Rethinking Consciousness: A Scientific Theory of Subjective Experience

by Michael S.A. Graziano

All vertebrates have a tectum, but no other animals do, at least as far as we know. We can make a good guess that around half a billion years ago, a species of small, jawless fish, the common ancestor of vertebrates, evolved a tectum, and all its descendants inherited that brain part from it.1

In a frog, the tectum takes in visual information and sorts the world into a literal map.

The frog’s tectum is not limited to vision. It also collects information from the ears and from touch receptors across the skin.4 A map of the frog’s body surface, of the auditory space around the animal, and of visual space converge and are partly integrated in the tectum. It’s the highest level of integration in the amphibian brain—

One of the most thrilling pastimes of a neuroscientist is listening in on the clicking of an individual neuron and wondering what role it plays in the brain.

When most people think about the word attention, they often mean overt attention. In that colloquial sense of the word, what you are looking at is what you are attending to. If you look away from an object, or turn your back on it, you’re not attending to it.

THERE IS NO point having attention, any kind of attention, whether overt or covert, if you can’t control it. But control is not an easy engineering problem. You need to closely monitor the thing you are controlling. For the first time in this evolutionary story, we will encounter not just cells that can process information, and not just animals that can direct attention, but brain systems that construct an attention schema—a bundle of information, called an internal model, that monitors attention. Our evolutionary story is drawing close to something that resembles consciousness—close, but not quite there yet.

An attention schema is a bundle of information that describes attention—not the object being attended, but attention itself. It monitors the state of attention, keeps track of how it can change dynamically from state to state, and predicts how it may change in the next few moments.

It’s a machine honed to sift a vast amount of information and winnow it down to a small subset. That subset is processed in a deep, thorough manner and ultimately guides behavior. Because of that constant winnowing process, the cortex is, fundamentally, an attention machine.

The information then leaves the eye along the optic nerve, a cable of about 1.5 million fibers, and reaches the thalamus at the base of the brain. The incoming visual information targets one specific part of the thalamus, a bump on its side called the lateral geniculate nucleus (Latin for, literally, “the nucleus sticking out the side that looks like somebody’s knee”). Once again, the information is passed through a competitive sieve in the thalamus.