The 2-dimensional model responsible for achromatic vision along with also two-dimensional color-opponent chromatic module provides separate inputs to the visual system.
What are the neural mechanisms of achromatic vision in the invertebrate visual system and how do they correspond to the neural mechanisms of brightness perception in vertebrates?
Geometric model of achromatic vision in the snail based on responses of B- and D-cells
Comparison of the independently constructed models of achromatic vision based on psychophysical data from the human (Figure 2 B), behavioral and neurophysiological studies in lower vertebrates and mammals (Evtikhin et al., 2008; Izmailov et al., 2006; Polianskii et al., 2008; Sokolov, 2003), these intracellular data (Figure 7A) and ERG responses (Figure 7 B) in the mollusk, confirms our hypothesis that, in the invertebrate visual system, as in the visual system of vertebrates,--the encoding of brightness can be represented by the direction of a 2-dimensional vector composed of the opponent responses of B- and D-cells.
Dvukhkanal'naya model' akhromaticheskogo zreniya lyagushki [Two-channel model of achromatic vision in frog].