Brain/behavior Asymmetry in Schizophrenia: A MEG Study of Cross-modal Semantic Priming

Language-Related Neurophysiological Deficits in Schizophrenia

Schizophrenia is a severe psychiatric disorder that affects all aspects of one's life with several cognitive and social dysfunctions. However, there is still no objective and universal index for diagnosis and treatment of this disease. Many researchers have studied language processing in schizophrenia since most of the patients show symptoms related to language processing, such as thought disorder, auditory verbal hallucinations, or delusions. Electroencephalography (EEG) and magnetoencephalography (MEG) with millisecond order high temporal resolution, have been applied to reveal the abnormalities in language processing in schizophrenia. The aims of this review are (a) to provide an overview of recent findings in language processing in schizophrenia with EEG and MEG using neurophysiological indices, providing insights into underlying language related pathophysiological deficits in this disease and (b) to emphasize the advantage of EEG and MEG in research on language processing in schizophrenia.

Magnetoencephalographic evidence of early right hemisphere overactivation during metaphor comprehension in schizophrenia

Whereas language processing in neurotypical brains is left lateralized, individuals with schizophrenia (SZ) display a bilateral or reversed pattern of lateralization. We used MEG to investigate the implications of this atypicality on fine (left hemisphere) versus coarse (right hemisphere) semantic processing. Ten SZ and 14 controls were presented with fine (conventional metaphor, literal, and unrelated expressions) and coarse (novel metaphor) linguistic stimuli. Results showed greater activation of the right hemisphere for novel metaphors and greater bilateral activation for unrelated expressions at the M170 window in SZ. Moreover, at the M350, SZ showed reduced bilateral activation. We conclude that SZ are overreliant on early-stage coarse semantic processing. As a result, they jump too quickly to remote conclusions, with limited control over the meanings they form. This may explain one of the core symptoms of the disorder-loose associations.

Neural correlates of semantic associations in patients with schizophrenia

Patients with schizophrenia have semantic processing disturbances leading to expressive language deficits (formal thought disorder). The underlying pathology has been related to alterations in the semantic network and its neural correlates. Moreover, crossmodal processing, an important aspect of communication, is impaired in schizophrenia. Here we investigated specific processing abnormalities in patients with schizophrenia with regard to modality and semantic distance in a semantic priming paradigm. Fourteen patients with schizophrenia and fourteen demographically matched controls made visual lexical decisions on successively presented word-pairs (SOA = 350 ms) with direct or indirect relations, unrelated word-pairs, and pseudoword-target stimuli during fMRI measurement. Stimuli were presented in a unimodal (visual) or crossmodal (auditory-visual) fashion. On the neural level, the effect of semantic relation indicated differences (patients > controls) within the right angular gyrus and precuneus. The effect of modality revealed differences (controls > patients) within the left superior frontal, middle temporal, inferior occipital, right angular gyri, and anterior cingulate cortex. Semantic distance (direct vs. indirect) induced distinct activations within the left middle temporal, fusiform gyrus, right precuneus, and thalamus with patients showing fewer differences between direct and indirect word-pairs. The results highlight aberrant priming-related brain responses in patients with schizophrenia. Enhanced activation for patients possibly reflects deficits in semantic processes that might be caused by a delayed and enhanced spread of activation within the semantic network. Modality-specific decreases of activation in patients might be related to impaired perceptual integration. Those deficits could induce and increase the prominent symptoms of schizophrenia like impaired speech processing.

Neural basis of semantic priming in schizophrenia during a lexical decision task: a magneto-encephalography study

Numerous behavioral and electrophysiological studies have provided evidence of abnormal semantic processing in schizophrenia. However, the neural basis of these deficits is poorly understood. We investigated magnetic cortical responses elicited by a word-pair lexical decision task in 20 patients with schizophrenia and 12 healthy control subjects. The task involved presentation of a prime word (200 ms), followed by a blank (250 ms), and then a target stimulus (1200 ms); the subject had to decide whether the target was a real word or not. During this task, bilateral temporal and left prefrontal activations were observed in both groups. However, in contrast to controls, patients with schizophrenia did not show increased activation in the left temporal and anterior cingulate cortices between 200 and 450 ms in response to semantic incongruity. These results suggested that schizophrenia was associated with a functional disturbance in some semantic regions that gave rise to the N400 component. Moreover, a significant modulation in the right temporal cortex was observed in patients, but not in controls. This suggested the existence of alternative processes in patients because both groups showed similar behavioral priming. Finally, we elucidated some functional abnormalities in the semantic network during prime word processing in patients, indicated by prolonged activation compared to healthy controls. Thus, in addition to context integration impairment, abnormal activations during the prime word provided new evidence of context processing deficits in schizophrenia.