Rajat Agarwala¹ (), Yannick Sauer^1,2, Elena Aggius-Vella³, Marco Bragaglia³, Klara Hoxha³, Annalisa Bosco^3,4, Patrizia Fattori^3,4, Siegfried Wahl^1,2; ¹ZEISS Vision Science Lab, Institute for Ophthalmic Research, University of Tübingen, Germany, ²Carl Zeiss Vision International GmbH, Germany, ³Department of Biomedical and Neuromotor Sciences, University of Bologna, Italy, ⁴Alma Mater Research Institute For Human-Centered Artificial Intelligence (Alma Human AI), University of Bologna, Italy

Visual blur degrades sensory input and may increase cognitive effort during tasks that require visuomotor integration, yet the minimum level of optical blur that measurably alters cortical haemodynamics remains unclear. Functional near-infrared spectroscopy (fNIRS) provides a non-invasive method to quantify changes in cortical oxygenation. This study investigated how induced optical blur modulates cortical activation across various brain regions during visuomotor and control tasks. Six participants performed four task conditions: (1) Visuomotor (VM): picking and placing beads according to a visual pattern; (2) Motor-only: bead placement with minimal visual demand; (3) Visual-only (VS): pattern observation without motor action; and (4) Baseline (BS): central fixation. Blur was induced using electronically tunable lenses set to 0, 1, and 2 diopters. Each trial included lens adjustment (~1 s), a 3-s cue, 10 s of task execution, and a 5-s recovery period. Cortical haemodynamics were recorded using the NIRSport fNIRS system, with optodes covering frontopolar, dorsolateral prefrontal, visual, and parieto-temporal cortices. Data were analyzed using a group-level random-effects GLM. Across task contrasts, VM vs BS produced the strongest and most widespread haemodynamic effects across frontal, parietal, and temporal regions, while VS vs BS showed smaller but reliable effects over left frontal and right parietal areas. VM vs VS yielded minimal differences, indicating highly similar haemodynamic responses between visuomotor and visual-only processing. Blur-response analysis revealed that both VM and VS exhibited significant haemodynamic changes already at 0.5D across Oxy-, Deoxy-, and Total-Hb signals, driven primarily by occipito-parietal visual association cortex, with additional fronto-parietal recruitment in VM. In contrast, baseline fixation showed significant Total-Hb effects only at 2D, indicating greater tolerance to blur. These findings demonstrate that task engagement dramatically lowers the neural blur threshold, with even mild optical degradation rapidly impacting cortical processing during active visual and visuomotor behavior.

Acknowledgements: This research is supported by the European Union’s Horizon Europe research and innovation programme under the Marie Skodowska–Curie grant PlACES: PLAsticity of perception in real and virtual spaCES, agreement No. 101086206.