Exploiting Verbal-Visual Synergy in Presentation Slides
http://www.ingentaconnect.com/content/stc/tc/2009/00000056/00000002/art00006
  access restricted (by the publisher) to members/subscribers/customers only
  peer-reviewed

Markel, Mike
Technical Communication Online
2009

Abstract:

Describes the most challenging aspect of creating slides for an oral presentation. Presents two principles for creating informative and persuasive graphics. Explains how to use drawing tools to communicate the schema of the slide and to emphasize important portions of the images.