Processing speed is associated with differences in IQ and cognitive profiles between patients with schizophrenia and their healthy siblings.

Background: Processing speed turns out to be the central area of research on cognition in schizophrenia. So far the relationship between this dimension and the IQ level of patients and their healthy siblings has not been investigated.

Aim: To investigate the differences in cognitive speed in patients with schizophrenia and their healthy siblings, and to determine whether cognitive speed as a covariate affects differences in IQ and cognitive profiles between groups.

Methods: Forty-seven inpatients diagnosed with schizophrenia according to DSM-IV (SCH) and their 36 healthy siblings (HSB) were tested with cognitive speed tasks according to Bartzokis et al. method and Wechsler Intelligence Scale. Additional control for the possible impact of antipsychotic drugs and selected demographic variables on the cognitive performance was taken into account.

Results: The siblings scored significantly higher in the cognitive speed task (p < 0.01) than patients, the WAIS-R cognitive test profiles were also significantly different in two ways: between groups, and between single test results in each of the assessed groups. The interaction effect: ANOVA, F(10, 770) = 2.798, p = 0.002. Similarly, the Performance and Full Scale IQs were significantly different, at p < 0.01. After controlling for cognitive speed, all significant differences no longer exist: e.g. Full Scale IQ, p = 0.459.

Conclusions: Significant differences in cognitive speed between patients and their healthy siblings generate the differences in the cognitive profile assessed with Wechsler Intelligence Scale. Some problems of cognitive speed diagnosis and further research on the cognitive schizophrenia endophenotype were discussed.