Cortical top-down processing is the brain function that simplifies and makes possible our ability to perform the complexities of moment-to-moment living. It would be too unwieldy for the brain to analyze the whole host of information from the senses and body nerves anew and connect it with our motor abilities, along with our intention, every moment. You wouldn’t be able to lift a spoon to your mouth, never mind balance a strawberry on it, or encompass your intention, or have a cohered sense of “you.” The cortex establishes a way to do all of this, with as little an expenditure of time, effort, and attention as possible. The cortex creates symbolic form by linking the brain-mappings of huge amounts or information. Cortical top-down processing operates from the highest levels of symbolic form.
To demonstrate cortical top-down processing, let us look at the phenomenon of phantom limbs. After an arm is amputated, the amputee may continue to see, feel, and have pain in the arm that is no longer there. How does this happen? Developmentally, as a baby, the amputee had organized and integrated the sensory and movement experience of his arm by establishing mappings in his cortex. This had established a sensory and motor neuronal mapping of the arm. Thereafter, sensory and motor input from the arms link to these existing cortical arm maps. My image of my arm, my feeling of my arm, and my experience of my arm come from this cortical mapping. The direct sensory input from my peripheral arm nerves triggers my top-down cortical arm. My actual experience of my arm does not come from a direct peripheral nerve processing of my arm. This is ordinary top-down processing—the way the brain is organized. However, when an arm is amputated, the previously established cortical mapping is not amputated. It continues to exist timelessly in the cortex. This cortical mapping continues to project the presence of the arm, even after the actual arm itself and its peripheral nerves are gone. Consequently, the amputee feels (and often literally sees) his missing arm that is clearly a pure illusion of the brain. The phenomenon of a phantom limb is a pure enactment of top-down cortical processing.