"Blindsight" is a medical condition where the patient sees nothing, yet is capable of pointing at an object or catching a ball thrown to them. What this tells us is that there are parts of our minds watching the same scene that we consciously see, extracting information from it, and making independent evaluations. This is happening in parallel to what we think we see, and presumably developed in times past when we were lower down the food chain and rapid response was key to survival. Actually, that describes corporate life pretty well too.
This skill is almost completely unexploited by boring paragraphs of text. Consider the following:
The block of text on the left tells us nearly nothing, because we don't understand the words. The only visible clue is that the underlined part might be important - and this information comes from a graphical device.
By comparison, the fragment of a map on the right is rich with information, even though the words are just as meaningless. Without interpreting the text, we can see that there is one main idea, plus three smaller ones (or at least they are smaller relative to the main one). These smaller ideas have some kind of consistent relationship with the bigger one, and appear to have some characteristics in common with each other (since they have a similar relationship to their parent). Perhaps they are three examples of something, or three characteristics of the bigger idea. Maybe there are more than three - if we could read them, we could perhaps think of a fourth. Or maybe one of them does not really belong with the others. And all this without understanding a single word - our mental model of this fragment of data is already primed with expectations for patterns that we can evaluate, question and improve upon.
The bottom line is this: there is more information in the lines of a map than in the words. The connecting lines in a map are the body language of its words. Body language that does not match the spoken word is instinctively and immediately recognised as deception. Sometimes we don't know exactly why we think the other person is trying to fool us - it's just a hunch. Perhaps this is blindsight at work, reading their body language without conscious effort, and sending alarm signals about inconsistency.
It's much harder to get away with deception in a map than in reams of dense text. This is a powerful characteristic that can be exploited by using maps as a team communication tool. It doesn't have to be deception - ambiguity or muddled thinking shines through just as clearly. And all without colours or images (yet!).