V. Srinivasa Chakravarthy, Indian Institute of Technology Madras,
Chennai
In the history of brain one notices a curious
long-drawn tug of war between two rival theories. One of these is known
as “Aggregate Field View,” among its many names, holds that the brain functions
as a single whole – damage to a part affects all behavior. The rival view
says that functionality is distributed locally in the brain – one part
for speaking, one part for walking, one part for seeing etc.
One of the early proponents the “holistic” view was
the French philosopher Des Cartes who loved freely speculating about everything
under the sun (and around it like, say, the solar system). He believed
that there is a supraphysical Soul supporting brain’s activities and the
pineal gland in the brain is some sort of a mouthpiece of the Soul. He
also spoke of “vital spirits” that flow through nerves producing mechanical
effects. Modern science could neither find any trace of the Soul, nor its
lesser children the “vital spirits.”
Then in the late 18th century there was this Austrian
physician and neuroanatomist called Franz Gall who founded the “science”
of phrenology (Phrenos = head, thus phrenology means the science of head
examination!!!). His idea is that since a person ’s character has many aspects
to it – honesty, intelligence, courage etc – each of these qualities has
a seat or a location on the brain’s surface. And the more a person is endowed
with a given quality, the greater is its seat on the brain’s surface. Further,
Gall taught, that the profile of the surface of the brain affected the head
surface too. Thus a person’s character can be estimated by examining the
bumps on the head! (People who didn’t like the whole affair called
this science “bumpology”! ) Thus Gall’s phrenology supported a crude form
of localization of brain function.
Later, in early 19th century, it was Pierre Flourens, who reacted to the
tenets of phrenology. However, unlike his predecessors (like Des Cartes
who glibly spoke of Soul, and Gall who linked head’s bumpiness with virtue),
Flourens took a strictly scientific approach to the problem. He studied the
behavior of experimental animals by systematically removing various parts
of their brains. He noted that the behavior of animals was affected as
a whole irrespective of the site of damage. Thus we are back again to a
holistic view of the brain, but with one very important difference – in
these brains there is no place for Soul! The tug of war did not end there.
In late 19th century, British neurologist, Hughlings Jackson did important
work on epilepsy – a nervous disorder commonly known as seizures or “fits.”
In a special variety of seizures studied by Jackson, the Jacksonian seizures,
convulsions start from the end of one limb and gradually spread to the entire
limb and then to the entire body. A good explanation for this phenomenon
is to imagine that abnormal neural activity started in the brain at a specific
location corresponding to the extremity of a limb. As the activity spread
in the brain from that location, convulsions spread over the entire
body. Since a local abnormal activity produced a local convulsion in the
body, these type of seizures are an evidence for localization of brain function.
More impactful results came from studies of language related disorders
known as aphasias. In 1861, French neurologist Paul Broca studied a patient
who could understand language but had problem speaking. The patient
could utter a few isolated words, and could even hum a melody. So his vocal
cords were intact. Postmortem analysis of the patient’s brain revealed that
a specific area of the brain – Broca’s area as it was later named – was
damaged.
In 1876, Carl Wernicke, a German neurologist, found another brain area
with language related function. Named Wernicke’s area, this area is essential
for language understanding. Patients with damage to this area could speak
but couldn’t understand language. These instances offer more evidence supporting
a localized view of brain function. Looks like, it is time for the see-saw
to swing to the other side. American (later Canadian) psychologist Karl Lashley
did experiments with rats learning to run through mazes. (Rats have terrific
spatial skills. You would have noticed when you chase a rat at home,
how it always escapes at lightning speed with
incredible accuracy through a specific remote hole). Lashley would slash
parts of rat’s brain and observe its maze-learning capabilities. He noted
that lesions impaired behavior in a global, overall way. Aggregate
field view again!
But support for localization came (sigh!) later with experiments of
a brilliant British neurosurgeon – Wilder Penfield (1891-1976). He electrically
stimulated exposed brains of patients (under local anesthesia of course,
because brain itself has no pain sensation ) and found that a stimulation
of a specific location produced a specific response, say, movement of a
specific body part, or even a single muscle.
More illuminating data came with sophisticated imaging data, where electrical
activity in the brain can be monitored without surgically exposing
the brain. In one experiment using Positron Emission Tomography (PET), a
special type of imaging tool, subjects were asked to 1) listen to, 2) read,
3) speak and just 4) think about a set of words. In each of these tasks
specific locations or groups of locations in the brain were found to be
active. It is when the subjects were thinking about the meaning of words
that the largest
number of regions were found to be active simultaneously, compared to
the other 3 tasks. So the moral of our PET story is that in a given
high-level task a group of brain regions work together, with extreme precision,
flexibility, adaptability and coordination.
With its 100 billion employees – the neurons – each one of them interacting
with about 1000 to 10,000 others, doing a variety of tasks from writing
poetry to bunjee jumping, brain is the largest and strangest corporate giant
ever. So the question of “local or global?” loses its meaning. It
is a situation where a large number of local units work with such perfect
coordination, that their assembly – the brain – seems to work like a single
unit. But don't go with an impression that we now understand how
brain functions
In fact, we are far from that. That is why Neuroscience is one of the
most active research fields today. Look for more on this in the next issue.
