. To override these semantic associations and determine that such sentences are unacceptable, participants must integrate semantic with syntactic information. These sentences were compared with congruous and pragmatically/semantically violated sentences that imposed fewer semantic-syntactic integration demands. At sentence-final words and decisions, patients showed smaller reaction time differences than controls to animacy/semantically violated or morphosyntactically violated sentences relative to pragmatically/semantically violated or nonviolated sentences. The relative insensitivity to these violations in patients with schizophrenia may arise from impairments in combining semantic and syntactic information to build up sentence context.

Despite decades of research, it remains controversial whether semantic knowledge is anatomically segregated in the human brain. To address this question, we recorded event-related potentials (ERPs) while participants viewed pictures of animals and tools. Within the 200-600-ms epoch after stimulus presentation, animals (relative to tools) elicited an increased anterior negativity that, based on previous ERP studies, we interpret as associated with semantic processing of visual object attributes. In contrast, tools (relative to animals) evoked an enhanced posterior left-lateralized negativity that, according to prior research, might reflect accessing knowledge of characteristic motion and/or more general functional properties of objects. These results support the hypothesis of the neuroanatomical knowledge organization at the level of object features: the observed neurophysiological activity was modulated by the features that were most salient for object recognition. The high temporal resolution of ERPs allowed us to demonstrate that differences in processing animals and tools occurred specifically within the time-window encompassing semantic analysis.