Assessing perceptual chromatic equiluminance using a reflexive pupillary response

Abstract

Abstract Equiluminant stimuli help assess the integrity of color perception and the relationship of color 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. One alternative, optokinetic nystagmus (OKN) of the eye in response to motion, is partially compromised by recent observations that changes in attentional fixation affect the OKN. 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 analyzes reflexive pupil oscillations induced by slow (< 2 Hz) temporal alternations between colored stimuli. Two equiluminant stimuli will induce a similar pupil dilation response regardless of color; therefore, an observer’s equiluminant point can be identified as the luminance ratio between two colors 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 novel technique can be successfully employed in non-verbal subjects.