A phenomenal world derives its structure from the dependencies between protophenomena, which correspond to connections between activity sites; thus the unity of a phenomenal world is a consequence of this connectivity. We see this in split-brain operations (cerebral commissurotomy), wherein severing the corpus collosum causes a split in consciousness: each hemisphere is unconscious of what the other is experiencing (Gregory, 1987, pp. 740-7). However, it is significant that these operations do not completely separate the hemispheres; at very least the brainstem is left intact. Therefore the protophenomena corresponding to the two hemispheres are not completely independent, and so the phenomenal world has separated into two loosely-coupled subworlds.
An analogy may clarify this. A picture is an emergent effect of its individual pixels and their relative positions. If we cut a picture in half, it becomes two pictures, because there is no longer a fixed relations between pixels in one half and those in the other. However, instead of cutting the picture, we may gradually separate it into two parts, pixel by pixel, by stretching and eventually breaking the connections between them. The gradual uncoupling of the pixels in the two halves causes the picture to change gradually from one to two. So also, consciousness is emergent from the individual protophenomena and the dependencies between them. As the neural connections are weakened or broken, the protophenomena in the two subworlds decouple from each other, and the one mind becomes two.
This thought experiment demonstrates that the unity of consciousness is a matter of degree. Indeed, in principle we can measure the unity of consciousness by the tightness of the coupling between its protophenomena, for it is this coupling that gives the phenomenal world its coherence. (The tightness of coupling can, in principle, be calculated from the characteristic patterns; it can be quantified in terms of mutual information.)
One may wonder what sort of coupling is sufficient to unify consciousness. For example, in split-brain patients it has been observed that one hemisphere may communicate with the other through transactions with the external world, for example, twitching the skin on one side of the face so that it can be felt on the other. (The patient is unconscious of doing this.) Can `external transactions' such as these effect the coupling of protophenomena? If so, then our individual phenomenal worlds may not be so independent as we commonly suppose, for any sort of communication couples protophenomena in one mind to those in another. I think the answer is, again, a matter of degree. There is an enormous difference between the bandwidth of the corpus collosum (approximately 800 million nerve fibers) and the narrow bandwidth of most external media. Nevertheless, the interconnection of phenomenal worlds by nonneural physical processes is a thought-provoking possibility.