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Reverse engineering the human mind 175

shut’ – I know you heard the door, but that was the image not the transla-
tion itself. Penfield and Rasmussen were aware of this problem and con-
cluded that in these cases stimulation ‘sheds no light upon the function of
an area unless the patient is making use of that area at the moment’. What
is needed, then, is some way of reverse engineering the brain in action – a
means of catching the brain in the act.
Another wave of reverse engineering, neuropsychology, began soon
after the first and got into full flight with the report by Pierre Paul Broca
(1824–1888) that damage to a part of the lower left frontal lobe rendered
patients unable to produce speech. The approach taken in neuropsychol-
ogy is to investigate the abilities of patients who have suffered brain
damage and from the pattern of their deficits to infer something about the
function of that region or about the general organisation of the system
under investigation. The study of patients with focal brain damage formed
perhaps the most important source of knowledge about the organisation
and function of the brain for the best part of the twentieth century and the
kinds of dissociations demonstrated were both informative and intellectu-
ally seductive. For example, one patient, called L.M., has been reported to
be severely impaired in the perception of movement but less so in the per-
ception of shapes and textures and not at all in the perception of colours.
Another patient perceived the world totally devoid of colour without suf-
fering any marked reductions in movement and form perception. Other
such functional dissociations abound: patient D.F. has very poor visual per-
ception of orientation but can nevertheless use information about orienta-
tion to grasp objects or put objects through differently oriented holes.
Other specific and curious deficits include the loss of awareness of one half
of the body, or of objects, or an inability to name an object presented to the
right hand side of the brain when it is disconnected from the left side. All
of these examples suggest that the brain is organised into groups of rela-
tively specialised areas.
In many respects the classic findings of neuropsychology have formed
the bedrock of much of what we know about how we see, hear, speak, move
and even feel. Nonetheless, neuropsychology has not always influenced
theories about how the intact brain carries out tasks. This is partly because
nature is a poor surgeon: accidental brain damage is usually spatially
diffuse, interrupts several functions, is irreversible and the time of its
occurrence cannot be predicted. Another problem with the lesion method
in general, even when specific areas can be removed from animals, is that