[In the case of a split brain patient,] “[w]hat is flashed to the right half of the visual field, or felt
unseen by the right hand, can be reported
verbally. What is flashed to the left half of the visual field ... cannot be reported, though if the word ‘hat’ is
flashed on the left, the left hand will retrieve a hair from a group of concealed object if the person is told to
pick out what he has seen. At the same time, he will insist verbally that he saw nothing. ... if two different
words
are flashed to the two half fields (e.g., ‘pencil’ and ‘toothbrush’) and the individual is told to retrieve the
corresponding object from beneath a screen, with both hands, then the hands will search the collection of objects
independently, the right hand picking up the pencil and discarding it while the left hand searches for it, and the
left hand similarly rejecting the toothbrush which the right hand lights upon with satisfaction."
- Nagel, 'Brain bisection and the unity of consciousness'
“Suppose first that I am one of a pair of identical twins, and that both
my body and my twin’s brain have been fatally injured. Because of
advances in neuro-surgery, it is not inevitable that these injuries will
cause us both to die. We have between us one healthy brain and one
healthy body. Surgeons can put these together.
If all of my brain continues both to exist and to be the brain of one
living person, who is psychologically continuous with me, I continue to
exist. This is true whatever happens to the rest of my body. ...
It is in fact true that one hemisphere is enough. There are many people
who have survived, when a stroke or injury puts out of action one of
their hemispheres. With his remaining hemisphere, such a person may need
to re-learn certain things, such as adult speech, or how to control both
hands. But this is possible. ... [So] I would survive if my brain was
successfully transplanted into my twin’s body. And I could survive with
only half my brain, the other half having been destroyed. Given these
two facts, it seems clear that I would survive if half my brain was
successfully transplanted into my twin’s body, and the other half was
destroyed.”
- Parfit, Reasons and Persons
“My Division. My body is fatally injured, as are the brains of my
two brothers. My brain is divided, and each half is successfully
transplanted into the body of one of my brothers. Each of the resulting
people believes that he is me, seems to remember living my life, has my
character, and is in every other way psychologically continuous with me.
And he has a body that is very like mine.”
- Parfit, Reasons and Persons
“At the near end of this spectrum is the normal case in which a future
person would be fully continuous with me as I am now, both physically
and psychologically. This person would be me in just the way that, in my
actual life, it will be me who wakes up tomorrow. At the far end of this
spectrum the resulting person would have no continuity with me as I am
now, either physically or psychologically. In this case the scientists
would destroy my brain and body, and then create, out of new organic
matter, a perfect Replica of someone else. Let us suppose this person to
be Greta Garbo. We can suppose that, when Garbo was 30, a group of
scientists recorded the states of all the cells in her brain and
body.
[One] assumes that, in each of these cases, the resulting person either
would or would not be me. This is not so. The resulting person would be
me in the first few cases. In the last case he would not be me. In many
of the intervening cases, neither answer would be true. I can always
ask, ‘Am I about to die? Will there be some person living who will be
me?’ But, in the cases in the middle of this Spectrum, there is no
answer to this question.”
- Parfit, Reasons and Persons