Boundaries of the I-Function in Twins
Identical, conjoined, and half-twins are all examples of intrinsic variability in humans. Intrinsic variability exists in all animals and is an adaptive mechanism built into the nervous system in response to input. This mechanism allows humans to distinguish the same inputs as different from one another and therefore, the possible outputs vary with time. It is possible that due to identical genetic input, the twins could share identical neural pathways and identical I-Functions. This hypothesis could explain the identical behaviors and inter-connectedness of feelings and thoughts that twins share. Differences that are seen in twin behaviors could simply be due to intrinsic variability causing differing output or behaviors.
During ovulation, when a woman releases an ovum, three different processes can occur. First, the ovum can remain in one piece all the way to the uterus where, if not fertilized, it will be shed out of the body along with the unused endometrium. Second, the ovum, if fertilized, can develop into a single embryo, which is the most common type of pregnancy in humans with about 99% of all births being singletons (1) . Finally, the ovum can split into two separate halves resulting in genetically identical twins. The three types of twins previously mentioned are identical, which are created when the fertilized ovum separates into two complete, identical parts, conjoined, which occur as a result of a fertilized ovum not completing the equal separation resulting in two fetuses fused together in some way, and half-twins or polar body twins, which are made when an unfertilized ovum splits into two complete, identical parts and is then fertilized by two different sperm. The frequency of identical twins is 3.5 per 100 births (1) and the frequency of conjoined twins is 1 birth per 50,000 with only 100 known cases surviving their first year (5) .
There is no scientific explanation for why an ovum splits into two or more parts. It has been observed that ovum splitting can be hereditary but can also occur spontaneously. This spontaneity or unexplained divergence from normal egg growth and fertilization could be due to intrinsic variability. This intrinsic variability could also be connected to certain differences in behavior of two genetically and culturally identical twins. Ideally, one could say that twins with identical genetic information and environments would have extremely similar, if not the same neural pathways and central pattern generators (CPGs). CPGs are innate responsive mechanisms that induce different types of outputs without the need for any input. Since it is hypothesized that within the neural pathways and CPGs lies the I-Function, it could be deduced that the I-Functions of both of the twins would be identical. This could account for the shared experiences, feelings, dreams, traumas, and thoughts between twins that are often studied.
The I-Function of a person is present from the beginning of a baby's life. It is thought that humans are born with a predisposition to see the world in a particular way and that the I-Function is involved in the idea of "choice." Infants firmly establish an image of themselves at around 18-24 months (6) . This sets up the idea of mental and physical boundaries for them in their minds and bodies. Boundaries are the "physical, emotional, cognitive, and spiritual edge of a person (6) . They help individuals distinguish between "self" and "non-self." The I-Function and sense of self are usually thought of together and are very important to an individual's identification and definition of themselves.
The shared experiences, dreams, traumas, feelings, and thoughts between genetically identical twins are described by some as mere coincidence. Others believe that there may be something concrete in the "twin connection". Similar or identical neural connections and I-Functions could explain this coordination of behaviors. Many twins report the ability to "communicate" using their "own" languages, many times without using any words or cues at all. They also report feelings of pain or anguish and "unspoken transmissions of anxiety and fear" when their twin is experiencing pain or discomfort (3) . Nathaniel from Ohio says that during a baseball game when he got hit in the back of the neck with a foul ball, his twin brother, Philip, felt a pain in the back of his neck as well (2) . Dan from California tells a story of shared pain with his brother, "Once I broke my leg at school and when I fell over, I looked at my watch...it was 12:24 p.m. On the other side of the playground I heard someone screaming...it was my brother. We both broke our legs at the same time (2) ." Stella from Indiana recalls the night when her twin sister went to a carnival and Stella stayed home. Stella burnt her hand while ironing, "felt a cloud of terror, followed by dizziness and nausea" and she went to the carnival drawn by "something she could not explain." Her sister was stuck at the top of the Ferris wheel. After they were safe, both knew exactly what had happened to the other one without any verbal communication about the incidents (3) . Martha of California tried to sue an airline company for the "pain and suffering that she experienced when her twin sister died thousands of miles away." She describes a "great burning and tearing sensation in her chest and abdomen at the exact moment of the collision... She felt as if she had been cut in two (3) ." Scott from New York talks about getting his twin brother back for being aggressive in ice hockey, "I whacked him in his arm with my stick, whack him on his left bicep...Pretty soon, Charlie's arm is all swollen, he has this big bruise on it...The next morning, I wake up with a sore arm, same arm, and I can't move it for like three days (4) ." Royce from Indiana says, "[O]nce when we were thirteen, we were playing baseball and I got hit on the head really hard with the ball and a few seconds later, [Brett's] nose started to bleed (4) ." These shared experiences are all very similar because non of these twins are conjoined, yet all feel a physical discomfort simultaneously or right after their identical twin experiences pain. This awareness of pain or activation of the I-Function is sometimes acknowledged by both sets of twins, many times simultaneously, suggesting one I-function working in two separate nervous systems. Intrinsic variability could account for the many differing and separate experiences and behaviors the twins demonstrate that do not activate the corresponding sibling's I-function.
A special set of conjoined twins, Brittany and Abigail Hensel, with a condition known as dicephalus, not only share the same genetic information but also an undivided torso, two arms, two legs, reproductive organs, and three lungs. Each has her own heart, spine, and head. The sensations and movement control of the limbs and trunk of the body are controlled by the corresponding twin on either side. If the twins were to be tickled on only one side of the body, only the twin controlling that side would laugh. Amazingly enough, Britty and Abby manage to move as a collective unit coordinating their body in complicated ways to successfully swim, bike, and run (5) . Emotionally, they are very supportive and understanding of one another. When Life magazine asked the girls to tell the world something about themselves they were very strong-minded, "[I]'m not going to be separated," Britty declares, while Abby pipes in, "And I don't have two heads (5) ." This sense of sameness and unity that the girls possess suggests a unified "self" or a singular I-Function. The Hensel twins are the most clear and obvious example of twins sharing experiences, behaviors, and thoughts leading to the support of one connected I-Function in twins.
The topic of the I-Function is still under debate by scientists, skeptics, and many neurobiology students. There is no definitive place that it resides in the nervous system and there are no answers as to why it can be "turned on" or "turned off" for no apparent reason. It seems that as it is one of the boundaries by which humans define where the conscious "self" and unconscious "self" meet, there is no justifiable definitive location or definition for it. Twins sharing the same genetic information, identical, conjoined, and even sometimes half-identical, seem to be one step closer in figuring out what the I-Function really is. Evidence suggests that they share parts of or the same I-Function and any deviation separating the I-functions of the two is due to intrinsic variability. Maybe, through more research on dicephalic conjoined twins like the Hensels, we will be able to put to rest all of the unanswered questions about the I-Function.
1 Basics of Twinning 
2 Personal Twin Stories 
5 Hensel Twins 
6 Understanding Body Image 
Personal Stories 
Nightmares and Twins 
Two of a Kind 
05/21/2005, from a Reader on the Web
Very interesting short article by Beth Varadian regarding the I-Function in twins. Though I find the concept of the I-Function in twins legitimate and credible, I would like to put forward the idea that in the particular case of the Hensel twins Abigail and Brittany, their statements are actually not the result of some transient sense of collective self from a "single twin" so to speak and the example does not apply. By throwing into their sentences some clarifying remarks and making them more verbose I think it will show what I believe is the more accurate interpretation of their quotes which are actually distinct statements about their very independence from each other: "[I]'m not going to be separated, [from my sister]..." (not "my other half") Britty declares "...as it would cause both our deaths, and we are not "freaks" in need of separation anyways." "And I [Abigail] (not Abigail and her sister collectively) don't have two heads. This is my head and the "other" head does not belong to me, it belongs to my sister Brittany. I caught a news piece about them several years ago on a show like 20/20 and just recently the thought came to me randomly "I wonder if they're still alive," so I researched con-joined twins on the internet. There's actually a fair amount of information available about them and after reading some on them I found your article. Just a thought. Thank you,