Once your spinal cord is cut and your head is severed you will continue to experience the full spectrum of pain, without the heavenly numb of shock-absorbing chemicals, which are back there with your body. You can’t talk, of course, but you can move your lips and appear to scream, and you can focus and blink your eyes, as proved by dozens of deathhouse deals.
A severed head is conscious, and in some ways hyperconscious. The head knows it's been picked up by the hair and shown to the crowd. The head sees the crowd, hears the crowd, smells the breath of the executioner, thinks happy thoughts, cannot believe how long 40 seconds is, because 40 seconds is how long the average head remains fully aware, if not alive. Forty seconds of indescribable pain and horror.