Relationship of color and polarization information in the butterfly lamina

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The eye of a butterfly Papilio consists of randomly distributed three spectrally distinct types of ommatidia. Each ommatidium contains nine photoreceptors (PRs) cells, which form a module called cartridge in the lamina with four second-order lamina monopolar cells (LMCs). In the cartridge, PRs of distinct spectral sensitivities are connected via histamine-gated chloride channels, forming characteristic spectral opponency. Some PRs and LMCs have long side branches penetrating neighboring cartridges, indicating possible spatial interaction. Here we aimed at demonstrating the spatio-spectral opponency in the Papilio lamina by intracellular electrophysiology, anatomy, and modeling. We could record spectral sensitivities of PRs from many (typically 50) different angles and indeed found that off-axis stimulations altered the sensitivity profile, indicating the spatio-spectral opponency. We made a computational model to analyze the responses, which supported our hypothesis of synaptic interaction underlying the complicated opponency. This model will be extended to estimate the function of interommatidial connections that produce spatial opponency in PRs and LMCs.