Page 199 - Visions of the Future Chemistry and Life Science

180 V. WALSH

Janet Eyre in Newcastle who stimulated the motor cortex in over 300 sub-
jects between the ages of 32 weeks and 52 years while recording electrical
activity in the biceps and the hand muscles. Eyre took notice of the time
between applying stimulation and the arrival of signals at the muscle
recording sites (a measure of the speed of nerve conduction) and also of the
magnetic stimulation power required to produce muscle activity. There
was a sharp decrease in both delay time and power required during the first
two years of life and by the time the children had reached five years of age
their delay time had reached the same level as that of adults. The impor-
tance of this is that the results correlate with the time taken for the muscle
nerve fibres involved to reach their maximum diameter and, because diam-
eter is a determinant of speed, their maximum conduction velocities. The
magnetic stimulation data also correlate with the time at which children
develop good fine finger and prehension skills.
Recording change is impressive enough but change can also be pro-
duced. A recent study by Alvaro Pascual-Leone at the Beth Israel Hospital
in Boston, MA, has shown that TMS applied at different temporal rates can
either impede or enhance one’s ability to learn certain kinds of tasks.
Remarkably low levels of stimulation (1 pulse per second) over the motor
cortex slowed down learning on a visuomotor association task but learn-
ing on the same task was faster than normal when magnetic stimulation
was applied at 10 pulses per second. Similar results have also been obtained
in the visual system and also in studies of language. The implications of
this kind of manipulation of learning function are far reaching and
attempts to apply this in the clinic are already underway: can we speed up
learning? Can we kick start the brain?
What will happen in the twenty-first century? As you no doubt
remember from all the ‘end of century’ pundits who soiled magazines and
newspapers as we entered the year 2000, prediction is no more than a veil
for predilection, so I’ll come clean and say what it is I would like to see
happen in the near future with magnetic stimulation. The emergence of
magnetic stimulation as a tool in neuropsychology has been slower than it
should have been. Other techniques, such as functional magnetic reso-
nance imaging, multi channel electroencephalography and magnetoen-
cephalography have all attracted more attention. They are in themselves
exciting developments and we have learned much about the human brain
from them. However, they all record brain activity in one form or another
and thus cannot reveal how the brain would function in the absence of a