Memory processes in schizophrenia: impact of comorbidity with substance use disorders

To compare verbal memory encoding, storage, and retrieval in patients with schizophrenia (SZ), SZ plus substance use disorder (SZ+), and substance use disorder only (SUD), testing the hypothesis that SZ + group exhibits greater impairment across all processes.

A total of 294 male patients under treatment (SZ = 72, SZ+ = 72, SUD = 150) meeting DSM-5 criteria completed the Rey Auditory Verbal Learning Test (RAVLT). RAVLT measures assessed encoding, storage, and retrieval. ANCOVA/MANCOVA, controlling for premorbid IQ, were used to explore group differences.

Significant differences among groups were observed in all RAVLT measures (F(2,291) > 9.25, p < 0.001, ηp² > 0.06) except retrieval. Post hoc analyses revealed that both SZ and SZ+ groups showed significant verbal memory impairments (learning trials and storage, interference, short and long-term recall and recognition) compared to the SUD group which performed within the normative range. The SZ and SZ+ groups showed altered values (Z ≥ -1.5) from the second learning trial onward and total learning, and the SZ+ group also for long-term recall and recognition.

This study confirms the existence of significant verbal memory deficits in both SZ and SZ+ groups compared to SUD. Verbal memory impairment appears as a central feature of SZ spectrum disorders, irrespective of SUD comorbidity. Exacerbated memory impairment in SZ+ compared to SZ on the RAVLT is subtle without reaching significant differences, although consideration of altered Z-scores suggests worse performance in SZ+ in encoding and consolidation processes. Further research should explore clinical variables and moderators of comorbidity effects in SZ.