Language lateralization in schizophrenia

Abstract

Schizophrenia is a severe chronic psychiatric illness that affects approximately 1-2% of the populations worldwide. Schizophrenia is characterized by episodes of psychosis, in which patients experience hallucinations (false perceptions) and delusions (false beliefs). Apart from the psychotic episodes, patients frequently have negative symptoms: they become apathic, loose interest in hobbies and friends and lack energy. The cause of schizophrenia is not clear, though several theories have been proposed. One of these theories states that language lateralization is decreased in schizophrenia. The relative high language activity in the right (non-dominant) hemisphere could be the source of language-related psychotic symptoms, such as the false perception of voices.

This hypothesis is tested in this thesis by applying functional Magnetic Resonance Imaging (fMRI) to measure language activity of each hemisphere. Language lateralization in right-handed men with schizophrenia was compared to that of healthy right-handed men. The patients had a lower mean degree of language lateralization than the controls. Patients who frequently experienced hallucinations, which were auditory verbal in nature, had lower degrees of language lateralization than patients without prominent hallucinations at the time of scanning. This finding was replicated in female patients.

In order to investigate whether decreased language lateralization is a result of this severe psychiatric condition or if it is a possible etiological factor a twin study was undertaken. Monozygotic twin pairs from which only one of the twin pair had schizophrenia were compared to healthy monozygotic twin pairs. Language lateralization was found to be decreased in both the affected twin and his/her unaffected twin brother/ sister. This implies that decreased language lateralization is already present before the outbreak of the disease and that it may constitute a genetic risk factor for schizophrenia.

In conclusion, some support was found that language lateralization is decreased in schizophrenia and that decreased lateralization may be a risk factor for schizophrenia. However, it remains unclear if decreased lateralization plays a role in the patho-physiological mechanism of psychosis.