Phototransduction in human cones measured using the a-wave of the ERG

To study human cone phototransduction, the alpha-wave of the ERG was recorded from color normals, dichromats, and patients with retinitis pigmentosa. A model of the activation phase of phototransduction, previously fitted to responses from single rods and the rod alpha-wave, was modified and fitted to the human cone alpha-wave. The modified model fits the cone alpha-wave well and allows questions about human cone phototransduction to be addressed. In particular, we conclude that: (1) the amplification of the activation phase of human cone transduction is comparable to that of the human rods. (2) Steady lights have relatively little effect on the amplification of cone transduction. (3) The normal alpha-wave elicited by red flashes is dominated by the L cones, consistent with a ratio of L:M cones of > 1. (4) Retinitis pigmentosa caused by mutations of the rhodopsin gene can affect cone phototransduction. Finally, a simpler computational expression is shown to approximate the modified model's responses.