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K. Mancuso, J. A. Kuchenbecker, M. Neitz, T. B. Connor, M. C. Mauck, A. Salzwedel, J. Neitz; Short- and Middle-wavelength Sensitive Cone Interactions via Horizontal Cells Examined in Primates Using the Electroretinogram. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3472.
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How do the three types of cone - short- (S), middle- (M), and long- (L) wavelength sensitive - ultimately relate to the four unique hue percepts: yellow, blue, red and green? Evidence from psychophysical testing in humans indicates that the S cones contribute to all four percepts. There is an S-OFF contribution to the sensations of yellow and green and an S-ON component to red and blue. The L and M cones synapse onto both ON and OFF bipolar cells; however, the S cones only have an ON bipolar pathway making it unclear how S cone signals are combined with those from other cones. We tested the hypothesis that signals from the S, L and M cones are combined via horizontal cell connections providing the substrate for S-cone input to both red/green and blue/yellow pathways as early as the retinal level.
We used the light-adapted, long-flash (or ON-OFF) ERG, a stimulus paradigm in which background photopic light is used to saturate the rods, effectively isolating cone photoreceptor pathways, and long duration flashes are used to separate downstream ON and OFF responses. We further isolated responses from S or M cones in dichromatic squirrel monkeys using a silent substitution technique, and animals were tested before and after intravitreal injections of 2-amino-4-phosphonobutyric acid (APB).
After complete blockade of the S-ON bipolar cells via APB administration, an S-cone ERG was still present with responses at both light onset and offset. However, the sign was reversed compared to that produced in response to M cone isolating stimuli, indicating that the S-cone signal was passed through horizontal cells that invert the response.
S-cones have access to the L/M pathway via horizontal cell connections such that signals from all three cone types are combined in the retina, at the first synapse. In trichromats, S-cones have the possibility of inputting to L or M cones, which in turn output to ON or OFF bipolar cells, producing four combinations corresponding to circuits for red, green, blue and yellow.
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