This study shows that people with Parkinson's disease have significantly reduced contrast sensitivity compared with age-matched controls across photopic, mesopic and glare conditions—most markedly under mesopic with mild glare—and that apparent stage-related declines are largely explained by age.

Findings highlight a real-world functional visual deficit in PD that could serve as a noninvasive outcome measure or symptom-target for interventions, but the paper offers limited mechanistic or therapeutic-discovery insight.

PURPOSE: Contrast sensitivity function (CSF) is a valuable tool for assessing visual function, which can be affected in neurodegenerative disorders such as Parkinson's Disease (PD). Visual perception in everyday life depends on the ability to detect objects across a wide range of spatial frequencies under different illumination conditions. However, the effect of disease progression and varying illumination on CSF in individuals with PD remains underexplored. The aim of this study was to investigate CSF in PD participants across six illumination conditions, examining disease stage influence and comparing results with age-matched healthy controls. METHODS: CSF was measured with the Functional Acuity Contrast Test. This prospective, comparative study evaluated CSF under photopic, mesopic and glare conditions, across disease stages, classified according to the Hoehn and Yahr Scale. The index of contrast sensitivity (ICS) assessed overall CSF. RESULTS: Seventy-one PD participants and 97 healthy controls were included. PD participants demonstrated significantly poorer CSF than controls across photopic, mesopic and glare conditions (mixed-model analysis, p < 0.001). The most pronounced impairment was observed under mesopic conditions with mild glare (ICS: 33 vs. 2.5, p = 0.02). Unadjusted analyses suggested a decline in CSF with advancing disease stage; however, this effect was attenuated after controlling for age. No significant relationship was found between disease progression and ICS after age adjustment under any illumination condition. CONCLUSIONS: This is the first study to assess CSF comprehensively with ICS data under six illumination conditions in PD, considering disease stage and comparing with controls. PD participants experience significant visual deficits, particularly under mesopic and glare conditions. While contrast impairments are robust in PD, stage-related differences appear partially influenced by age. These findings highlight the clinical importance of evaluating CSF under ecologically relevant lighting environments to better characterise functional visual impairment in PD and its impact on activities of daily living.