He first did an outline drawing of a familiar face (President Bush) which was almost unrecognizable although an accurate drawing of the "profiles" of every form on the face. He then did an outline drawing of the shadow shapes which didn't read either. The picture that worked best was a rather crude black and white posterized version of the photograph which allowed one to read the three dimensional shapes of the face without much detail. This was the best likeness.
His conclusion was that what the feature detectors in the brain's facial recognition centers are looking for is three dimensional forms. For example, a nose is a protuberance from the face with a very particular three dimensional form.
It's a very interesting idea, particularly important when working from photographs which temp one to draw linearly. In a portrait workshop with Leslie B. Demille he made a point of never drawing a line, but drawing an edge between two forms. For an extreme example an elderly person with lots of "lines" on his or her face is best done by putting down multiple wide marks on one side of the "line" and other broad marks on the other side of the "line" with a little space between the two series of marks that allows us to see the underdrawing. The underdrawing peaking through the edge between the two passages is, of course, what we call a "line". In reality a line of the face is a three dimensional fold, and this method represents them best.
I think Bitterman is on to something here about how the brain reads portraits and how we should think about what we are doing. This brings up the question of what it means to "draw three dimensional forms" with lines. It's not just putting in the form shadows or the cast shadows. In fact, I think it may have less to do with the shadow shapes than we think. For some strange reason, when you think about drawing some form in three dimensions, you draw it differently than when you think of drawing the idea of the feature itself (nose, arm, etc).
What interests me is that I have seen innumerable didactic figure drawings in which there is, in addition to the figure, an arm or leg off to the side which the teacher has cut in half to show that it is a cylinder. However, if you really were to draw a cylinder with the sides exactly parallel to each other it would look more like a robots arm than a human arm. If you put in too much technical shading with the core shadow and the highlights running up and down the arm, it also looks like it's made of metal because that's how very reflective surfaces work, not the way skin works.
In fact, when you think "the arm I'm drawing is a cylinder-like form", what you do is related the curves of the two outside edges to each other . They bow out a bit like a soft shape, perhaps like the long thin balloon, not like a metal cylinder. So, perhaps, what we do is to integrate the lines all over the form more when we think "three dimensions." Consider how one might draw a nose. The wrong way would be to draw from point to point down one side, around the bottom, and up the other side even 'though that is the easiest to do and would accurately outline the form. This is what you would do with tracing paper over a photograph. It would look like Blattermann's first try. The right way would be to draw both sides of the nose at the same time. Sounds impossible, but you would come close if you would keep you pencil on the paper and circle it around the biggest nose shape, lets say the overall length of the nose, then add another aspect of the form the same way, the circular bulb at the base of the nose, keeping the lines light. There would be some lines that you will have to take out, for example at the root of the nose because, in keeping your pencil on the paper, you have left a line there, but the end result is more three dimensional and reads like a better likeness. In doing it this way there are some fairly faint lines that you would never think to include that help to make it three dimensional.