Chapter
Six
The unity
Scottish neuroscientist Donald MacKay (1987) was deter-
mined to find out whether split-brain patients are really
two persons or one, and devised an ingenious test. He
taught each hemisphere, separately, to play a ‘twenty ques-
tions’-type guessing game with him. One person chooses a
number from 0 to 9 and the other has to guess what it is by
saying ‘up’, ‘down’, or ‘OK’ until the correct answer is reached.
Both halves of patient J.W. learnt the game easily. Then they
were asked to play against each other, with J.W.’s mouth
(controlled by his left hemisphere) making the guesses, and
his left hand (controlled by his right hemisphere) pointing
to cards saying ‘go up’, ‘go down’, or ‘OK’. With this game, it
proved possible for the two half-brains to play against each
other, and even to cooperate and pay each other winnings
in tokens, but MacKay concluded that there was still no evi-
dence of two separate persons or of true ‘duality of will’.
How, he asked, could anything play a game of twenty
questions without being conscious? He noted all the
intelligent actions we can carry out unconsciously, and
the artificial systems that can play games, and came to the
following conclusion. To understand human behaviour
we must distinguish between the executive and super-
visory levels of brain function. The executive level can
(unconsciously) control goal-directed activities and eval-
uate them in terms of current criteria and priorities, but only the self-supervisory
system can determine and update those priorities. We are conscious only of those
features of our world that engage this self-supervisory system.
With this theory, MacKay provides his own answers to some of our recurring ques-
tions about consciousness. Question: what makes some things conscious and oth-
ers not? Answer: whether they engage the self-supervisory system or not (though
he admits that how the activity of this system gives rise to conscious experience
remains totally mysterious). Question: what makes each of us a psychological
unity? Answer: that we have only one self-supervisory system to determine our
overall priorities. As for the split-brain patient: he has only one self-supervisory
system and therefore is still only one conscious person.
So who was right? Do split-brain patients have one consciousness or two? At first
sight, it seems as though there must be an answer to this simple question, but
perhaps we need to think again.
You may have noticed that in the paragraphs above we sometimes described
one hemisphere or the other as knowing, seeing, or even being conscious. This
kind of language is hard to avoid when confronted with the strange findings we
have described, but the philosophers Maxwell Bennett and Peter Hacker (2003)
accuse Sperry, Gazzaniga, and other neuroscientists of making conceptual errors
and causing ‘profound confusion’ by talking about a half brain as though it were
a person.
It should be obvious that the hemispheres of the brain can neither see
nor hear. They cannot speak or write, let alone interpret anything or makeFIGURE 6.8 • The human visual system is
organised as shown. Information
from the left visual field of both eyes
(in this case the flowers) goes to the
right hemisphere, and information
from the right visual field of both
eyes (in this case the cat) goes to
the left hemisphere. Note that by
this partial crossing-over of fibres in
the optic chiasm, the effect is that
the two sides of the brain deal with
opposite sides of the world, not with
opposite eyes.