LESSON : 2
Mapping the Human Brain
The idea of mapping the human brain is not new. The “father
of neuroscience,” Santiago Ramon y Cajal, argued at the turn of the 20th
century that the brain was made up of neurons woven together in a highly
specific way. We have been trying to map this exquisite network since then.
In fact, scientists in other settings have called the wiring
diagram a Grand Challenge of neuroscience in and of itself. It appears on the
Grand Challenges of the Mind and Brain list for the National Science Foundation
(NSF, 2006), on the Grand Challenges list of the National Academy of
Engineering (NRC, 2008), and on the wish lists of at least a half-dozen major
scientific fields, from genetics to computer science.
If we are interested in how the mind works, then we
definitely need to know the physical instantiation of brains and function,
remarked Jeffrey Lichtman, professor of molecular and cellular biology, Harvard
University. This effort will require some mechanism to obtain the connectional
maps that will integrate anatomy, neuronal activity, and function. Until those
are available, the field will not be able to move forward to its full
potential.
The challenge is similar, in many ways, to mapping the human
genome: We might not know exactly what we will learn, but we have a strong
belief that we will learn a lot, commented Leshner.
So why has it not happened?
Because neurons are very small and the human brain is
exquisitely complex and hard to study. Eve Marder, professor of neuroscience at
Brandeis University and president of the Society for Neuroscience, noted that
scientists have been working on circuit analysis for nearly 40 years, primarily
with smaller organisms, particularly invertebrates, because their simpler
neurological systems are more amenable to study and analysis.
The classic approach, in place since the 1960s, has been
simple: Define behaviors, identify neurons involved in those behaviors,
determine the connectivity between those neurons, and then excite individual
neurons to understand their role in influencing behavior. This approach is
called “circuit dynamics,” and it has been tremendously helpful to
understanding how these simple neurological systems work.
But as you move from sponges and anemones to primates and
humans, each step of that analytical process becomes infinitely more
challenging.
As Marder noted, the impediments, until today, to
understanding larger circuits and vertebrate brains include difficulty in
identifying neurons, difficulty in perturbing individual classes of neurons in
isolation, and difficulty in recording from enough of the neurons at the same
time with enough spatial and temporal resolution.
In other words, difficulty arose in every step of the
circuit dynamics process.
But the key words in Marder’s statement are “until today.”
If you look at the three things Marder identified as stumbling blocks, major
technological breakthroughs over the past few years have solved or are close to
solving each one, starting with a new technique born from the lab of Lichtman:
“the Brainbow.”
Lesson 1
Lesson 1
