Consciousness is like the top window, with other functions running in the background. While I select my words I am also aware of my aching muscles from an earlier workout and of rising tension as the hour advances and time begins to run out on this task. With all that and more going on in the periphery of consciousness, my brain is also regulating my heart rate, blood pressure, rate of respiration and other bodily functions. In Dissociative Identity Disorder, the personalities are taking turns occupying consciousness, but they are all simultaneously present and processing experience. And while only one window is on top, sometimes fragments of the other windows can be seen around the edges.
Many forces determine which personality commandeers consciousness for the moment.
If I drift from my task of writing, my screen saver will suddenly obscure this text. And even if I am intently persistent in my task, AOL will preempt the screen if it ever actually connects. Similarly, an alternate personality might take over when the attention of the personality occupying consciousness begins to drift or when an alter develops an urgent agenda that demands conscious attention. DID is an extreme version of this fragmentation.