Handedness is related to formal thought disorder and language dysfunction in schizophrenia

The handedness of 29 schizophrenics, 42 manic-depressives, and 25 normal controls was examined in relation to formal thought disorder and language dysfunction. Subjects demonstrated hand preference (Preference Index) and also performed four tasks of manual proficiency. On each of the four proficiency tasks a lateral proficiency score was computed to quantify direction and degree of manual superiority and their mean constituted the Lateral Proficiency Index. A Variability Index, consisting of their standard deviation, was also computed. Sinistrality, as measured by the Preference Index, was related to severity of formal thought disorder in male schizophrenics. The Variability Index was significantly related to severity of formal thought disorder and to language dysfunction in the schizophrenic group as a whole. These relations were not demonstrated in the control groups. These findings support the hypothesis that, in schizophrenia, atypical handedness is a marker of left-hemisphere dysfunction that also disrupts language processes, including those implicated in the manifestation of formal thought disorder.