Semantic bias, homograph comprehension, and event-related potentials in schizophrenia

Abstract

Objectives: It is controversial whether a semantic processing bias for strong associates is present in schizophrenia, and unknown whether the language abnormalities observed in schizophrenia can be attributed to dysfunctions early or late in cognitive processing. Combined behavioral and event-related potential (ERP) data can indicate the nature and timing of such abnormalities.

Methods: Sensibility judgements of dominant and subordinate homograph sentences were measured in 12 schizophrenia patients and 13 normal controls. ERPs were recorded to the disambiguating sentence-ending word.

Results: All subjects showed greatest misinterpretation of subordinate homograph sentences, but schizophrenia patients particularly misinterpreted these sentence types. For control subjects, subordinate homograph sentences that were classified as nonsensical showed greater N400 than those classified as sensible. By contrast, the N400 of patients was large, regardless of the sensibility judgement--patients' brains initially responded to all subordinate sentences as if nonsensical. These data are consonant with a semantic bias. However, the patients' N400 to dominant homograph sentence endings was also larger than that of controls, a finding not consonant with a semantic bias.

Conclusions: The behavioral results indicate a selective comprehension abnormality in schizophrenia dependent on the content of verbal memory. The ERP results suggest a pervasive contextual memory failure. A semantic activation decay model is proposed to explain these results.

Fig. 1

Percent of comprehension errors for the two groups for each type of sentence. Panel A shows that patients make more errors to all sentences, both groups make more errors in comprehending subordinate homograph meanings, and that patients make disproportionately more errors to subordinate sentences than controls. Panel B presents the error rates to homograph sentences controlling for the base error rate by subtraction of the error rate on one meaning noun sentences. The persistently increased error rate to subordinate sentences indicates a semantic bias.

Fig. 2

Grand averaged waveforms to the last word of subordinate sentences from each group at sagittal midline electrode sites (Fz, Cz, Pz )and lateral sites (TCP1 and TCP2). Sensible: ERPs to subordinate homograph sentence endings judged sensible by the subjects. Nonsensical: ERPs to subordinate homograph sentence endings judged nonsensical by subjects.

Fig. 3

Grand averaged waveforms to the last word of sentences judged sensible from each group at sagittal midline electrode sites (Fz, Cz, Pz) and lateral sites (TCP1 and TCP2). Dominant: ERPs to dominant homograph sentence endings judged sensible by the subjects. Subordinate: ERPs to subordinate homograph sentence endings judged sensible by subjects. These subordinate ERPs are the same waveforms as in Fig. 2, presented for ease of comparison along this different dimension.

Fig. 4

Model of interaction between initial semantic activation and verbal memory maintenance of activations. Strong associates are initially activated to a greater degree than weak associates. A faulty maintenance function in schizophrenia would cause the content of verbal memory to be determined by the decay rate of the initial excitation. Given a constant decay rate behavior would be solely a function of the semantic relatedness of associates until the point where no initial activation remained, beyond which behavior would be random. A bias toward strong associates would be greatest in the interval where weak associate activation approaches zero but strong associate activation remains above threshold.