As a child grows up, the body schema adjusts to the changing body, but its adaptability has limits. A phantom limb is a stark reminder of the errors that the body schema can make. The truth is, we all have phantom limbs superimposed on our real ones. Lord Nelson lost the real one but kept the phantom.

After studying the body schema for many years, I became interested in how the brain models another part of the self. Not a physical part like an arm, but a computational part. If the brain describes the physical body in ghostly, incomplete terms, how much more mystically would it describe a non-physical trait like computation?

One of the most crucial computational processes in the brain is attention. The word “attention” has many colloquial connotations, but in neuroscience it has a specific meaning. Attention is the selective enhancement of some signals over others, such that the brain’s resources are strategically deployed. In some ways attention is like a computational hand—it’s how the brain grasps things.

The brain needs to control its attention, just as it controls the body. To understand how, we can gain some insight from control theory, a well-developed branch of engineering theory that deals in the optimal ways for complex systems to work—whether those systems dictate the airflow in a building, traffic patterns in a city, or a robot arm. In control theory, if a machine is to control something optimally, it needs a working model of whatever it’s controlling. The brain certainly follows this principle in controlling the body. That’s why it computes a body schema. Since the brain can control its attention exquisitely well, it almost certainly has an attention schema, a simulation of its own attention.

But what kind of information is in that attention schema? How would it describe something as hard to pin down as attention? Presumably it would describe abstracted properties, not precise or microscopic details. It might describe attention as a kind of mental grasp, a personal experience, something located inside me that seizes on the things around me and empowers me to react to them. But it wouldn’t contain information about neurons or synapses. It wouldn’t describe the actual physical mechanisms of attention. Just as the body schema is a surreal description of the body, so the attention schema would be a surreal description of attention.

Here’s how a brain with an attention schema might behave. First, it would have a nuanced control of attention. Second, if it had an ability to translate internal information into words, it might make some strange, physically incoherent claims based on that attention schema. It wouldn’t claim, “Well look at that, my cerebral cortex has an attentional enhancement of the visual signal of that sandwich in front of me.” Instead, going off the incomplete information in its attention schema, it might say, “I’ve got a non-physical, subjective experience of that sandwich. You know, the feely thing inside me. Consciousness.” That’s the brain’s caricature of attention. Of course the process is not limited to sandwiches. The same logic applies to consciousness of any object in front of you, consciousness of a memory that you’ve just recalled, or consciousness of yourself as a person.