FoundationalConceptsNeuroscience

ning with the photoreceptor, bipolar, and ganglion cells in the retina
—is the receptive field. Because of the light-focusing properties of the
eye, photoreceptor cells will respond only to light stimuli that come
from specific regions of visual space. All of the neural cells in the
retina have visual receptive fields, and so also do the brain neurons
that receive signals from the retina.
What happens to the information represented by the neural signals
after exiting the eye and traveling along the optic nerve toward the
brain? A short distance behind the eyes, the two optic nerves intersect
in a structure called the optic chiasm (named after the Greek letter chi,
which looks like an X, a crossing). At the chiasm, fibers from the two
optic nerves divide into two new groups, with axons from each of the
eyes that gather information from the left half of visual space (the left
visual field) going to the right half of the brain, and axons from each
of the eyes that gather information from the right visual field going to
the left half of the brain.
About 10 percent of the optic nerve axons go into a part of the
midbrain called the superior colliculus. This pathway is heavily in-
volved in very rapid responses to sensory stimuli in ways that do not
involve awareness. You notice something in the periphery of your vis-
ual field and begin to turn toward it to get a better look, before you are
even aware you saw anything—that’s the retina—midbrain pathway at
work.
Almost 90 percent of the optic nerve axons head to the thalamus
in the diencephalon, where they enter a pair of structures called the
lateral geniculate nuclei (LGN; Fig. 14.7). (The name comes from the
Latin genu = knee; the LGN resemble a bending knee—a genuflect.)
Axons carrying information from the right visual field go to the left-
side LGN, and axons carrying information from the left visual field go
to the right-side LGN.