Krista’s and Tatiana’s brains are partly conjoined, allowing them to share nerve signals. This means that any sight seen by the eyes of one twin can be perceived by the other twin’s consciousness.
Canadian twins Tatiana and Krista Hogan are unique. Not only are their heads conjoined down to the neck, but they also, quite exceptionally, share major parts of their brains. The sisters’ sight centers are conjoined, allowing nerve signals to pass between their brains, apparently enabling one twin to perceive what the other one is seeing, even though their eyes are looking in different directions. The twins’ parents recount how a cartoon will sometimes make Tatiana laugh even when it is on the television behind her and only Krista can actually see it.
Tatiana and Krista are extremely rare. The majority of twins who are conjoined by the head share only the skull and some of the blood-circulation system. This type accounts for 6 percent of all conjoined twins, occurring in only one of 2.5 million conjoined-twin births each year. Just under half of those are stillborn, while another 25 percent die within the first 24 hours. But Tatiana and Krista are full of life. They are now 6 years old and are developing in a normal manner.
Neurosurgeon Douglas Cochrane, of BC Children’s Hospital in Vancouver, Canada, has followed the twins since they were born and has examined them several times. When they were 2 years old, Cochrane conducted an experiment in which he covered Krista’s eyes and put electrodes on her head to monitor her portion of the twins’ conjoined sight center. When he flashed a light in front of Tatiana, the electrodes on her sister’s head indicated a strong reaction, suggesting that Krista’s sight center had been alerted to what Tatiana’s eyes saw.
He never published the results, but Cochrane is convinced that the girls not only share visual experiences, but other senses as well. This is anecdotally confirmed by many people who have spent time with the girls. Their mother remembers how placing a pacifier in the mouth of one twin could make them both calm down when they were younger, while some of their doctors say that both girls would cry even when only one had had a blood sample taken.
Linked brains and shared senses
Brain scans show that the girls’ brains are conjoined in the central part of the occipital and parietal lobes, which control visual perception and play a vital role in the coordination of impressions from all of the senses, respectively. But the other parts of their brains, including the frontal lobe, which harbors our thoughts, opinions and decisions, are separated, and that accords neatly with the fact that the girls have very different personalities. Their mother says that Tatiana has a sunny disposition, whereas Krista is a hothead who flies into a temper when she is unhappy, often hitting or scratching her twin sister.
Cochrane thinks that the sisters’ shared sensory impressions are not due to their conjoined occipital and parietal lobes, but are the result of a phenomenon called the thalamic bridge. The thalamus is a walnut-size structure buried deep inside the central part of the brain. It functions like a kind of switchboard, sending nerve impulses from the body’s sensory cells to the proper places in the cerebral cortex. For example, when light falls on our retinas, nerve signals are generated and sent to the sight center in the occipital lobe, but before they can get that far, they must first go past the thalamus. This is also true of most of our other senses (all except the sense of smell), as well as for many of the nerve impulses that flow to and from our muscles.
Although Tatiana and Krista have different thalami, Cochrane thinks that they are linked via the thalamic bridge. This means that all of the sensory impressions that one twin experiences will flow through her thalamus and into her sister’s, before continuing on to her centers of sight, hearing and touch. Thus, one sister can see, hear and feel everything that the other one does.
But the thalamus is also involved in our consciousness: the part of the brain’s functions that scientists know the least about. They hope that conjoined twins can help them understand how the brain is able to process the vast amount of information with which it is constantly bombarded in order to identify and relate which pieces of that information are the most important. It is this ability that allows us to recognize ourselves relative to the surrounding world and function as conscious human beings.
Unique test subjects
Todd Feinberg from New York’s Albert Einstein College of Medicine is intrigued by conjoined twins. He has studied the consciousness of patients in which the connection between the two cerebral hemispheres has been severed, meaning that the sensory impressions or actions controlled by one of the brain’s hemispheres can’t be communicated via nerve signals to the other hemisphere, which is sometimes necessary in order to embed the sensory impressions or actions in the patient’s consciousness. Such a condition causes a disconnect within the person’s perceptivity: For example, one hand may do something that the patient is not aware of or does not want it to do. But with Tatiana and Krista, the situation is reversed: One twin can apparently become aware of something that only her sister, not herself, has experienced.
By studying the girls, scientists hope to find out how one of them experiences the sensory impressions she receives from the other’s eyes, ears or fingers. If Tatiana hugs a teddy bear, will Krista then also experience the feeling of comfort that the hug would have triggered if she had done it herself? Scientists haven’t yet been able to carry out all of the experiments that they would like to. The scanners used for these types of studies are not designed to accommodate conjoined twins, and the girls have been too young to fully participate, because they have been unable to speak or understand what to do.
But now the twins have started school, and they are looking forward to learning new things. And scientists are also looking forward to the twins being able to teach them new things about the human brain and consciousness.
How conjoined twins develop
An egg that is fertilized by a sperm cell begins to develop into an early embryo. For reasons that are still unknown, the embryo sometimes starts to divide in two within the first 15 days; both of these halves will try to develop into a baby. If the division starts before 12 days have passed, it often continues until the two embryos have separated into identical twins.
1. Fission theory
If the division begins after 12 days have passed, it may be incomplete: The two embryos never fully separate, but still continue to develop, resulting in conjoined twins.
2. Fusion theory
Presented in 2000, this theory holds that conjoined twins occur when two identical twins who were initially completely separated are somehow joined back together.