There was a posting a while back in Wiley Life Sciences Blog (here) about conjointed twins that share some parts of the brain, a unique condition. It is from the NYT original article by Susan Dominue and worth a read and a look at the video.
From the standpoint of understanding consciousness it has some interest. Anatomically the twins have fused skulls. Brain images reveal a connecting line between the two brains, a thalamic bridge. The thalamus is involved in sensory input, levels of brain activation and consciousness. There is a possibility that sensory input to one twin can cross over to the other twins. One girl drinks, another girl feels it. And yet despite a level of shared awareness, they appear to by very separate people with no real shared ‘self’.
It is a little more complicated because they have unusually short corpus callosum (the large tract of nerves connecting the left and right hemispheres) and the hemispheres are not symmetrical. One twin has an unusually small left hemisphere and the other an unusually small right one. This is a little like partially split brain individuals each with a different strengths and weaknesses that complement each other, being somewhat connected. So it is difficult to know what effect this has on each twin’s mental processing as well as their shared processing.
Sometimes they appear to be very separate, doing their own thing and ignoring the other. At other times they appear to be close to a single individual with a very complex body. It will be interesting to hear how they describe their lives when they are a little older and better able to make such descriptions.