Vancouver- Twins Tatiana and Krista Hogan were a medical marvel when they were born sharing a skull. Now, seven weeks later, doctors say they have discovered a baffling "bridge" of tissue connecting the girls' brains, raising the spectre that they can transmit certain brain signals to one another.

Recent scans show that part of the infants' upper brain stems are connected by a corridor of tissue, doctors told reporters yesterday at the British Columbia Women's Hospital and Health Centre.

However, pediatric neurosurgeon Doug Cochrane said it could take months for doctors to determine what kind of information is carried along this brain bridge and what it means for the girls' future.

"One of our tasks in the next few months will be to define what information is communicated from one twin to the other and vice versa," Dr. Cochrane said.

The conjoined twins are growing like weeds -- each weighing exactly 6.6 pounds -- and have already developed distinct personalities.

Krista cries more than Tatiana and demands more attention from her mother and nurses at the health centre, where the tiny pair have lived since they were delivered by cesarean section seven weeks ago.

The conjoined girls have distinct brains as well; each has her own frontal lobe, lower brain stem and cerebellum.

But with this new "bridge" discovery, they are more intertwined than originally thought.

For example, blood that originates in one girl's brain, travels through the other's, and vice versa.

The new findings cast doubt on whether the twins, who are anatomic mirrors of one another, can ever be successfully separated.

Dr. Cochrane used the analogy of a traffic maze to describe the web of connected brain tissue shared by the girls.

"It [the connecting tissue] is sort of the No. 1 highway that brings information to the surface of the brain, then delivers it down through the more basic functions and through the spinal cord," Dr. Cochrane said.

"So it's likely that there's important wiring, so to speak, in that bridge," he added.

The neurosurgeon said doctors still don't know what kind of brain signals travel along this corridor. Right now, the infants, who were born two months prematurely on Oct. 25, are at the same developmental stage as a newborn.

"One of our tasks in the next few months will be to define what information is communicated from one twin to the other and vice versa," Dr. Cochrane said. "I'm not sure I know how to interpret it, as yet," he said, adding there is no medical precedent for this kind of brain fusion.

The twins are scheduled to be flown home to Vernon, B.C., today with their mother, Felicia Simms, and a team of medical professionals.

The daily cost of caring for the twins over the seven weeks has been $1,000 a baby. The tab, billed to British Columbia's Medical Services Plan, is a basic one and doesn't include the cost of diagnostic tests.

The girls have thrived in their first weeks of life, and tests show their hearing and vision are normal. Both infants are being bottle-fed their mother's breast milk.

Despite their relatively good health, Ms. Simms, 21, faces a monumental task in caring for her girls, the doctors said. Feeding, bathing and even carrying the infants is a job for two, said Brian Lupton, clinical director of the health centre's neonatology unit.

A specially designed car seat was donated to the family.

During Ms. Simms' pregnancy, there was some public speculation about how the young woman would cope with the stress and work involved in raising conjoined twins.

Ms. Simms has two other young children, ages 2 and 3, with her partner, Brendan Hogan. Ms. Simms's mother and sister have pledged to help care for the twins.

"Felicia is doing very well," Dr. Lupton said. "She is an experienced mother and it shows," he said, noting that Ms. Simms spends nights in her daughters' hospital room.

Dr. Cochrane said he was surprised at the emotional impact the girls' birth has had on him.

"I've come to know a family who is absolutely committed and absolutely strong. They are dedicated to these children. . . . That, from my perspective, has been the principal learning experience.

"I find the definition of anatomy and function all very interesting . . ., but I think the more important thing is the rejoicing of this family."

As for the future, Dr. Cochrane said it's still too early to say if separation is medically possible, especially with the discovery of the brain bridge.

"It would raise the question of knowingly creating an injury to the twins as a result of separating. . . . We're trying to understand the traffic in that particular nerve bundle."

The girls are to return to Vancouver for more tests in the spring.

The twins defied the odds simply by being born. Even though they were delivered early to avoid late-term complications, the two still had only a one-in-four chance of surviving for 24 hours.