Surprisingly, it turns out not to be always the case. It seems that these Siamese twins are sufficiently joined to share each other sensations, but remain separate persons:
"[...]Their brain images reveal what looks like an attenuated line stretching between the two organs, a piece of anatomy their neurosurgeon, Douglas Cochrane of British Columbia Children’s Hospital, has called a thalamic bridge, because he believes it links the thalamus of one girl to the thalamus of her sister. The thalamus is a kind of switchboard, a two-lobed organ that filters most sensory input and has long been thought to be essential in the neural loops that create consciousness. Because the thalamus functions as a relay station, the girls’ doctors believe it is entirely possible that the sensory input that one girl receives could somehow cross that bridge into the brain of the other. One girl drinks, another girl feels it. [...]"
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Surprisingly, it turns out not to be always the case. It seems that these Siamese twins are sufficiently joined to share each other sensations, but remain separate persons:
http://www.nytimes.com/2011/05/29/magazine/could-conjoined-twins-share-a-mind.html?nl=todaysheadlines&emc=tha210&pagewanted=all
"[...]Their brain images reveal what looks like an attenuated line stretching between the two organs, a piece of anatomy their neurosurgeon, Douglas Cochrane of British Columbia Children’s Hospital, has called a thalamic bridge, because he believes it links the thalamus of one girl to the thalamus of her sister. The thalamus is a kind of switchboard, a two-lobed organ that filters most sensory input and has long been thought to be essential in the neural loops that create consciousness. Because the thalamus functions as a relay station, the girls’ doctors believe it is entirely possible that the sensory input that one girl receives could somehow cross that bridge into the brain of the other. One girl drinks, another girl feels it. [...]"