I believe the horizontal cells combine information laterally across the retina, they communicate between neighboring groups of photoreceptors and bipolar cells enhancing contrasts and adjusting photoreceptor sensitivity to light levels. That is; they integrate and regulate the input from multiple photoreceptor cells. Additionally, they are responsible for allowing eyes to adjust so as to see well under both the dim and bright conditions. They also provide inhibitory feedback to cone and rod photoreceptors.
