This myth has been debunked through a massive study of 1000 brains by Plos One with more articles HERE and HERE. So if the foundation of Betty Edwards' book is false, does that mean one should throw it away?
Hardly. Many of the exercises are very helpful in teaching student to truly "see" what they are drawing as opposed to drawing what they think they know. Though I don't teach from the book, I still use several of the techniques, like drawing upside-down when my students do grid portraits. It slows them down and their work looks better. Others love the negative space drawings, or the line drawing approaches. Betty Edwards offers a lot of tools to add to your toolbox.