Form and Function: Which one comes first?

As I continue my path along the story of neurogenesis, I've come across the historical argument between two relatively famous scientists over one of the most basic questions.  These two men, Roger Sperry and Donald Hebb, debated over the significant  question of how neurons actually know where to connect.

This is a nissl stain depicting the cellular
layering found in the V1 of the vision
processing part of your cortex.

In the developing brain, neurons face three major problems:
1. Which pathway to take?
2. Which regional target does the neuron need to reach?
3. What is the actual cellular target?

Each man had his own theory as to how neurons find the right path.  Hebb believed the model that experience forms the connections in the brain.  That because certain portions of the brain receive certain types of stimuli, they wire in a way that is most efficient to process that data and hence you get the specific "cytoarchitecture" (way cells are structured in a tissue) for different areas of the brain.

Sperry, on the other hand, saw the paradigm as form proceeding function due to chemical cues.  He believed that neurons knew where to migrate because of chemical signals which were excreted by the tissues which were to be the axonal destinations.  Imagine a mother holding an apple pie and wafting the scent towards her children who she wants to attract.  That's the basis of Sperry's view.

As it turns out, there are components of both in the actual biological system.  But what I find astounding is one of the famous experiments conducted by Sperry in the 1960s to support his hypothesis.  I like to call it "the kermit chemoaffinity test."

Apparently frogs were big as animal models
for science done in the 60s.

Essentially, he tested his chemical signaling theory by taking a live frog and rotating its eye 180 degrees.  Mind you this is with the optic nerve still attached and the eye was just rotated in its socket.  This literally turned the amphibian's world upside down, and the frog was unable to accurately catch flies because up was down and left was right to the poor kermit.

The next step was to sever the optical nerve and leave the eye rotated.  It's key to mention that amphibians are some of the few creatures who can actually reconnect some of their nerves.  If we were to believe Hebb's hypothesis in this particular instance, then the eye would rewire to turn things right side up- because use denotes how it will connect and that is how the frog would like to use the eye.  However, Sperry was proven correct. The eye rewired along the same lines as it started (due to chemical messages) and the frog stayed in an upside down world.