Electroretinograms in S-cone monochromacy using S-cone and rod isolating stimuli

To study the electrophysiological properties of the S-cone pathway, we adopted a silent substitution paradigm for ERG measurements in an S-cone (blue-cone) monochromat. ERG responses to pure S-cone and rod square wave modulation were measured for different temporal frequencies (1-36 Hz) at 66 cd/m² mean luminance and additionally to 1 and 12 Hz modulation at 6.6 cd/m² mean luminance. We found rod ERG signals up to a temporal frequency of 30 Hz and S-cone ERG signals up to 36 Hz. For 1 and 2 Hz temporal modulation, the S-cone ERG showed a negative defection after stimulus onset and a positive deflection (OFF-response) after stimulus offset, whereas for the rod ERG the reverse was the case. At temporal modulations of 6 Hz and above, the first harmonic response showed a phase difference between S-cone and rod modulation of about 180°. The S-cone ERG response amplitude for frequencies above 12 Hz with similar photoreceptor contrast was larger than the rod ERG response amplitude. At the lower mean luminance (6.6 cd/m²), the response amplitude of the S-cone-ERG was smaller than that found at the higher mean luminance (66 cd/m²), whereas the rod ERG response was larger. The response phase of the S-cone ERG lagged compared to that at the higher mean luminance (66 cd/m²), whereas the rod ERG response was phase advanced. Our approach can serve as a method to detect differences between the rod and S-cone pathways and to evaluate protocols of the human S-cone ERG.