The reason the psychology test below is important in marketing is because of a concept known as eye movement research. According to this research described in The Tipping Point, the human eye is only capable of focusing on a very small area at any time, known as the perceptual span. This is because most of the sensors in your eye are clustered in a small area in the middle of the retina called the fovea. We can not take in much visual information unless we focus our fovea directly on an object or word. When we read, we focus on about 4 characters to the left and 15 characters to the right. In the basketball pass experiment (posted below) we focus on the small area where the basketball moves and don't notice the gorilla come into the center of the screen.
If you can track where a person's foveas are moving you can know, with high certainty, what information they are receiving. The Tipping Point uses the example of a beer commercial with a model. Advertisers would be very interested in whether the viewer focuses only on the model or if their fovea moves to the beer.