Analysis of 41,053 pupillary light reflex (PLR) recordings demonstrates that specific pupillary dynamics — particularly maximum and average constriction speeds and maximum and minimum diameters — vary predictably with age. These effects are robust across individuals aged 10–80 years and show high explanatory power (R² ≈ 0.80–0.89) for multiple parameters. We propose that the PLR constitutes a fundamental, involuntary measure of neural processing speed, awareness, and perceptual integration. As such, PLR metrics enable estimation of "cognitive age" — a biologically grounded construct distinct from chronological age.
Introduction
The subjective cognitive experience is not constant across the human lifespan. Older adults consistently report that time feels like it passes faster — a phenomenon documented across cultures and historical periods. Traditional explanations, such as proportional theory, posit that each successive year represents a smaller fraction of total lived experience. While intuitively appealing, such explanations remain descriptive and lack a direct neurophysiological linkage.
The pupillary light reflex offers a unique solution. Because the PLR operates outside conscious control, it provides a direct assay of neural transmission speed, synaptic efficiency, and neuromuscular execution — making it ideal as a foundation for biomarker development.
Methods
Data were aggregated and anonymized from 13,762 neurologically diverse individuals aged 10–80 years, taken across 361 test administrators. Pupillary responses were recorded using Reflex, a mobile app produced by Brightlamp Inc. operating in the visual light spectrum at 30 Hz. Nine PLR parameters were extracted: latency, maximum diameter, minimum diameter, constriction amplitude, maximum constriction speed, average constriction speed, constriction time, average diameter, and release amplitude.
Key Findings
Pupil diameters decrease strongly with age
Maximum diameter (R²=0.89), minimum diameter (R²=0.88), and average diameter (R²=0.89) all showed strong decreases with age — reflecting the well-documented age-related decrease in pupillary size. These were among the strongest age associations in the dataset.
Constriction speeds decline with age
Average constriction speed declined linearly with age (R²=0.85), while maximum constriction speed followed an exponential decay (R²=0.80). These speed measures exhibited some of the strongest age associations in the dataset alongside diameter parameters.
Latency shows no significant age relationship
Response latency showed no significant relationship with age (R²=0.01). This may reflect that pupillary response latency does not vary meaningfully with age, or that the 30 Hz sampling rate limits measurable variation.
PLR as a Measure of Neural Processing Speed
Processing speed is widely regarded as a foundational constraint on higher cognition. The strong age-related decline observed in PLR constriction speed suggests that pupillary dynamics capture this constraint at a fundamental physiological level. Unlike reaction-time tasks, PLR measures are involuntary and free from strategic compensation, making them especially well suited for biomarker development.
A Neurophysiological Account of Time Dilation
We speculate that age-related slowing of PLR dynamics reflects a broader slowing of neural "clock speed." If perceptual and cognitive events are sampled more slowly, subjective experience remains internally coherent while objective time advances further between sampled events.
Cognitive Age Estimation
The high R² values observed for multiple parameters suggest that PLR metrics could serve as biomarkers for "cognitive age." Unlike chronological age, cognitive age reflects actual neurophysiological function and may better predict cognitive performance, disease risk, and functional capacity. Individuals whose PLR profile resembles younger cohorts may possess "younger" neural function regardless of birth date. Conversely, accelerated PLR aging could identify individuals at risk for cognitive decline before clinical symptoms emerge.
Conclusion
Age-related changes in pupillary light reflex demonstrate measurable neurophysiological slowing that parallels cognitive decline across the lifespan. We propose that PLR-derived metrics can estimate "cognitive age," potentially distinguishing chronological age from neurophysiological function. This approach offers a rapid, non-invasive biomarker for cognitive aging that may enable early detection of decline and monitoring of interventions aimed at preserving cognitive function.