Roger Sperry surgically rotated an amphibian's eye, severed the optic nerve, and allowed the retinal ganglion cells to re-grow into the brain. Based on the animal's movements in the opposite direction from its prey, the RGC axons made connections to the same part of the brain as before. If normal vision had been restored, what would he have concluded?

When Sperry and colleagues ablated the dorsal RGCs, the remaining ventral RGCs projected to the dorsal tectum. Conversely, when they ablated the ventral RGCs, the remaining dorsal RGCs projected to the ventral tectum.
A. What are some plausible models for dorsoventral patterning suggested by these results?
B. Ephrins and Ephs are distributed in dorsoventral gradients in both structures. EphB1 is expressed in the tectum, in a low-dorsal to high-ventral gradient. Ephrin-B1 and ephrin-B2 are expressed in the retina in a high-dorsal to low-ventral gradient. What do these expression patterns suggest about the mechanism used for dorsoventral patterning of RGCs projecting to the tectum?
C. A stripe assay was prepared in which stripes of artificially clustered EphB1 receptor extracellular domain were prepared on a polylysine coated coverslip. Based on your model, how would you expect the axons of dorsal retinal explants to behave in this assay?

In chick, ephrin-A2 is found in an increasing gradient from anterior to posterior tectum. Researchers virally misexpressed ephrin-A2 in patches throughout the tectum and then visualized the temporal RGC axons that expressed high levels of its receptor EphA3. What result would demonstrate the sufficiency of ephrin-A2 for repulsion?