Disentangling Cognitive Load From Visual Reflexes: An Iso-Luminant Framework for Virtual Reality (VR)-Based Pupillometry

Pupillometry is a robust, non-invasive indicator of cognitive load; however, its application in virtual reality (VR) is compromised by luminance-driven pupillary light reflexes (PLR) that mask subtle task-evoked pupillary responses (TEPR). We introduce a strict iso-luminant VR protocol to isolate cognitive load-related autonomic responses. The framework comprises a PLR validation phase establishing reflex dynamics via controlled luminance alternations, followed by a cognitive isolation phase where visual luminance is clamped constant and cognitive load is modulated exclusively through auditory tasks. Analysis of 22 sessions confirmed PLR sensitivity (mean amplitude: 1.52 ± 0.33 mm, constriction velocity: 19.1 ± 11.2 mm/s). Critically, under iso-luminant conditions, TEPR was successfully isolated with significant dilation during high-load versus low-load conditions (mean difference: 0.38 ± 0.28 mm). The framework differentiated temporal signatures: PLR exhibited rapid velocity (19.1 mm/s), while cognitive load induced gradual dilation (3.86 mm/s), demonstrating a response velocity increased by a factor of 5.2 compared with the cognitive condition. This temporal dissociation confirms that pupillary changes under iso-luminant conditions reflect cognitive processing through the locus coeruleus-norepinephrine (LC-NE) pathway rather than brainstem-mediated light reflexes. The methodology establishes a reproducible foundation for VR-based neuro-ergonomic research with potential applications in attention-deficit/hyperactivity disorder (ADHD) assessment and neurodegenerative disease monitoring.