FoundationalConceptsNeuroscience

to grow and how synapses decide to form remained unknown. Major
progress in addressing these questions was made by Roger Sperry
(1913-1994), who back in the 1930s and 1940s conducted clever
investigations of the process by which neurons form connections.
Sperry did his early research with frogs and salamanders, amphibians
that have a remarkable capacity to regenerate after sustaining physi-
cal damage to their bodies. Salamanders, for example, have famously
been known to regrow a leg after amputation. This regenerative ca-
pacity also takes place in the nervous system; if a nerve fiber is cut, it
frequently will regrow.
One of Sperry’s classic experiments involved cutting the optic
nerve in a frog, the connection between the frog’s eye and the frog’s
brain. If the optic nerve is cut in a person, that person will be blind for
life (in the eye for which the nerve has been cut). Sever the optic nerve
in a frog and the frog will be completely blind, also. However, wait sev-
eral weeks and the severed optic nerve regenerates—completely nor-
mal vision is restored to the frog. This much had been known. It had
also been established that, in the regeneration process, axons from
the eye regrew from the eye toward the frog’s brain. The question that
Sperry addressed was this: how did axons from the eye know where to
form synapses in the brain, so that normal vision was restored?
To begin, Sperry noted the following: if the muscles holding the
frog’s eyeball in place are delicately cut, the eyeball can be rotated it
its socket by a full 180 degrees. If the eye muscles are then repaired
and allowed to heal, the frog will see the world upside down and back-
ward. This can be demonstrated by introducing a morsel of food (such
as an insect) to the frog: the frog strikes at the insect in the wrong
direction (Fig. 10.6). No matter how many weeks are allowed to go by,
the frog never learns to correct this behavior and adjust to its upside
down and backward world.