INFORMATION TECHNOLOGY SYMPOSIUM
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Prior to the introduction of these novel methods, there were only two
very limited methods available for the neurological study of cognitive
brain functions. One of them consisted of examining the performance
of people with particular cognitive deficits while they were alive and
then, after their death, performing brain autopsies to identify their neu–
roanatomical abnormalities. This was the method by which Wernicke
and Broca identified their eponymous cerebral areas involved in the
interpretation and production of speech in the latter part of the nine–
teenth century. The other method, electroencephalography, or EEG, was
developed in the
1930S
by Hans Berger.
It
consists of placing electrodes
on the scalp and recording the medley of electrical signals emanating
from the underlying brain areas. The EEG method can reveal whether
an unconscious person is or is not brain-dead, but it can only provide
very crude information about the localization of cerebral activity.
Both PET and fMRI involve the recording of patterned emission of
photons from brain tissue by a spherical array of radiation detectors sur–
rounding the subject's head. A computer program converts the photon
emission pattern recorded by the detector array into a two-dimensional
image of the cerebral activity pattern. This method of recording and
computerized interpretation of the data is called tomography. Both
tomographic methods can provide images of brain structure as well as of
brain function, and both represent a milestone in the history of neuro–
psychology because they can be performed on awake human subjects
without requiring any surgical interventions. Their advent was presum–
ably one of the main reasons why former President George Bush declared
the years
1990-2000
as the "Decade of the Brain."
The principle of the PET method is the detection of the emission of
two gamma ray photons travelling in opposite directions upon the
encounter and mutual annihilation of a positron (emitted upon the
decay of a radioisotope) and an ordinary (negatively charged) electron.
The gamma ray photons emitted are recorded by the array of radiation
detectors surrounding the subject's head. This method of photon detec–
tion permits a fairly precise localization of the site of gamma ray emis–
sion, and hence of the source of the emitted positrons, with a spatial
resolution of about 4 to 8 mm.
To perform a PET scan, the subject is injected with deoxyglucose
labeled with an unstable (radioactive) isotope of fluorine, which emits a
positron upon its decay. Deoxyglucose is an analogue of glucose, the
main source of energy fueling the operation of the brain. Deoxyglucose,
which lacks one of the five hydroxyl groups of ordinary glucose, is
taken up but not metabolized by the cerebral nerve cells. Consequently,
