Clinical correlates of parametric digit-symbol substitution test in schizophrenia

Processing speed deficit, ascertained by digit-symbol substitution test (DSST), is considered as a fundamental impairment in schizophrenia. Clinical correlates of processing speed abnormalities, especially using the parametric version of DSST is yet to be evaluated comprehensively. In this study, we examined schizophrenia patients (N = 66) and demographically matched healthy controls (N = 72) using computer-administered parametric DSST (pDSST) with fixed (pDSST(F)) as well as random (pDSST(R)) conditions and analysed the relationship between pDSST performance and clinical symptoms. Psychopathology was assessed using Scale for Assessment of Positive Symptoms (SAPS)/Negative Symptoms (SANS) with good inter-rater reliability. In comparison with healthy controls, patients demonstrated significantly lesser number of correct responses (C(N)) in pDSST(F) (t = 8.0; p < 0.001) and pDSST(R) (t = 7.8; p < 0.001) as well as significantly prolonged reaction time in pDSST(F) (t = 7.1; p < 0.001) and pDSST(R) (t = 7.0; p < 0.001). The difference in C(N) between pDSST(F) and pDSST(R) [ΔC(N)] was significantly lesser in patients than healthy controls (t = 2.61; p = 0.01). The pDSST reaction time had significant positive correlation with negative syndrome scores as well as bizarre behaviour score. Significantly greater processing speed deficits in pDSST suggest potential relational memory/visual scanning abnormalities in schizophrenia. Furthermore, pDSST deficits demonstrated a significant association with the psychopathology, especially with the various negative symptoms and bizarre behaviour.