is like a view of the earth from space. Just as brain areas light up during activity, individual
cities may be lit up at night, but that light doesn’t say anything about why the inhabitants
of that city have their lights on. There may be people shopping, working, or driving from
one place to another, each with a specific purpose or intent. Such a macroscopic viewpoint
may be useful for some purposes, but it obscures the rich detail and meaning of the activity.
Clearly to understand what is going on we must use other tools as well.
In contrast to the top down approach used in fMRI studies, another way to study the
brain is to begin by analyzing some of its neurons in isolation, where detailed electrical
recordings can be made, and the effect of drugs or other neuromodulatory substances can
be tested. This bottom up approach has traditionally been employed by researchers who
study the hippocampus or simple invertebrates such as the sea slug or roundworm. One of
the difficulties in studying brain sections in vitro, however, is that the normal inputs to the
neurons have been removed. The slice can be stimulated with a microelectrode, but such an
artificial stimulus may not have any similarity to the physiological inputs to these neurons.
On the other hand, the retina is a part of the central nervous system (CNS) whose phys-
iologic inputs are familiar: two dimensional patterns of light that form images. As a high-
throughput source of information for the visual cortex and higher level brain centers, it is
our brains window to the external world. Part of the visual system, it arguably provides
more information about our external environment than any of our other senses. In addi-
tion, it appears to function on its oyvn in isolation, although it may rely on some level of
feedback from the brain.