Chapter 6: Imaging the Brain’s Activity
• Angelo Mosso (1846– 1910) was the first to experiment with the
idea that changes in the flow of blood in the brain might provide
a way of assessing brain function during mental activity. Mosso
knew that, in newborn children, the fontanelles— the soft areas
on a baby’s head where the bones of the skull are not yet fused
— can be seen to pulsate with the rhythm of the heartbeat.
• Electrical recording methods detect changes in the electrical
activity of neurons.
• Brain stimulation methods induce changes in the electrical
activity of the brain.
• X- ray imaging methods are sensitive to the density of different
parts of the brain, the ventricles, nuclei, and pathways.
• Dynamic imaging methods record and manipulate ongoing
changes in brain activity, including the electrical activity of cells,
biochemical events, differences in glucose consumption, and the
flow of blood to various regions.
Recording The Brain’s Electrical Activity
• The techniques for recording the brain’s electrical activity
include ( 1) single- cell recording; ( 2) electroencephalographic
recording; and ( 3) event-related potential recording.
• Action potentials are the currency with which the brain operates.
The sensation of a mosquito landing on your arm is conveyed
from one neuron to the next in the form of action potentials:
somatosensory neurons convey action potentials to the spinal
cord, and spinal neurons convey them to the cortex. In the
cortex, action potentials record the perception that a mosquito is
on your arm. When the cortex instructs the hand to swat at the
mosquito, it sends the message in the form of action potentials.
• Single- cell recordings at different levels show that ganglion cells
and LGB cells respond only to dots of light, whereas the cells in
the primary visual cortex respond to bars of light of specific
orientation. Cells in higher visual areas respond to more-
complex stimuli, including the position and movement of objects,
and perhaps even to the specific features of the face such as “
Halle Berry” or “ Grandmother.” In some way, the visual cortex
takes information encoded as dots by numerous cells and bars in
fewer cells and translates it into the complex, ongoing visual
experience that tells us the “ look” of our world. Electroencephalographic Recording
• A simple technique for recording the electrical activity of large
regions of the human brain was developed in the early 1930s by
German physiologist Hans Berger. He found that voltage
fluctuations, or “ brain waves,” could be recorded by placing the
leads from a voltmeter onto the skull. These recordings, called
electroencephalograms (electro, for “ electrical,” encephala, for “
brain,” and grams, for “ graphs”) or EEGs, are a valuable tool for
(1) studying sleep, (2) monitoring the depth of anaesthesia, (3)
diagnosing epilepsy and brain damage, and (4) studying normal
• Polygraph: Apparatus for simultaneously recording blood
pressure, pulse, and respiration, as well as variations in electrical
resistance of the skin; popularly known as a lie detector.
• The neurons of the neocortex are arranged in horizontal layers,
and a substantial part of the EEG signal comes from the large
pyramidal neurons of layers V and VI. Pacemaker cells ensure
that these neurons undergo graded potentials at the same time,
presumably so that they can synchronize their action potentials.
The signal recorded by the EEG consists of the rhythmical graded
potentials on many thousands of neurons.
• Volume conducted: Refers to electrical potential re-corded in
tissue at some distance away from its source.
• Beta rhythm: Irregular electroencephalographic activity ranging
from 13 to 30 Hz and generally associated with an alert state.
• When a person is calm and resting quietly, especially with eyes
closed, the rhythmical brain waves often emerge. These so-
called alpha ( a) waves are extremely rhythmical but with waxing
and waning amplitude and a frequency of approximately 11
cycles per second.
• Some forms of epilepsy, called petit mal (from the French words
mean-ing “ little bad”) epilepsy, are generally associated with
brief losses of consciousness, perhaps lasting only a few
seconds. Other forms of epilepsy may be associated with a loss
of memory lasting for many minutes. Still other forms, called
grand mal (meaning “ big bad”) epilepsy, are characterized by
convulsions of the body, falling down, and loss of consciousness.
Event –Related Potentials
• Event- related potentials, or ERPs, are brief changes in a slow-
wave EEG signal in response to a discrete sensory stimulus. An
ERP is not easy to detect, be-cause the signal is “ hidden” in the
EEG. The ERP, which consists of a graded potential generated by the sensory stimulus of interest, is mixed with many other
electrical signals and so is impossible to spot just by visually
inspecting an EEG. One way to detect an ERP is to produce the
stimulus repeatedly and average the recorded responses.
Averaging tends to cancel out any irregular and unrelated
electrical activity, leaving only the graded potentials generated
by the stimulus event.
• Readiness potential: Evoked potential that occurs just before a
• Neural activity, by generating an electrical field, also produces a
magnetic field. Although the magnetic field produced by a single
neuron is extremely small, the field produced by many neurons
is sufficiently strong to be recorded on the surface of the skull.
Such a record is called a magnetoencephalogram (MEG), and it is
the magnetic counterpart of the EEG or ERP.
• Calculations based on MEG measurements not only provide a
description of the electrical activity of neurons but also permit a
three- dimensional localization of the cell groups generating the
measured field. Magnetic waves being conducted through living
tissue undergo less distortion than electrical signals do, and so
an MEG can have a higher resolution than an ERP. Thus, a major
advantage of the MEG over the EEG and ERP is its ability to more
precisely identify the source of the activity being recorded.
• The heart of a magnetoencephalogram probe is a sensing device
containing the special superconducting coils needed to detect
the brain’s very weak magnetic fields. This so- called SQUID
(superconducting quantum interference device) is immersed in
liquid helium to keep it at the low temperature necessary for
superconductivity. One or more probes are moved across the
surface of the skull, sending signals to the SQUID.
• Parkinson’s disease is characterized both by tremors and by
akinesia, an absence or poverty of movement. When electrodes
are implanted in the brain so that deep brain stimulation (DBS)
can be applied to a number of brainstem regions, both tremors
and akinesia are lessened.
Transcranial Magnetic Stimulation
• The relation between magnetism and electricity forms the basis
of transcranial magnetic stimulation (TMS), a noninvasive
method that allows the brain to be stimulated through the skull.
In the TMS technique, a small wire coil is placed adjacent to the
skull. A high voltage current is passed through the coil in pulses as rapid as 50 times per second.
• Transcranial magnetic stimulation was originally used by
neurosurgeons to stimulate brain tissue to monitor its functional
condition during brain surgery and to identify the function of the
tissue. From this initial use, it became clear that TMS did not
harm tissue, even after thousands of pulses of stimulation, and
so could be used to stimulate the normal brain through the skull.
X-Ray Imaging Techniques
• The first method for producing a visual image of the brain,
conventional radiography, consists of passing X- rays through the
skull onto an X- ray- sensitive film. As the X- rays travel through
the head, they are absorbed to different degrees by different
tissues: to a great degree by dense tissue such as bone, to a
lesser degree by neural tissue, and less still by fluid such as that
in the blood vessels and ventricles.
• Pneumoencephalography (literally, air– brain graph) is a method
for enhancing conventional X- ray ra