Abstract
Purpose :
Inner retinal melanopsin signalling can interact with outer retinal cone-opsin and rhodopsin signals to alter human visual performance. The sites of interaction within the visual pathway are unknown. Here we investigate whether the origin of melanopsin-cone interactions in humans occur in the retina or/and the visual cortex.
Methods :
The cone-directed (rod and melanopsin silent) full-field ERG and VEP were recorded using temporal white noise (wn) stimuli (TWN) in 10 healthy observers. We applied a photoreceptor silent-substitution technique to independently control the melanopsin, rod and three cone photoreceptor (L-, M-, and S-cone) excitations using a 5-primary photostimulator. Cone-directed TWN stimuli generated under retinal equilibrium at a mean adaptation level of ~8,000 Td, with either high or low melanopsin excitation (i = 0.2 and 0.3), were presented in 1 s epochs containing 1024 normally distributed photoreceptor excitations (evenly distributed in the 0-64 Hz frequency range). Observer calibrations accounted for individual differences in pre-receptoral filtering and photoreceptor spectral sensitivities. The wnERG and wnVEP impulse response functions (IRF) were derived by cross-correlation between the TWN stimulus and the electrophysiological recordings.
Results :
While consistent in shape with the typical photopic flash ERG and VEP waveforms, the cone-directed wnERG and wnVEP responses showed different effects of melanopsin adaptation. Increasing melanopsin adaptation from low (i = 0.2) to high (i = 0.3) decreased the cone wnERG N1P1 amplitude by 21% from 367.5 ± 5.0 μV (mean ± SEM) to 289.7 ± 5.1 μV, but increased the cone wnVEP N2P2 amplitude by 16% from 89.8 ± 1.5 μV to 104.1 ± 1.5 μV. The ratio of the response amplitude with high to low melanopsin adaptation was 0.8X for the wnERG (i.e., suppression) and 1.2X for the wnVEP (i.e., enhancement). Melanopsin adaptation did not alter the implicit times of either the cone wnERG (N1 = 11.3 ± 0.1 ms for i = 0.2 and 11.4 ± 0.1 ms for i = 0.3) or wnVEP (N2 = 65.1 ± 0.3 ms for i = 0.2 and 65.0 ± 0.3 ms for i = 0.3).
Conclusions :
Melanopsin-cone interactions occur both in the retina and cortex, with the resultant effect on cone signalling being of opposite polarity. The facilitatory interactions observed in the cortex provide a mechanism for the melanopsin-mediated enhancement of cone vision.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.