Human factors 101.

WHILE THINKING ABOUT THIS column, it occurred to me that I've never explained some of the more interesting aspects of human perception. These properties are so fundamental that most human factors people take it for granted that everybody understands them. This allows us to talk over everybody's head and then wonder why nobody believes what we say. Maybe this short explanation will help.

If you can't tell the difference, it doesn't matter. Human perception is a funny thing. Not funny meaning "ha ha," but funny meaning "kind of strange." The objects we see and touch, sounds we hear, and odors we smell have to provide enough energy for us to perceive them. For example, the type on this page has to be big enough and have enough contrast with the background for us to see and read it. Below that size and contrast, human vision just isn't capable of detecting and recognizing the characters. The minimum size and contrast are called threshold values.

Beyond the threshold value, things get a little dicey. How different do two things have to be in order to be perceived as different? For example, if I show you two slightly different colored paint chips, how different do they have to be for you to see them as different colors? Whatever those values are, they're called just noticeable differences.

This is an important phenomenon for all kinds of consumer products. Can you tell the difference between a 1080p and a 1080i high-definition TV picture? Neither can I.

People are A/C coupled. To clarify, the electrical engineers who worked on our bizarre equipment in the human factors lab were fond of saying that humans are "A/C coupled." (No, it doesn't mean that. Get your mind out of the gutter.) It means that people perceive things more readily (or at all) when they change, rather than when they are constant.

Our eyes are constantly making small jerky movements called saccades. For many years, nobody understood why saccadic movement existed. An enterprising researcher built a viewing apparatus that compensated for saccadic movements. This had the effect of stabilizing images on the retina. Want to guess what happened to those images? Yep, they disappeared. People just didn't see them anymore.

This is all part of a common effect called habituation. If a perceptual stimulus is constant, after a while, we just don't perceive it anymore. Do you feel your wedding ring on your finger? Do you feel your belt? OK, bad example. This also applies to more complex, real-world occurrences. Does $2.50 per gallon seem expensive anymore?

Yeah, it's big, but you have to eat it in small bites. Finally, let's briefly look at the dynamic range of human senses. We can see things in rooms that are almost totally dark (and in which alcohol might be sold). We can also see things in bright sunlight. That's a difference in illumination of 0.1 lux to 100,000 lux--or a dynamic range of one million to one. There are very few scientific instruments that can come close to that dynamic range.

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How do we do that? Well, there's a trick. Our vision works in only a portion of that range at any given time. That's why it takes us a while to see things when we walk into a dark movie theater and to readjust when we walk back outside. This phenomenon is called adaptation and it works for every sense except hearing. We never adapt to loud noise.

OK, you're all human factors people now. I'll cover the secret handshake in a future column.

Michael Maddox, Ph.D., is a senior scientist for HumanCentric Technologies. A certified human factors professional with 20 years of experience, he specializes in human error reduction and risk analysis. He can be reached at mmaddox@humancentrictech.com