Persistence of contralateral innervation

[David Franklin]

We have been puzzling over the following:

	Why is contralateral innervation so prominent and conserved a 
feature of the chordate nervous system? What is the functionality of this 
feature in the "higher" vertebrates? How does one explain the exceptions 
to this general rule, e.g. bilateral innervation of the upper half of the 
face in man (contrasted with largely unilateral innervation of the lower 
face)?

	Can anyone suggest any answers, or references where answers might 
be found?