Why all the confusion? Experimental task explains discrepant semantic priming effects in schizophrenia under "automatic" conditions: evidence from Event-Related Potentials

Abstract

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.

Figure 1

Electrode montage and regions used in analysis. Mid regions: A – Anterior frontal, B – Frontal, C – Central, D – Central-posterior, E – Posterior. Lateral regions: F – Anterior, G – Posterior.

Figure 2

(Top) ERPs to directly-related and unrelated target words at 2 parietal-occipital sites by group (patient, control) and task (implicit, explicit). Linearly interpolated voltage maps showing the scalp distribution of differences in ERPs elicited by target words produced using EEGLAB v4.512 for MatLab software: scalp distributions of comparisons between unrelated-directly related in controls and patients in the N400 time window. (Bottom) ERPs to indirectly-related and unrelated target words at 2 parietal-occipital sites by group (patient, control) and task (implicit, explicit). Linearly interpolated voltage maps showing the scalp distribution of differences in ERPs elicited by target words produced using EEGLAB v4.512 for MatLab software: scalp distributions of comparisons between unrelated-indirectly related in controls and patients in the N400 time window.

Figure 3

Bar graph depicting N400 difference scores (unrelated-directly-related; unrelated-indirectly-related) by group (patient, control) and task (implicit, explicit). Negative is plotted upward for ease of comparison with ERP plots.