14 June 2011
A recent NYTimes article (sent to me by fellow grad Chris Kanan from computer science) details the very special case of conjoined twins who not only share parts of their head, but actually share parts of their brain. They share more than just supporting structures, such as ventricles--their neurologist claims that a band of grey matter (that he's labeled a "thalamic bridge") connects the thalamus of each twin.
The thalamus is classically characterized as a sensory "relay-station"; all senses (other than smell) are routed from the sensory receptors and through the thalamus on it's way to the cerebral cortex. It is thought that this "thalamic bridge" is a way for these conjoined twins to share conscious sensory experience.
For example (as can be seen in the video embedded in the NYTimes article): if one twin's eyes are covered while an object is presented to the other, when the twin with covered eyes is asked what she sees, she identifies the correct object. Similar tests are shown for feeling the location of a tickle; one twin with covered eyes can locate the spatial position of a tickle on the other twin's body.
Other possible explanations: http://6thfloor.blogs.nytimes.com/2011/05/31/the-twins-connection-other-theories/
While no definitive studies have been done, no publications have been completed, and no MRI scans have been made publicly available (other than the one linked to above), these reports and images are suggested young twins stand to inform us about the amazing flexibility and workings of the human brain.