General intellectual ability does not explain the general deficit in schizophrenia

Patients with schizophrenia demonstrate a generalized deficit across multiple cognitive domains. However, it is unknown whether this deficit is largely due to lower intelligence, or if there is an impact of schizophrenia which cannot be accounted for by measures of general intellectual ability (GIA). We created four IQ-matched strata of equal width between 89 healthy volunteers (HC) and 77 patients with schizophrenia (SZ) who had very similar IQ and reading scores within each stratum, then compared each stratum's performance on the MATRICS Consensus Cognitive Battery (MCCB). We hypothesized that any patient impairment on the MCCB after matching on IQ would be evidence that GIA does not fully explain the general deficit seen in schizophrenia. We found that patients showed evidence of greater neuropsychological impairment than what would be expected based solely on their IQ and reading ability scores. Further, this deficit was stronger in some cognitive domains than others, namely, processing speed and social cognition. These results suggest the presence of a distinction between GIA and generalized neuropsychological impairment that was consistent in magnitude across all patients, regardless of IQ.