YOUR BRAIN
we can use brain imaging to
get a good idea while they are
still alive.
To chart the localization
of function in the brain,
neuropsychologists look
for what they call “dissocia-
tions”, which is when damage
to one part of the brain has
specific functional conse-
quences, whereas damage to
another region does not, thus
implying some kind of func-
tional independence between
the two areas. The holy grail
for this kind of work is the
“double dissociation” which is
when one patient has damage
to one area, another patient has damage to a different area, and each
patient exhibits distinct patterns of behavioural impairment.
Probably the best known example of a double dissociation was uncov-
ered in the nineteenth century by the French surgeon Paul Broca and
the German neurologist Carl Wernicke. Broca described a patient who
suffered damage to the rear of his left frontal lobe. His comprehension
was unaffected but he was subsequently only able to utter the syllable
“tan”, hence his nickname Tan Tan. By contrast, Wernicke worked with
a patient who, after suffering damage to the temporal lobe, seemed to
have lost the ability to understand speech. Wernicke’s patient could still
utter words, but because his comprehension was destroyed, his speech
was garbled nonsense. Today, these regions of the brain are still referred
to as Broca’s and Wernicke’s areas, respectively, although the functional
distinction is seen as being between syntax (Broca’s area) on the one
hand, and semantics or meaning (Wernicke’s area) on the other, rather
than between production and comprehension.
Further clues about brain function were provided by a series of studies
conducted in the 1960s, 70s and 80s on so-called “split brain” patients,
who’d had their corpus callosums severed in an attempt to help relieve
intractable epilepsy. Michael Gazzaniga and colleagues presented words
and images to one hemisphere of these patients but not the other, to
see what would happen. This is possible by getting a participant to stare
Phineas Gage poses proudly with the
tamping iron that shot through his brain.