Meta-Analysis Confirms Processing Speed as Core Cognitive Deficit in Schizophrenia
Key Clinical Summary
- Processing speed remains profoundly impaired in people with schizophrenia, with symbol coding tests among the most affected cognitive measures.
- Symbol coding deficits exceeded impairments in most other domains, outperforming 34 of 49 additional cognitive tests in severity.
- Demographic and clinical factors, including age, IQ, inpatient status, and diagnostic composition, moderated impairment magnitude.
Processing speed remains one of the most severely impaired cognitive domains in people with schizophrenia, according to a large systematic review and meta-analysis published in JAMA Psychiatry. Analyzing data from more than 23,000 participants, researchers found that symbol coding tests showed among the greatest deficits compared with controls, exceeding impairments in most other cognitive measures.
Study Findings
The meta-analysis synthesized data from 115 studies published between February 2009 and November 2023. To be included, studies had to compare individuals with schizophrenia with control participants, use contemporary diagnostic criteria, and report symbol coding tests alongside at least 2 other cognitive measures from different domains.
In total, data from 10,114 people with schizophrenia and 13,235 controls were analyzed. Symbol coding tests of processing speed showed a large degree of impairment, with a pooled Hedges g of −1.52 (95% CI, −1.65 to −1.40). This placed symbol coding among the most impaired cognitive tests assessed.
Across 50 cognitive tests included in the analysis, symbol coding was reliably more impaired than 34 of the other measures. However, it did not significantly differ from 15 tests, indicating that while processing speed is highly sensitive to schizophrenia-related deficits, it is not uniquely impaired in all comparisons.
Moderator analyses revealed that several factors influenced the magnitude of symbol coding impairment. These included intelligence quotient (IQ), age differences between patient and control groups, sex assigned at birth composition, inpatient versus outpatient status, and whether samples included individuals with schizoaffective or schizophreniform diagnoses.
Clinical Implications
The study suggests that processing speed impairment may reflect a central feature of broader cognitive impairment rather than an isolated deficit. The authors note that the consistency and severity of symbol coding impairment raise the possibility of an underlying mechanism contributing to global cognitive dysfunction, such as altered brain connectivity, to which processing speed measures may be particularly sensitive.
These findings have important implications for how cognition is evaluated in patients with schizophrenia; this meta-analysis demonstrates that symbol coding tests remain among the most impaired cognitive measures, supporting their continued use as indicators of cognitive dysfunction in both research and clinical settings. Understanding moderators such as age, IQ, and inpatient status may also help clinicians interpret test results more accurately and tailor interventions.
Expert Commentary
“In this meta-analysis, we found that despite evolving research methods and clinical interventions and better inclusion of research from understudied parts of the world, processing speed continues to emerge as particularly impaired for people with schizophrenia,” wrote Danielle N. Pratt, PhD, Department of Psychology, Northwestern University, Evanston, Illinois, and coauthors. “This work added to the substantial evidence that slowed information processing is a central feature of the cognitive impairment seen in schizophrenia, which is potentially a reflection of the altered brain connectivity in this population.”
