Rod-pathway pupil responses measured with luminance versus rod-isolating contrast stimuli

Abstract

Purpose : The pupillary light reflex (PLR; response of the pupil to a flash of light) measured under dark-adapted conditions is useful for assessing rod pathway function. However, clinical implementation of rod-mediated PLR measures is limited by the requirement of dark-adaption and by cone pathway intrusion for moderate to high retinal illuminance stimuli. This study sought to overcome these limitations by assessing the PLR using stimuli that isolate the rod pathway by silent substitution.

Methods : PLRs were recorded from the right eye of 6 normally-sighted subjects (mean age, 32 years) using infrared videography. A four primary LED ganzfeld system was used to generate and present the stimuli. Two paradigms were used: first, subjects were adapted to a 0.3 scot cd/m² field for 20 sec and the four primaries were modulated in counter-phase to achieve a rod isolating pulse by means of triple silent substitution (rod contrast of 0% to 60%). Second, subjects were dark-adapted and short-wavelength (465 nm) luminance pulses (-4.0 to 2.6 log cd/m²) were presented in the dark. PLRs were normalized to the baseline pupil size and plotted as a function of stimulus luminance or contrast. These data were fit with Naka-Rushton functions to derive R_max (maximum pupil response) and S (PLR sensitivity).

Results : PLR amplitude increased as both stimulus luminance and contrast increased; the relationships between PLR amplitude and stimulus contrast/luminance were well described by Naka-Rushton functions. Mean (±SD) R_max was 56% ± 8% for luminance modulation and 20% ± 9% for contrast modulation; R_max values measured under the luminance and contrast modulation paradigms were correlated significantly for the 6 subjects (r = 0.89, p = 0.02). Mean S was -2.91 ± 0.53 log cd/m² for luminance modulation and 19% ± 4% for contrast modulation; S measured under the luminance and contrast modulation paradigms for the 6 subjects did not achieve statistical significance (r = 0.56, p = 0.25).

Conclusions : The relationship between pupil response and stimulus luminance under dark-adapted conditions is similar to the relationship between pupil response and rod-isolated contrast measured without dark adaptation. Approaches based on silent substitution to assess rod pathway function have clinical potential, as robust PLRs can be driven without dark adaptation and concerns of cone-pathway intrusion are minimized.

This is a 2020 ARVO Annual Meeting abstract.