# Assessing perceptual chromatic equiluminance using a reflexive pupillary response

## TL;DR

This paper introduces a new method to determine equiluminance using reflexive pupil responses, which works without verbal input from participants.

## Contribution

The Pupil Frequency-Tagging Method (PFTM) enables non-verbal assessment of equiluminance with minimal training.

## Key findings

- PFTM successfully identified equiluminance points in humans and non-human primates.
- PFTM results correlated with those from traditional methods like minimum flicker and minimum motion.
- The method is suitable for pre-verbal or non-verbal observers.

## Abstract

Equiluminant stimuli help assess the integrity of colour perception and the relationship of colour to other visual features. As a result of individual variation, it is necessary to calibrate experimental visual stimuli to suit each individual’s unique equiluminant ratio. Most traditional methods rely on training observers to report their subjective equiluminance point. Such paradigms cannot easily be implemented on pre-verbal or non-verbal observers. Here, we present a novel Pupil Frequency-Tagging Method (PFTM) for detecting a participant’s unique equiluminance point without verbal instruction and with minimal training. PFTM analyses reflexive pupil oscillations induced by slow (< 2 Hz) temporal alternations between coloured stimuli. Two equiluminant stimuli will induce a similar pupil dilation response regardless of colour; therefore, an observer’s equiluminant point can be identified as the luminance ratio between two colours for which the oscillatory amplitude of the pupil at the tagged frequency is minimal. We compared pupillometry-based equiluminance ratios to those obtained with two established techniques in humans: minimum flicker and minimum motion. In addition, we estimated the equiluminance point in non-human primates, demonstrating that this new technique can be successfully employed in non-verbal subjects.