The Brain

56
Chapter 2 Biology and Behavior
FIGURE 2 .6
A Reflex Pathway
Sit on a chair, cross one leg
over the other, and then use
by
the handle of a butter knife or
some other solid object to gently tap your
top knee, just below the joint, until you
get a “knee jerk” reaction. Tapping your
knee at just the right spot sets off an almost instantaneous sequence of events
that begins with stimulation of sensory
neurons that respond to stretch. When
those neurons fire, their axons, which end
within the spinal cord, cause spinal neurons to fire. This, in turn, stimulates the
firing of motor neurons with axons ending
in your thigh muscles. The result is a contraction of those muscles and a kicking of
the lower leg and foot. Information about
the knee tap and about what the leg has
done also goes to your cerebral cortex,
but the reflex is completed without waiting for guidance from the brain.
Spinal cord
To brain
Sensory neuron
from muscle
doing
2
learn
Knee tap
Reflex motor
output, causing
thigh muscle
to contract
Single
synapse
in reflex
To other
leg
consequences of an action goes back to the source of the action for further adjustment.
That is a feedback system.
The Brain
When pain messages from that hot burner reach your brain, you don’t become aware
just of being burned. You might also realize that you have burned yourself twice before
in the past week and get annoyed at your own carelessness. The brain is the most complex element in the central nervous system, and it is your brain’s astonishing capacity
for information processing that allows you to have these thoughts and feelings. A variety of new brain-scanning techniques, combined with some older measures, are giving
scientists ever better views of the workings of the human brain (Amaro & Barker, 2006;
Miller, 2003; see Table 2.1).
Each technique can indirectly measure the activity of neurons firing, and each has
different advantages and disadvantages. One of the earliest of these techniques, called
the electroencephalograph (EEG), measures general electrical activity of the brain. Electrodes are pasted on the scalp to detect the electrical fields resulting from the activity
of billions of neurons (Figure 4.3 in the consciousness chapter shows how EEG can be
used to record brain activity during sleep). Although this tool can associate rapidly
changing electrical activity with changes in the activity of the brain, it cannot tell us
exactly where the active cells are.
A newer technique, called the PET scan, can locate brain cell activity by recording
where radioactive substances become concentrated when injected into the bloodstream.
PET stands for positron emission tomography. It records images from the brain that indicate the location of the radioactivity as the brain performs various tasks. For instance,
PET studies have revealed that specific brain regions are activated when we look at fearful facial expressions or engage in certain kinds of thoughts (Morris et al., 1998; Wharton et al., 2000). PET scans can tell us a lot about where changes in brain activity occur,
but they can’t reveal details of the brain’s physical structure.
A detailed structural picture of the brain can be seen, however, using magnetic resonance imaging, or MRI. MRI exposes the brain to a magnetic field and measures the
resulting radiofrequency waves to get amazingly clear pictures of the brain’s anatomical details (see Figure 2.7). Functional MRI, or fMRI, combines the advantages of PET
and MRI and is capable of detecting changes in blood flow and blood oxygen that
reflect ongoing changes in the activity of neurons—providing a sort of “moving picture” of the brain (e.g., Shu et al., 2002). The newest techniques offer even deeper
insight into brain activity, structure, and functioning. These techniques include a variant on fMRI called diffusion tensor imaging (DTI), as well as a procedure called transcranial magnetic stimulation (TMS).
57
The Central Nervous System: Making Sense of the World
TA B L E
2.1
Techniques for Studying Human Brain Function and Structure
Technique
What It Shows
Advantages (ⴙ) and
Disadvantages (ⴚ)
EEG (electroencephalograph):
Multiple electrodes are pasted
to the outside of the head
Lines that chart the summated
electrical fields resulting from the
activity of billions of neurons
Detects very rapid changes in
electrical activity, allowing analysis
of stages of cognitive processing
Provides poor spatial resolution
of the source of electrical activity;
EEG is sometimes combined with
magnetoencephalography (MEG),
which localizes electrical activity by
measuring magnetic fields associated
with it.
PET (positron emission tomography)
and SPECT (single-photon emission
computed tomography): Positrons
and photons are emissions from
radioactive substances
An image of the amount and localization
of any molecule that can be injected
in radioactive form, such as neurotransmitters,
drugs, or tracers for blood flow or glucose
use (which indicates specific changes
in neuronal activity)
Allows functional and biochemical
studies
Provides visual image
corresponding to anatomy
Requires exposure to low levels of
radioactivity
Provides spatial resolution better
than that of EEG but poorer than
that of MRI
Cannot follow rapid changes
(faster than 30 seconds)
MRI (magnetic resonance imaging):
Exposes the brain to a magnetic
field and measures radiofrequency
waves
TMS (transcranial magnetic stimulation):
Temporarily disrupts electrical
activity of a small region of brain by
exposing it to an intense magnetic
field.
Traditional MRI provides high-resolution
image of brain anatomy. Functional
MRI (fMRI) provides images of changes
in blood flow (which indicate specific
changes in neural activity). A new variant,
diffusion tensor imaging (DTI), shows water
flow in neural fibers, thus revealing the
“wiring diagram” of neural connections
in the brain.
Requires no exposure to
radioactivity
Normal function of a particular brain region
can be studied by observing changes after
TMS is applied to a specific location.
Shows which brain regions are
necessary for given tasks.
Provides high spatial resolution of
anatomical details ( 1 mm)
Provides high temporal resolution
1
( 10
second)
Long-term safety not well
established.