Reappraising the functional implications of the primate visual anatomical hierarchy.

The primate visual system has been shown to be organized into an anatomical hierarchy by the application of a few principled criteria. It has been widely assumed that cortical visual processing is also hierarchical, with the anatomical hierarchy providing a defined substrate for clear levels of hierarchical function. A large body of empirical evidence seemed to support this assumption, including the general observations that functional properties of visual neurons grow progressively more complex at progressively higher levels of the anatomical hierarchy. However, a growing body of evidence, including recent direct experimental comparisons of functional properties at two or more levels of the anatomical hierarchy, indicates that visual processing neither is hierarchical nor parallels the anatomical hierarchy. Recent results also indicate that some of the pathways of visual information flow are not hierarchical, so that the anatomical hierarchy cannot be taken as a strict flowchart of visual information either. Thus, while the sustaining strength of the notion of hierarchical processing may be that it is rather simple, its fatal flaw is that it is overly simplistic.