Healthcare and Medicine Reference
In-Depth Information
BOX
WATCHING NEURONS THINK: FUNCTIONAL
PROPERTIES OF NEURON ENSEMBLES
Amajor goal of neurophysiology is to demonstrate a
causal relationship between CNS function and animal
behavior. While our success has been limited, there have
been a number of exciting strategies that should bring us
closer to the goal. For almost a century, neurophysiolo-
gists have been recording electrical activity from the
nervous system, at first from large populations of cells
with scalp electrodes (cf. electroengephalograms) and
eventually with small extracellular electrodes that
monitor the action potentials from a single neuron. For
those interested in sensory coding and perception, the
extracellular electrodes are usually lowered into the brain
of an anesthetized animal, and a neuron's activity is
recorded while stimuli are delivered to the ears, eyes, or
other receptor populations. In this way, we learn how
environmental stimuli are converted into a neuronal code.
For example, if an electrode is placed in the visual cortex
and stimuli are delivered to each eye, we find that some
neurons respond to bars of light that are vertically ori-
ented, whereas others are driven best by horizontal bars.
The neurophysiologist would call this “orientation selec-
tivity,” and such response properties usually find their
way into theories on the neural basis of visual perception.
Therefore, single neuron recordings provide an extremely
sensitive measure of whether the building blocks have
been assembled correctly during development.
Of course, it would be most compelling to record neural
activity while the animal is actually processing a stimulus
or moving a limb. In an early approach, animals were
injected with a tritium-labeled sugar molecule, 3 H-2-
deoxyglucose, that was taken up by nerve cells that were
very active and required energy (Kennedy et al., 1975). It is
now possible to measure neural activity and behavior
simultaneously using several different techniques. Elec-
trodes can be permanently mounted in the nervous system
during an initial surgery, and these electrodes are then
used to monitor neural activity when the animal recovers.
Arrays of such electrodes are now used to record from the
hippocampus of freely moving rats as they explore their
environment and learn new tasks. It is also possible to
stimulate or inactivate a region of the brain in awake-
behaving animals, including humans during the course of
neurosurgical treatment, and to monitor the effects on
motor function or sensory perception (Penfield and
Rasmussen, 1950; Riquimaroux et al., 1991).
There are several ways to monitor brain activity in
awake animals, including humans, that can be performed
without exposing the brain. Although these techniques
have not been applied widely to developing animals, they
will probably play an important role in our future under-
standing of plasticity. One technique that offers <1mm
spatial resolution, called function magnetic resonance
imaging (fMRI), uses a very strong magnetic field (15,000
times the earth's magnetic field) to detect oxygen content.
Since deoxyhemoglobin is paramagnetic relative to oxy-
hemoglobin and surrounding brain tissue, brain activity
commonly produces a local increase in oxygen delivery.
For example, it has recently been possible to visualize
activity in a single barrel field in rat somatosensory cortex
(Yang et al., 1996). Another technique, magnetoen-
cephalography (MEG), uses superconducting detectors to
monitor the magnetic fields produced by a population of
active neurons. This technique provides information
about the timing, location, and magnitude of neural activ-
ity. For example, word-specific responses in the inferior
temporo-occipital cortex are slow or absent in dyslexic
individuals compared to control subjects, suggesting a
specific neural impairment in this developmental disor-
der (Salmelin et al., 1996). Finally, there are changes in
light absorbance that are well correlated with neuronal
activity, and it is possible to illuminate the surface of
the brain and measure the reflected light while the system
is processing information, referred to as differential
optical imaging. By using these signals, many features of
visual cortex development have been observed, including
ocular dominance orientation selectivity (Blasdel et al.,
1995).
Obviously, the challenge to find causality between
brain function and behavior is magnified during devel-
opment when nonspecific behavioral factors (cf. level of
arousal) and the fragility of nerve cell function (cf. rapid
fatigue) introduce great restraints. However, the study of
immature brains, and the behaviors that they manufac-
ture, should prove useful because it is the only means of
correlating changes in the two without imposing surgery,
drugs, or bad genes.
Search Pocayo ::




Custom Search