Page 197 - Visions of the Future Chemistry and Life Science

178 V. WALSH

however, one can apply a single pulse (which lasts for less than 1ms) at any
time while a subject performs a task. The effect of the TMS is to cause
neurons to discharge at random in and around the area stimulated and thus
to impede the normal functioning of that area. Thus the subject ‘suffers’
from a temporary ‘lesion effect’ which lasts for a few tens of milliseconds.
Theoretically we are now able to disrupt information transmission in spe-
cific circuits at specific moments in time in the same way as a debugger
needs to be able to access parts of a computer program at a particular point
in its execution: a reverse engineer’s dream. This has become known as the
creation of ‘Virtual Patients’ and takes us into the realms that Penfield and
Rasmussen could not enter – those elaboration areas. But the first chal-
lenge for magnetic stimulation is to show that it can recreate the effects
seen in real brain damaged patients.
The patient L.M., mentioned above, suffered brain damage, caused by
a thrombosis, which affected those regions of her brain, known as the V5
complex, that are important for the perception of movement. According to
the rationale of the virtual patient approach, magnetic stimulation applied
to the visual motion areas of the brain should make subjects experience the
same difficulties as L.M. Indeed several laboratories have now shown that
magnetic stimulation over human area V5 specifically impairs the percep-
tion of movement. So magnetic stimulation has the face validity conferred
by replication of others’ findings (an important step in science) but it needs
also to be able to extend the findings of others.
In their investigations of patients, Penfield and Rasmussen observed
that stimulation of the brain regions responsible for seeing led patients to
experience phosphenes which they described in terms such as ‘I saw just
one star’ , ‘Silver things to the left of me’ or ‘red and blue wheels’. Penfield
and Rasmussen were aware that seizures of the occipital lobe were asso-
ciated with blindness in the parts of the visual field represented therein and
they surmised that with their more localised electrical stimulation the
patient ‘may be blind only in that portion of the field where he seems to
see the light’. This kind of focal blindness is known as a scotoma and mag-
netic stimulation has since been able to show that the prediction was
correct. Thomas Kammer, working at the Max Planck Institute in
Teubingen, applied magnetic stimulation to the visual cortex and gave sub-
jects a task in which they were required to detect the presence of a target
in different parts of the visual field. He found that the location of the tran-
sient scotoma coincided with the location of the phosphene produced by