Duplicity theory and ground squirrels: linkages between photoreceptors and visual function.

The presence of rod and cone photoreceptors has traditionally been linked to well-defined classes of visual capacity by the generalization known as duplicity theory. This paper summarizes results obtained from studies of vision and the visual system in ground squirrels (Spermophilus sp.) that reveal instances where structure/function linkages depart from expectations based in duplicity theory. The details of these exceptions are reviewed and their possible mechanisms discussed.