Two models of the brain
Until five years ago, the dominant model of the brain was
hierarchical, focused on individual neurons and patterned
on the way digital computers work. In brief, the hypothesis
held that single neurons operated by detecting features in
the environment. At the base level, individual neurons in
the optic region of the brain, for example, would detect simple
features, such as lines and edges. At the next level, higher
order neurons integrated data from the hundreds of lower order
neurons to which each was linked to detect more sophisticated
features — a red ball, for example, or green squares. At the
highest level, cardinal neurons would integrate sensations
of higher order neutrons to recognize concepts such as "grandmother's
face."
Problems confronted the hypothesis from the start. First,
there was the "binding" problem — how could information
obtained by different means be bound together into a single
perception? How could the neuron that recognized grandmother's
face, the neuron that recognized her voice, and the one that
recognized the word "grandmother" all work together
to create a single perception of grandmother? Closely linked
with this was how billions of individual neurons could produce
a single consciousness.
A competing model, now becoming dominant, contends that the
brain stores and processes information only when millions of
neurons work together, with their electric potentials correlated
or synchronized in patterns at various frequencies. Large-scale
electrical fields produced by the brain and recorded as electroencephalograms
can only be produced by the cooperative actions of many neurons,
the theory holds. These cell assemblies, as they are called,
are not anatomical entities. Rather, they are temporary functional
collections of neurons scattered across wide areas, or even
throughout the brain, whose firings are correlated or synchronized
at a given frequency.
The newer model makes it much easier to understand the data
from brain mapping which show that close coordination of several
relatively large regions of the brain are essential for almost
any activity. In turn, since the model hypothesizes that it
is the patterns of brain activity in space and time that are
important, not the states of individual neurons, it makes brain
mapping essential to understanding how the brain works.
Also:
Two models of the brain
Brain maps old and new
|
