Yannick Sauer^1,2, Rajat Agarwala¹, Jenny Loren Witten^1,2, Antonia Roth^1,2, Patrick Michel², Judith Ungewiss², Siegfried Wahl^1,2; ¹University of Tübingen, ²Carl Zeiss Vision International GmbH

Blurry vision from uncorrected refractive errors, like presbyopia, affects the daily lives of billions of people worldwide. Many presbyopes report increased effort when performing visually guided tasks, suggesting an impact of reduced visual performance on cognitive demand. To study the functional impact of optical blur, we investigated how defocus blur affects visuomotor performance, subjective cognitive load, and EEG activity. Twenty-nine participants performed a pegboard task with tweezers under three different conditions: vision was corrected or intentionally blurred with two different levels of defocus lenses. The three-minute task was repeated three times per condition in random order. Cognitive load was assessed both subjectively using the NASA-TLX questionnaire and objectively using EEG. Results show a significant decrease in task performance and a significant increase in subjective cognitive load with increasing levels of defocus blur. EEG analysis revealed condition-dependent changes, specifically a decrease in theta and alpha activity in the occipital lobe and an increase in theta band power in the prefrontal cortex. These findings are consistent with elevated cognitive and attentional demands under reduced visual quality. Our results support the use of cognitive load measures to quantify the functional impact of blurry vision and to evaluate optical correction strategies, for example, different spectacle lens designs.