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Reverse engineering the human mind 179

magnetic stimulation at the same site. Thus, in the case of the visual
cortex we now know that the mechanism of suppression appears to be
excitatory. This is an important step forward because the production of a
deficit does not of itself say anything about the neural mechanism.
Penfield called areas from which they could not elicit a response ‘elab-
oration areas’ and surmised that these could only be studied in action. In a
recent series of experiments in Oxford, Matthew Rushworth has not only
shown this to be true but has demonstrated the temporal structure of inter-
actions between the motor cortex (which Penfield and Rasmussen could
study) and the premotor cortex (an elaboration area which could not be
studied by direct stimulation). Subjects were required to carry out a simple
visual discrimination task (discriminating between large and small rectan-
gles and circles) and to press an appropriate button. Magnetic stimulation
was applied to one of three cortical areas at different times after the stimuli
were presented. If TMS was applied to the motor cortex around 300ms after
the stimuli were presented, subjects were slower to make their responses;
if magnetic stimulation was applied to the pre-motor cortex around 100ms
after stimulus onset the subjects were slower to make their response; and
if an area between these two sites was stimulated, the time to respond was
slower when the TMS arrived around 180ms after the visual stimuli were
presented. Here we have an example of three links in a chain of motor
signals being segregated by magnetic stimulation across a gap less than one
fifth of a second. This millisecond-level power shows that the pre-motor
elaboration area is important for selecting which movements to make over
100ms before the lower level motor cortex is instructed to execute the
movement.
Correlating excitation with temporary blindness, recreating the effects
of brain damage and elaborating the fine temporal structure of the interac-
tions between different areas within a system all seem to be reasons for
brain engineers to be cheerful. But the brain cheats. Like no other machine
it changes the way it performs a task over time. One may have given a
detailed and even accurate account of the function of an area, but the
details of the function can change: an area which is crucial to learning a
task may not be necessary once the task has been learned and even if it is,
its role may have changed. Studies using magnetic stimulation have
approached the issue of plasticity by either measuring the functional cor-
relates of it or by actually manipulating it. A particularly pleasing example
of charting the changing functions of the nervous system is the work of