Reality

I open my eyes and a world appears. I’m sitting on a plastic chair on the deck of a tumbledown wooden house, high in a cypress forest a few miles north of Santa Cruz, Calif. It’s early morning. Looking straight out, I can see tall trees still wreathed in the cool ocean fog that rolls in every night, sending the temperature plummeting. I can’t see the ground, so the deck and the trees all seem to be floating together with me in the mist. There are some other plastic chairs, a table, and a tray arranged with coffee and bread. I can hear birdsong, some rustling around in the back — the people I’m staying with — and a distant murmur from something I can’t identify. Not every morning is like this; this is a good morning. I have to persuade myself, not for the first time, that this extraordinary world is a construction of my brain, a kind of “controlled hallucination.”

Whenever we are conscious, we are conscious of something, or of many things. These are the contents of consciousness. To understand how they come about, and what I mean by controlled hallucination, let’s change our perspective. Imagine for a moment that you are a brain.

Really try to think about what it’s like up there, sealed inside the bony vault of the skull, trying to figure out what’s out there in the world. There’s no light, no sound, no anything — it’s completely dark and utterly silent. When it forms perceptions, all the brain has to go on is a constant barrage of electrical signals that are only indirectly related to things out in the world, whatever they may be. These sensory inputs don’t come with labels attached (“I’m from a cup of coffee.” “I’m from a tree.”). They don’t even arrive with labels announcing their modality, whether they are visual or auditory or sensations of touch, temperature, or proprioception (the sense of body position).

How does the brain transform these inherently ambiguous sensory signals into a coherent perceptual world full of objects, people, and places? The essential idea is that the brain is a prediction machine, so that what we see, hear, and feel is nothing more than the brain’s best guess of the causes of its sensory inputs. Following this idea all the way through, we will see that the contents of consciousness are a kind of waking dream — a controlled hallucination — that is both more than, and less than, whatever the real world really is.

Here’s a common-sense view of perception. Let’s call it the “how things seem” view.

In this view, there’s a mind-independent reality out there, full of objects and people and places that have properties like color, shape, texture, and so on. Our senses act as transparent windows onto this world, detecting these objects and their features and conveying this information to the brain, whereupon complex neuronal processes read it out to form perceptions. A red coffee cup out there in the world leads to a perception of a red coffee cup generated within the brain. As to who or what is doing the perceiving — well, that’s the “self,” isn’t it?, the “I behind the eyes,” one might say, the recipient of wave upon wave of sensory data, which uses its perceptual readouts to guide behavior, to decide what to do next. There’s a cup of coffee over there. I perceive it and I pick it up. I sense, I think, and then I act.

This is an appealing description. Patterns of thinking established over decades, maybe centuries, have accustomed us to the idea that the brain is some kind of computer perched inside the skull, processing sensory information to build an inner picture of the outside world for the benefit of the self. This picture is so familiar that it can be difficult to conceive of any reasonable alternative. Indeed, many neuroscientists and psychologists still think about perception in this way, as a process in which the brain works from the “bottom up” to discern features of things in the world.

Here’s how the bottom-up picture is supposed to work: Stimuli from the world — light waves, sound waves, molecules conveying tastes and smells, and so on — impinge on sensory organs and cause electrical impulses to flow “upwards” or “inwards” into the brain. These sensory signals pass through several distinct processing stages, and at each stage the brain picks out increasingly complex features. Let’s take vision as an example. At first the brain might detect features like luminance or edges, and later it might detect the parts of discrete objects — such as eyes and ears, or wheels and side-view mirrors. Still later stages of this processing system would respond to whole objects, or object categories, like faces and cars.

In this way, the external world with its objects and people and all sorts of everything becomes recapitulated in a series of features extracted from the river of sensory data flowing into the brain. Signals flowing in the opposite direction — from the “top down” or the “inside out,” serve to refine or otherwise constrain the bottom-up flow of sensory information.

This bottom-up view of perception fits well with what we know about the anatomy of the brain, at least at first glance. Perceptual systems of all modalities are organized in the brain as hierarchies. In the visual system, for example, the primary visual cortex of the brain is close to sensory inputs, while the parietal and frontal cortices, where later stages of processing are believed to occur, are further away.

Studies of brain activity also seem friendly to this bottom-up view. Experiments going back decades — investigating the visual systems of cats and monkeys — have repeatedly shown that neurons at early stages of visual processing respond to simple features like edges, while neurons at later stages respond to complex features like faces. More recent experiments using methods like functional magnetic resonance imaging have revealed much the same thing in human brains.

You can even build artificial “perceiving systems” this way. Machine vision systems based on artificial neural networks are nowadays achieving impressive performance levels, in some situations comparable to those of humans. These systems, too, are frequently based on bottom-up theories.

With all these points in its favor, the bottom-up “how things seem” view of perception seems to be on pretty solid ground.


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