Early visual processing in schizophrenia spectrum disorders: a systematic review and meta-analysis of the P1 component

Background: Growing evidence indicates that schizophrenia spectrum disorders (SSD) are associated with abnormalities in early visual processing, including retinal impairments, oculomotor and perceptual dysfunctions [1]. These findings suggest that early visual pathways may be broadly affected, potentially involving cortical processing. Visual evoked potentials (VEPs), particularly the P1 component, provide a reliable index of early visual cortical activity and represent a valuable tool to investigate these neural deficits [2,3]. Alterations in P1 amplitude and latency have been reported in SSD; however, findings remain heterogeneous. To date, no meta-analysis has examined the P1 component of the VEPs targeting early visual processing.

Methods: This systematic review and meta-analysis followed PRISMA guidelines and was registered in PROSPERO (CRD420251023312). A systematic search of Web of Science, PubMed, Embase, and APA PsycINFO was conducted on April 3, 2025, to identify studies comparing SSD patients and healthy controls on P1 amplitude and/or latency elicited by simple visual stimuli (e.g., checkerboards, gratings) without cognitively demanding tasks. Methodological quality was assessed independently by two reviewers using the Newcastle-Ottawa Scale. Effect sizes (Hedges’ g) were computed from group means and standard deviations; when necessary, standard errors were converted, and numerical data were extracted from graphical representations. Random-effects models using restricted maximum likelihood (REML) with Hartung-Knapp adjustment were applied to estimate pooled effects. Heterogeneity was assessed using I² statistics. Publication bias, sensitivity analyses, and exploratory moderator analyses were also performed.

Results: Nineteen studies were included, of which fourteen were eligible for the P1 amplitude meta-analysis (453 SSD patients, 663 controls) and ten for the P1 latency meta-analysis (258 SSD patients, 255 controls). Patients with SSD showed reduced P1 amplitude compared with healthy controls (g = −0.77, 95% CI [−1.22, −0.33], p = 0.002), as well as a significantly prolonged P1 latency (g = 0.35, 95% CI [0.09, 0.62], p = 0.015). These findings indicate both attenuated visual responses and delayed cortical processing in SSD. Heterogeneity was high for amplitude and moderate for latency, suggesting substantial variability across studies, particularly for amplitude measures. Exploratory moderator analyses identified EEG low-pass filter settings as a significant methodological factor influencing both amplitude and latency effects, highlighting the importance of signal processing choices in VEP research.

Conclusion: Overall, these findings suggest altered visual cortical processing in SSD, as reflected by reduced P1 amplitude and delayed latency. By limiting higher-order cognitive demands, this meta-analysis sharpens evidence for early sensory processing abnormalities in SSD. These results are consistent with the hypothesis of dysfunction along early stages of the visual system, possibly involving both retinal input and early cortical processing mechanisms. P1 alterations may therefore represent a potential electrophysiological biomarker of SSD. Future research should investigate their clinical relevance, including associations with symptom dimensions and functional outcomes, as well as their sensitivity to treatment effects, which could not be assessed in the present meta-analysis.

References

1. Jurišić, D., Ćavar, I., Sesar, A., Sesar, I., Vukojević, J., & Ćurković, M. (2020). New Insights into Schizophrenia : A Look at the Eye and Related Structures. Psychiatria Danubina, 32(1), 60‑69. https://doi.org/10.24869/psyd.2020.60
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