S-cone ERGs Elicited by a Simple Technique in Normals and in Tritanopes

Purpose: To measure changes in the relative spectral sensitivities of the dark adapted and light adapted ERG and thus to establish the possible contribution of rods to the 'blue cone' ERG elicited by flashes of blue light.

Background: Short wavelength stimuli in the light-adapted eye evoke small rounded b-waves which have been considered to be S-cone responses. We have recorded such responses from tritanopes, which called the assumptions into question.

Methods: Small ERGs were recorded to blue and green flashes. The stimulus was a Ganzfeld which employed light emitting diodes. ERGs were obtained in both the dark-adapted eye and after light adaptation to intense orange light (peak wavelength 610 nm). The change in sensitivity with light adaptation and the relative spectral sensitivity was determined from the voltage/log light intensity functions, using a 10 microV criterion.

Results: (1) peak times and changes in sensitivity did not help distinguish light-adapted rod from possible S-cone responses; (2) analysis of the change in the ratio of blue:green sensitivity from darkness to 4.4 log Td. 610 nm background suggests that in seven normal subjects, 90% or more of the ERG evoked by 440 nm flashes is generated by S-cones; (3) three tritanopes have insignificantly reduced S-cone responses.

Conclusions: (1) clinical techniques used to isolate S-cone ERGs are appropriate; (2) there are at least two types of tritanope and in those we investigated, functional S-cones are probably displaced into the retinal periphery.