Evidence for spatial regularity among retinal ganglion cells that project to the accessory optic system in a frog, a reptile, a bird, and a mammal

The vertebrate retina contains only five major neuronal classes but these e mbrace a great diversity of discrete types, many of them hard to define by classical methods. Consideration of their spatial distributions (mosaics) h as allowed new types, including large ganglion cells, to be resolved across a wide range of vertebrates. However, one category of large ganglion cells has seemed refractory to mosaic analysis: those that project to the access ory optic system (AOS) and serve vestibulocerebellar mechanisms of motion d etection and image stabilization. Whenever AOS-projecting cells have been a nalyzed by nearest-neighbor methods, their distribution has appeal ed almos t random. This is puzzling, because most aspects of visual processing requi re the visual scene to be sampled regularly. Here, spatial correlogram meth ods are applied to distributions of large ganglion cells, labeled retrograd ely From the AOS in frogs, turtles, and rats, and to the AOS-projecting dis placed ganglion cells of chickens. These methods reveal hidden spatial orde r among AOS-projecting populations, of a form that can be simulated either by superimposing a single regular mosaic on a random population or, more in terestingly, by overlapping three or more regular, similar but spatially in dependent mosaics. The rabbit is known to have direction-selective ganglion cells (not, however, AOS projecting) that can be subdivided into functiona lly distinct, regular mosaics by their tracer-coupling patterns even though they are morphologically homogeneous. The present results imply that the d irection-selective AOS-projecting ganglion cells of all vertebrates may, li kewise, be subdivided into regular, independent mosaics.