Your brain is currently expending about a fifth of your body’s energy, and almost none of that is being used for what you’re doing right now. Reading these words, feeling the weight of your body in a chair—all of this together barely changes the rate at which your brain consumes energy, perhaps by as little as 1 percent.
The other 99 percent is used on the activity the brain generates on its own: neurons (nerve cells) firing and signaling to each other regardless of whether you’re thinking hard, watching television, dreaming, or simply closing your eyes.
Even in the brain areas dedicated to vision, the visuals coming in through your eyes shape the activity of your neurons less than this internal ongoing action.
In a paper recently published in Psychological Review, we argue that our imagination sculpts the images we see in our mind’s eye by carving into this background brain activity. In fact, imagination may have more to do with the brain activity it silences than with the activity it creates.
Imagining as Seeing in Reverse
Consider how “seeing” is understood to work. Light enters the eyes and sparks neural signals. These travel through a sequence of brain regions dedicated to vision, each building on the work of the last.
The earliest regions pick out simple features such as edges and lines. The next combine those into shapes. The ones after that recognize objects, and those at the top of the sequence assemble whole faces and scenes.
Neuroscientists call this “feedforward activity”—the gradual transformation of raw light into something you can name, whether it’s a dog, a friend, or both.
In brain science, the standard view is that visual imagination is this original seeing process run in reverse, from within your mind rather than from light entering your eyes.
So, when you hold the face of a friend in mind, you start with an abstract idea of them—a memory or a name, pulled from the filing cabinet of regions that sit beyond the visual system itself.
That idea travels back down through the visual sequence into the early visual areas, which serve as your brain’s workshop where a face would normally be reconstructed from its parts—the curve of a jawline, the specific shade of an eye. These downward signals are called “feedback activity.”
A Signal Through the Static
However, prior research shows this feedback activity doesn’t drive visual neurons to fire in the same way as when you actually see something.
At least in the brain regions early in the vision process, feedback instead modulates brain activity. This means it increases or decreases the activity of the brain cells, reshaping what those neurons are already doing.
Even behind closed eyes, early visual brain areas keep producing shifting patterns of neural activity resembling those the brain uses to process real vision.
Imagination doesn’t need to build a face from scratch. The raw material is already there. In the internal rumblings of your visual areas, fragments of every face you know are drifting through at low volume. Your friend’s face, even now, is passing through in pieces, scattered and unrecognised. What imagining does is hold still the currents that would otherwise carry those pieces away.
All that’s needed is a small, targeted suppression of neurons that are pulled by brain activity in a different direction, and your friend’s face settles out of the noise, like a signal carving its way through static.
Steering the Brain
In mice, artificially switching on as few as 14 neurons in a sensory brain region is enough for the animal to notice it and lick a sugar-water spout in response. This shows how small an intervention in the brain can be while still steering behavior.
While we don’t know how many neurons are needed to steer internal activity into a conscious experience of imagination in humans, growing evidence shows the importance of dampening neural activity.
In our earlier experiments, when people imagined something, the fingerprint it left on their behavior matched suppression of neuronal activity—not firing. Other researchers have since found the same pattern.
Other lines of evidence strengthen our theory, too. About one in 100 people have aphantasia, which means they can’t form mental images at all. One in 30 form these images so vividly they approach the intensity of images we actually see, known as hyperphantasia.
Research has found that people with weaker mental imagery have more excitable early visual areas, where neurons fire more readily on their own. This is consistent with a visual system whose spontaneous patterns are harder to hold in shape.
Taking all this together, the spontaneous activity reshaping hypothesis—our new theory that imagination carves images out of the steady stream of ongoing brain activity—explains why imagination usually feels weaker than sight. It also explains why we rarely lose track of which is which.
Visual perception arrives with a strength and regularity the brain’s own internal patterns don’t match. Imagination works with those patterns rather than against them, reshaping what is already there into something we can almost see. 
This article is republished from The Conversation under a Creative Commons license. Read the original article.
