FOCUS!

Unfortunately for advertisers, we have a lot of stimuli in our modern environment, especially the kind that assaults our eyes. Mental and physical resources are limited, and our brains must decide in an instant which stimulus is the most important to attend to. This is called selection for action. For primitive man, the question was, "Where is it?" He heard a rustling in the bushes, saw a twig move. His mind attended to the task of determining where "it" might have been so that his body and senses could focus on a target for potential action.

This very low level of engagement isn’t just something that keeps us out of the way of danger or attracts us to food. When we read and reach the end of a line of text, our brains prepare our eyes to move quickly to the left to search for the rest of a sentence. When it comes to engagement, if your brain chooses to attend to a stimulus that it deems significant, you are engaged.
To date, the exploitation of this in computer applications has been limited to changes in highlighting or flashing words and icons. Even so, when used properly this kind of attention capturing has proved useful to users. In interface design there is a technical term for changing certain colors in the borders of desktop windows, depending on their active or non-active status. It is called giving "focus" to a particular window, and it helps computer users keep track of what can be acted upon.

There are limitations to the exploitation of this low level mental process, and these limitations are shared with another human perceptual trait.