Intensity and hue are secondary, and can be filled in later. As you can see in the following eye chart. The retina is a mesh of neurons and photoreceptors, wired and programed to extract change from the image on the retina. Indeed this is one part of the brain that we do know pretty well how it works. To illustrate this, Try out the next two eye charts.
Edges are important too .
But those lines are the edges of the facial features. A line sketch simplifies things down the same way the retina does. Which leads to the next eye chart. This chart is sorta neat. Stare long enough and the fuzzy edged circle goes to gray .