Theory of Mind in Schizophrenia and Asperger's Syndrome: Relationship with Negative Symptoms

Objective: Although previous studies have shown that the theory of mind (ToM) ability is impaired in Asperger's Syndrome (AS) and in schizophrenia, few controlled studies compared the ToM performance between the two disorders. Besides, the relationship between the degree of ToM impairment and symptom dimensions is unclear, and presence of ToM impairment in remitted patients with schizophrenia is controversial. Here, we tested the hypothesis that schizophrenia patients with prominent negative symptoms were closer to AS patients and different than schizophrenia patients without prominent negative symptoms and healthy controls in terms of ToM functioning. Method: Fourteen patients with AS, 20 with schizophrenia and 20 healthy controls, matched by age, educational level and IQ scores were enrolled. AS was diagnosed according to the DSM-IV criteria and independently confirmed by two psychiatrists. Schizophrenia patients were diagnosed by the Turkish version of Structured Clinical Interview for DSM-IV Diagnosis (SCID-I) and symptom severity was evaluated with the Scale for the Assessment of Negative and Positive Symptoms. Schizophrenia group consisted of clinically stable patients. The ToM battery included stories to assess first and second order false belief tasks (ToM1 and ToM2). The full-scale IQ, Verbal Comprehension, Freedom from Distractibility and Perceptual Organization scores were assessed by Weschler Adult Intelligence Scale-Revised (WAIS-R). Non-parametric tests were used to compare the neuropsychological performances of the three groups. In order to investigate whether schizophrenia patients with prominent negative symptoms were similar to AS patients, schizophrenia patients were divided into high (Sch-HN) and low (Sch-LN) negative-symptom subgroups by median split. For these four groups (AS, Sch-HN, Sch-LN, and controls) between group comparisons were performed. Correlations between the clinical measures and ToM performance were assessed by Spearman correlation test. Results: AS and schizophrenia patients performed significantly worse than controls in the ToM2 task, while the AS group had worse ToM1 performance than both schizophrenia patients and healthy controls. The Sch-HN subgroup had significantly lower ToM2 scores than the Sch-LN patients, and worse ToM1 functioning than the controls. Conclusions: These results suggest that clinically stable schizophrenia patients have ToM impairments. Sch-HN group performed comparably poorly as the AS group, while the Sch-LN group was relatively spared. The roost profoundly impaired patients with schizophrenia in terms of ToM functioning were represented by those with high negative symptoms (Sch-HN). Similar to AS, as a neurodevelopmental impairment, these patients may not have developed ToM ability, or they may have lost their ToM capacity as a result of a neurodegenerative process during the illness. Supplementary studies using other methods (e.g., neuroimaging, neurophysiology) may highlight the brain regions that are affected differentially in AS and schizophrenia, the relationship of ToM impairments and negative symptoms, and the role of ToM impairments in the neurodevelopmental or neurodegenerative hypothesis of schizophrenia.