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December 07, 2005
New Study Finds Similarities Between Brain's Spontaneous and Evoked Activity
For centuries, philosophers have engaged in a rollicking debate over
the role the mind plays in processing and comprehending human
experiences. Are our thoughts simply a reflection of the world as
perceived through our senses? Or does the mind itself construct the
features of our experience?
In recent years, as neuroscientists have deployed new techniques to
observe the brain in action, they have discovered that the brain is
spontaneously active. Their studies have begun to decipher not just how
we perceive the world around us, but how the information gathered
through our senses trips neural circuits and sets the mind in
motion.
“Such a notion indicates a dominant role for the cortex in perception, and it flies in the face of a long-held view of the brain that holds that the purpose of the cortex, or brain circuits in general, is more passive.”
Rafael Yuste
Now, a new report by a team of Howard Hughes Medical Institute
(HHMI) researchers at Columbia University goes a step further in this
debate, disputing the idea that our minds are pure reflections of the
world. Rather, the new data suggest that our brains might be better
defined as a rich dynamical system with internal states that could be
the templates of our thoughts or memories.
The cortex is the largest part of our brain and is the site of most
mental functions. The cortex receives input from the thalamus, which
itself receives inputs from sensory systems. Thus, practically all
information about the outside world reaches the cortex through the
thalamus. Writing in the December 8, 2005, issue of the journal
Neuron, a team of researchers working in the laboratory of
Rafael Yuste, an HHMI investigator at Columbia University, report that,
when the thalamus is stimulated, cortical neurons react in a way that
precisely mirrors the patterns of cortical activity that occur
spontaneously, without any kind of input.
“The brain is not a reflex machine. What the thalamus is doing
is waking up internal cortical states,” Yuste explained.
“The brain is humming along and the world (through the thalamus)
appears to be selecting one of those states.”
Since the invention of electroencephalography (EEG) by Hans Berger
in the 1920s, scientists have known that the brain is always on. It is
a blur of electrochemical activity, even when we are asleep or not
gathering information about the world through our senses. This
spontaneous activity has traditionally being considered the
“noise in the machine” and neuroscientists have mostly
ignored it and have instead concentrated on examining how the brain
responds to sensory stimulation or how it generates behavior.
However, during the last 20 years it has become clear that the
spontaneous activity is very prominent, even during sensory stimulation
or motor behavior. Moreover, in earlier studies, Yuste's and other
groups have shown that the spontaneous firings of neurons in vitro or
in vivo were not random, but instead possessed exquisite spatiotemporal
patterns of activity. Nevertheless, it was unclear whether this
spontaneous activity was at all important, and the relation between
those patterns of spontaneous activity and the sensory inputs remained
unknown.
The new work from Yuste, first author Jason MacLean, Brendon O.
Watson and Gloster B. Aaron, used calcium imaging techniques to monitor
the activity of cortical circuits in brain slices of mouse cortex that
were connected to the thalamus. With this optical technique, pioneered
by Yuste, they reconstructed with unprecedented resolution the
sequential turning on and off of large neuronal populations under
different experimental conditions. By stimulating the thalamus, the
HHMI team observed that, neuron by neuron, the triggered patterns in
the cortex were indistinguishable from those that occur when the cortex
is spontaneously active. Moreover, these cortical patterns occurred
with great temporal fidelity, suggesting that the cortex can somehow
preserve and faithfully replay temporal sequences of activity and that
the thalamus can turn this intrinsic program on.
The results of the new experiments from Yuste's group have wide
implications. For example, some of the ideas they impact were discussed
in Immanuel Kant's philosophical treatises in the 18th
century. Kant was interested in the role of the mind in perception, and
he suggested that the brain possesses its own internal categories, such
as the sense of space and time, through which the external world is
perceived. But the new findings also impinge on modern debates in
neuroscience discussing the relative contributions of the thalamus and
the cortex in determining patterns of brain activity and the role of
recurrent excitatory circuits in sustaining intrinsic states. These
intrinsic activity states or “attractors” have long been
postulated to exist by theorists as potential mechanisms that implement
memories or thoughts.
Yuste said the new work proves that the spontaneous patterns of
neural activity that occur in the brain are not random phenomena.
“Spontaneous activity, rather than being noise in the machine, is
actually engaged by the outside world,” according to Yuste. In
other words, the brain seems to harbor its own predefined dynamical
states, which are jumpstarted by thalamic activity. Such a notion
indicates a dominant role for the cortex in perception, and it flies in
the face of a long-held view of the brain that holds that the purpose
of the cortex, or brain circuits in general, is more passive, and
responds faithfully to sensory inputs in order to generate
behavior.
“The results demonstrate that the thalamus is not necessary
for the generation of spontaneous cortical activity and it is likely
only modulating this intrinsic activity,” Yuste and his
colleagues conclude.
Neuroscientists, Yuste predicts, will next attempt to understand the
internal logic that governs those intrinsic states and trace the
spontaneous internal states of the cortex in behaving animals in an
effort to see how they relate to sensory experience and motor planing.
Their goal: To understand how internal mental states map out to the
world.
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Versión en español
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Jennifer Michalowski
