1. It’s all about the constraints
John K. Tsotsos 
Current Biology 
Volume 24, Issue 18, Pages R854-R858 (September 2014)
DOI: /j.cub
Copyright © 2014 Elsevier Ltd Terms and Conditions

2. Figure 1
A Venn diagram representing the defining characteristics for vision systems.
Each little light blue blob represents a characteristic. Each oval represents a class of vision system (human, bird, and so on) while the largest oval represents the space of all possible vision systems, whether biological or otherwise. Where ovals or classes overlap it means that the characteristics within the overlap region are shared by the overlapping classes of systems.
Current Biology  , R854-R858DOI: ( /j.cub )
Copyright © 2014 Elsevier Ltd Terms and Conditions