Abstract
Purpose: :
There is uncertainty about the distribution of L to M cones across the retina. For example, the relative level of L vs. M messenger RNA increases with eccentricity, however, it has not been possible to directly test for a higher density of L cones in the periphery. In addition, we have been working on a gene therapy approach to produce cones that express L pigment in the eyes of dichromatic animals that initially have M cones but lack L cones. A wide–field multifocal electroretinogram (mf–ERG) apparatus capable of producing a functional map of the relative numbers of L and M cones would provide a tool for studying the chromatic topography of the retina and it would provide a method for mapping the region of transduced cones in experiments using viral–gene delivery to modify cone vision.
Methods: :
A RETIscan mf–ERG instrument made by Roland Consult, Inc. was adapted for the purpose of mapping the relative contributions of L and M cones to ERG signals. We built an LED display having 2048 paired green and red LEDs controlled by the RETIscan stimulator. To obtain a wide–field map, the LEDs were mounted on the inside of a concave surface that approximates a hemisphere. With this arrangement, a multifocal map covering about 150°of visual angle can be obtained.
Results: :
The completed system has been tested on both humans and animals. The stimulator was designed to allow LEDs to be activated in any subset of the segments. ERGs carried out with only a subset of the segments active allow measurements of signal–to–noise ratio. When any one segment is active, the signal is about 30 times higher for that segment, than the average for all the dark segments (signal–to–noise ∼30:1). Because of the concave surface, the LEDs of each segment are pointed along a line parallel the optical axis of the cones in each retinal area illuminated. In conventional mf–ERGs the signal amplitudes are reduced for the periphery, in part, because with increasing eccentricity the illumination from a flat display falls off roughly as the cosine of visual angle. The peripheral decrease in illumination is pronounced for wide fields. As done previously, in our design, the number of LEDs in each segment increases with eccentricity to compensate for the decrease in cone density in the periphery. Because of the curved configuration of the stimulator there is minimal fall off in intensity from individual LEDs. In tests on humans and animals, our stimulator produces relatively constant ERG amplitudes at all retinal locations.
Conclusions: :
A wide–field, color–LED mf–ERG system shows promise for measuring the relative distribution of L and M cones across the retina.
Keywords: color vision • electroretinography: non-clinical • photoreceptors