Consciousness and Psychopathologywithout having any conscious memory of the previous learning episodes. A “double dissocia-
tion” is found when A and B can each function independently of the other.
It is worth mentioning that neither the dualist nor the materialist can easily explain what hap-
pens in DID. For example, if we take DID to show that there can be more than one conscious
mind associated with a body, then, for example, a substance dualist would be even more hard
pressed to explain how more than one nonphysical mind comes to be causally connected to a
single body. On the other hand, materialists have the difficulty of explaining how two people, often
exhibiting very different personalities and behaviors, can co-exist within a single brain.^2
Another intriguing abnormality of self and consciousness comes from patients who have
undergone a commissurotomy or a “brain bisection” operation (Nagel 1971; Sperry 1984). They
were performed decades ago as a last resort to relieve the seizure symptoms of severe epilepsy.
During the procedure, the nerve fibers connecting the two brain hemispheres (the corpus callo-
sum) are cut, resulting in so-called “split-brain” patients. So “split-brain cases” are those patients
where severing the corpus callosum blocks the inter-hemispheric transfer of, for example, per-
ceptual and motor information. The human retina functions in such a way that the left half
of each retina is primarily connected to the left hemisphere of the brain and the right half of
each retina is primarily hooked up to the right hemisphere of the brain. The visual system takes
information from the left visual field of both eyes to the right hemisphere and information from
the right visual field of both eyes to the left hemisphere. Given the crossing over of fibers in the
optic chiasm, the effect is that the two sides of the brain reflect opposite sides of the outer world.
In some laboratory conditions, these patients seem to behave as though two “centers of
consciousness” are present, each associated with one of the two cerebral hemispheres. Some
puzzling results were found in controlled laboratory presentations where patients were forced
to look straight ahead without being able to move their heads as usual. Different stimuli were
shown to patients’ left and right visual fields, revealing striking dissociations of consciousness and
behavior. Their left hands literally do not know what their right hands are doing. Shown one
stimulus on the left and a different one on the right, each hand will respond appropriately to its
respective stimulus but not to the other. Moreover, patients typically showed verbal knowledge
only of the stimuli shown to their right field and not to those in their left field, even though
their left hand is able to respond appropriately to the left field stimuli. Given the usual location
of primary language function to the left hemisphere, the right hemisphere, which receives visual
input from the left visual field, remains unable to verbally describe what it sees. During an object
recognition task, a subject might report seeing a bottle, due to the left hemisphere, while his
left hand (controlled by his right hemisphere) was searching to find a hammer from a group of
objects. Patients would report seeing one thing but some of their bodily behavior would indi-
cate that they saw something else.
Split-brain cases often have more to do with “synchronic identity,” that is, how many selves
are present at a given time, as opposed to DID where “diachronic identity” is the main issue, that
is, how many selves are present over time. Nonetheless, even for split-brain cases, Bayne (2010)
for example argues for a “rapid-switching model,” in which selves alternate in existence. So only
one self exists at a given time although which one exists may switch rapidly and repeatedly over
short intervals of time. The rapid-switching hypothesis seems to have the advantage of a kind of
“unity thesis,” such that all the experiences had by a given human at a given time would remain
phenomenally unified.^3
Somatoparaphrenia is a very strange “depersonalization disorder” and body delusion where
one denies ownership of a limb or an entire side of one’s body. Anosognosia is a related condi-
tion, in which a person who suffers from a disability seems unaware of the existence of the dis-
ability. A person whose limbs are paralyzed will insist that his limbs are moving and will become