Monocular Gap Stereopsis

Cross-fuse left and middle images. You will see two rectangles in depth, the right one further. Cross fuse the left and middle images. The depth will now reverse. There are no matched images and therefore no disparity where the gap is.

• Gillam, B.J., Blackburn, S., & Nakayama, K. (1999). Stereopsis based  on monocular gaps: Metrical encoding of depth and slant without matching contours. Vision Research, 39, 493-502.
  
  
• Pianta, M.J., & Gillam, B.J. (2003). Paired and unpaired features can  be equally effective in human depth perception. Vision Research, 43, 1-6.
  
  
• Spang, K., Gillam, B., & Fahle, M. (2012). Electrophysiological correlates of binocular stereo depth without binocular disparities. PLoS One, 7(8), 1-14.