Processes in a computer can create input/output between each other. If the brain is just another program, why couldn't some other program running inside the same computer send it inputs?
But the brain exists in a certain state at any given time, and this state theoretically can be uploaded (from some individual person) to a computer simulating a brain so that we don't always have to "grow" the computer brain.
Your brain wouldn't be worth much without your body. You'd be surprised how much your body does. Think of typing for example. You couldn't do it without the exact nerve/muscle pathways in your hands. Hook up robot hands (even crudely modelled after your existing nervous/muscle/bone system in your current hands) and you'd basically have to relearn to do anything. You'd be a baby again.
I think you'd be surprised how much this applies to. Speech, even things like counting we rely on our bodies for.
That's the opposite of my understanding; Sandberg and Bostrom discusses this and at least as far as compute resources go, simulating the brain is by far the harder problem.
That may be true of the body, although our nervous system in our body is quite large, not to mention the spinal chord. But it's certainly not true of the developmental environment. For one thing, simulating that would require simulated thousands of other human brains.
