Christopher W Tyler
City University of London, UK
Title: Melanopsin pathway control in the human retina and the relationship with traumatic photalgia
Biography
Biography: Christopher W Tyler
Abstract
Introduction: The melanopsin light-signaling pathway is a recently-discovered addition to the light-processing capabilities of the vertebrate eye. Human melanopsin responses have been measured largely through pupillometric responses. Spectral analysis of electroretinographic (ERG) responses provides a further capability of assessing the functionality of the melanopsin pathway in human. Our results suggest that there is a fast melanopic response contributing to the ERG at high intensities.
Methods: To gain insight into the process of visual information in photalgic eyes, we recorded electroretinographic (ERG) from facial electrodes. High-quality ERG responses for full-field stimulation as a function of wavelength and intensity were obtainable with the approach of recording wavelength-specific ERG responses to whole-field chromatic stimuli under a full range of light-adaptation conditions. Stimulation conditions were full-field R (610 nm), G (540 nm), B (480 nm) and W (R+G+B) stimulation of 2.5 Hz square-wave modulation (200 ms on/ 200 ms off) with a maximum intensity of 265 cd/m2 (W). Signals were recorded from controls and a population of individuals with photalgia subsequent to mild traumatic brain injury.
Results: Light-adapted ERGs in controls exhibit similar properties to dark-adapted ERGs as a function of flash intensity; with the b-wave speeding up from a peak time of ~250 ms when dark-adapted to ~40 ms when maximally light-adapted with a compressive reduction in the amplitude function at the higher intensities. The results at all test wavelengths conform to a single pair of functions (amplitude and time-to-peak) of melanopic intensity under most conditions, which we term the “main sequence” for ERG amplitude and times-to-peak as a function of intensity. These functional properties were significantly different across the population of photalgic individuals, who showed markedly reduced amplitudes and delayed b-wave peaks relative to the controls.
Conclusion: To extend the measurement of human melanopsin responses beyond pupillometric responses, we have developed an approach for spectral analysis of ERG responses to expand the capability of assessing the functional role of the melanopic pathway in humans. Our results suggest that there is a fast melanopic response contributing to the light-adapted ERG, with a peak time comparable with the rod and cone responses under these conditions. This melanopic response showed marked amplitude reductions and delays in the photalgic patients, potentially providing the first objective biomarker for photalgia.