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Peripheral and Foveal Vision.

Bill Hill

To write software that’s friendly and easy to use, first you have to really understand who’s using it.

We think we’re so civilized, but we’ve only been doing this civilization stuff for about 13,000 years, not nearly enough time for our perception system—the way we interpret input from the world outside ourselves—to have evolved far from where it started. We’re still Homo Sapiens version 1.0. Our "operating system" first shipped about 140,000 years ago, and there’s still no upgrade in sight. HS 1.0 is the most important OS for which you’ll ever write code.

You wouldn’t dream of writing code for Windows® without understanding the Windows event-handling model or APIs, would you? If you want to write code that’s usable by people, you have to understand their event-handling model.

When we feel information overload as we sit in front of our computer display, it’s because the flow of information is not well matched to our perception system. Our primary input comes through our vision. We have about 207 degrees of peripheral vision. We use our ears to monitor sounds, which tells us when to turn our heads to give us 360 degree visual coverage. There’s a high resolution area of our vision, slightly smaller than a sheet of paper, held at normal reading distance. Our peripheral vision uses the whole retina, but this high-resolution area uses only the foveal area—and that’s only about 0.2mm in diameter.

As hunter-gatherers, our brains developed to prioritize inputs. An unexpected sound from behind, for instance, often signals danger. We are incredibly sensitive to sudden movement on the periphery of our vision; it’s a "Priority 0 Interrupt" because to a hunter-gatherer it means either threat—or lunch.

We use our peripheral vision to triage information, and our brain can pattern-match and sort huge amounts of data to give attention only to what’s important. Once we decide to focus in on something, we pick it up or get close enough to use our high-resolution foveal vision.

That’s the key to information overload. Our computer display generally sits right in our foveal vision. There’s some periphery there, but not much. It’s like we took this 64-bit channel, then put an 8-bit nozzle on it—and wonder why we feel increased pressure. As we stream more and more information through that nozzle, the channel gets overloaded. As the computer becomes our window on the world, this gets to be a problem. If the world is so big, why is my window so small?

The lessons are pretty straightforward. Use bigger displays or multiple displays. But whatever the display size, keep what’s going on in the foveal area as stable as possible—unless you REALLY need to get the user’s attention. It’s the Priority 0 Interrupt nature of movement that makes pop-ups or flashing adverts on a Web page so irritating—they’re bound to distract you from reading or any other task. Advertisers know this—that’s exactly what they want.

Our perception system is very sophisticated and highly developed—for basic survival. But we’re really just beginning to use computers as information interfaces. We still have a lot to learn and a long way to go before this way of acquiring information will feel natural. We can help by trying to better match information flow to human perception.

There’s a practical exercise you can do to help understand your own natural perception system. Go out to a park or, even better, a natural area. Relax and try to just look without focusing on anything. After a few minutes, you’ll be amazed at how much visual input you can easily handle. This is a technique my animal tracker friend Jon Young calls "owl vision."

If you want to practice this while moving, you have to combine it with a second technique Jon calls "fox walking" which helps remove unwanted noise from the visual signal. Most of us bounce up and down a little as we walk, which introduces a lot of signal noise and confuses the input. You need to glide along, keeping your eyes on the same plane. Imagine you’re carrying a tray of very full glasses of water which you don’t want to spill. When you’ve practiced enough to get it right, you can leave the tray behind. You’ll get some pretty strange looks at first, until it all becomes natural. You’ll be stunned at how much you’ve never noticed before.

This is your "free laboratory." Once you’ve reset your perception and understand its baseline or natural state, you can use that as a benchmark. And if you think this has nothing to do with software or computers, just remember: you can take the Man out of Nature—but you never take Nature out of Man.



Bill Hill joined Microsoft in 1995 because he believed the company would lead the transition from reading on paper to reading on the screen. He is one of the inventors of the ClearType technology which dramatically improved the screen readability of text.