Semantic processing in schizophrenia

While investigating brain structure in schizophrenia is one line of research, others have focused on behavioral and cognitive correlates of schizophrenia. Early studies used memory tasks to examine semantic categorization processes. Semantic categorization was considered to be an encoding

strate-3 As indexed by the Thought Disorder Index (TDI, Johnston & Holzman, 1979). This test elicits speech samples by Rorschach cards.

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B.1.4 : Introduction - Semantic processing in schizophrenia

gy in order to reduce the volume of the materials to be remembered (Craik & Lockhart, 1972).

Words lists are recalled in groups of categories and reflect the organization of memory storage.

Comparing healthy controls, non-schizophrenic and schizophrenic psychiatric patients, Koh et al.

(1973) found that schizophrenic patients recalled in a less categorically organized way. However, when patients where asked to classify words of a list before learning, patients with schizophrenia used categories similar to normals ( Koh et al (1974)). Moreover, other studies found similar recall performance for schizophrenic patients and controls (Koh & Peterson, 1978; Russel & Beekhuis, 1976).

More recently, possible anomalies in semantic processing have been investigated within the frame-work of connectionism. In early connectionistic models of the semantic system, each word is repre-sented as a node in an interconnected network. Models assume a distributed representation of differ-ent aspects of a word or concept by corresponding nodes within a network (McClelland & Rumel-hart, 1987). These networks can be assessed by the phenomenon of semantic priming. Semantic priming refers to the effect that a word (e.g. dog) is recognized faster if it is preceded by a semanti-cally related one (e.g. cat) (Meyer & Schvaneveldt, 1971).

The assessment of semantic networks is of particular interest as abnormal or disinhibited semantic associative networks have been suggested as a possible explanation for the psychopathology of schizophrenia (Spitzer, 1997) and consequently priming paradigms have been applied to study se-mantic associations in schizophrenia. Most of these studies have been interpreted within the frame-work of information processing models that make a distinction between automatic and controlled processes (Schiffrin & Schneider, 1984). The study of automatic vs. controlled semantic priming is achieved by manipulating the stimulus onset asynchrony (SOA) between a prime and a target stimu-lus. Automatic processes (short SOA) are rapid and unconscious. Longer SOAs are used to examine controlled processes, which are slower and can be either conscious or unconscious (Posner, 1986).

B.1.4 : Introduction - Semantic processing in schizophrenia

Several studies examining automatic priming found an enhanced spread of activation (e.g. Moritz et al., 2003; Spitzer et al. 1994; Weisbrod et al., 1998). Moritz et al. (2003) used a) indirectly related (e.g. stone-soft), b) directly related (e.g. sun-moon), c) unrelated (e.g. shelter-paint) word pairs in a word pronunciation task with a short SOA of 200 ms. Patients with thought-disorder exhibited stronger semantic priming for the indirect task compared to controls and non-thought-disordered pa-tients. Such an enhanced spread of activation in thought-disordered patients for indirectly related stimulus materials was also reported earlier by Weisbrod et al. (1998). A study by Spitzer et al.

(1994) showed semantic priming effects in a lexical decision task which were larger for a group of 70 schizophrenic patients than for 44 normal controls. These group differences did not depend on SOAs (200, 400, 700 ms) and priming effects were largest for the subgroup of thought-disordered patients. However, larger priming effects in TD patients were not found in all studies. Aloia and coworkers (1998) found no overall differences between controls and patients with mild thought dis-orders in a word pronunciation task with varying degrees of relatedness (high, medium, low) and word pairs taken from the same semantic category. While controls and patients with low levels of thought disorder showed significant priming in the high- and medium association conditions, but not in the low-association condition, a different pattern emerged for patients with high levels of thought disorder. This subgroup showed inhibited responses for the high and medium associates, i.e. reduced semantic priming.

Though the majority of studies show enhanced semantic priming in schizophrenia, the topic remains contradictory as some studies have not found larger priming in schizophrenia (e.g. Barch et al, 1996^; Chapin et al., 1992;Vinogradov et al, 1992). This might be related to methodological differ-ences, as not all studies included patients with thought disorder or examined only direct semantic priming.

An electrophysiological measure of the neural mechanism underlying semantic priming is the 62

B.1.4 : Introduction - Semantic processing in schizophrenia

N400 component in the human EEG. The N400 is a negative ERP deflection following semantic targets that are not primed by the prior context. Congruent with its indexing of deviations of se-mantic predictabilities of a word, the N400 was found to be larger to unrelated than to related words (Kutas & Hillyard, 1989).

The N400 component was examined in several studies with schizophrenic patients and a recent re-view was published by Kumar & Debruille (2004). A reduced N400 in schizophrenia was initially reported by Adam and coworkers (1993). The authors examined the ERP evoked by sentences with semantically correct or anomalous endings. Compared to normal subjects, a group of 12 chronic schizophrenic patients had a significantly increased latency and reduced amplitude of the N400 to the incorrect endings. An earlier study by Koyama et al. (1991) also found a prolonged N400 laten-cy for schizophrenic patients in a lexical decision task, but did not show any differences in N400 amplitude compared to control subjects. More recently, Condray et al. (1999; 2003) recorded the N400 during a lexical decision task in which semantic relationship (high vs. non-associated word pairs) and SOA (350 ms vs. 950 ms) were varied. While for control subjects the N400 discriminat-ed highly significant between the levels of semantic context/relatdiscriminat-edness, the schizophrenic group showed reduced discrimination of semantic context in the N400. Similar findings were reported by Mathalon et al. (2002) for a picture-word matching task with a short interstimulus interval. The N400 was reduced for schizophrenic patients following unprimed words, but did not differ from normals if the word was primed by the previous picture.

Within the framework of semantic network models these results are consistent with the assumption of an abnormally large spread of activation and/or inadequate use of context information in patients with schizophrenia (McCarley et al., 1999b).

B.1.5 : Introduction - Thought disorder in schizophrenia