The Discovery of the Occluding Edge and Its Implications for Perception (Gibson, 1979, Chapter 11)

Gibson introduced us to the notion of occlusion in Chapter 5. Occlusion is when one surface is progressively covered up by another surface, and the details of how optical texture is deleted specifies the occluding edge and the fact that occlusion is happening. In this chapter, Gibson details the radical implications of this fact of the optic array for theories of perception. Most importantly, given occlusion, vision cannot be based on images, series of images, or perspective structure, and it is not limited to working when sensations fall on the retina. Vision is a process that extends back and forwards in time, and this is enabled by information. 

The Information for Progressive Occlusion

Gibson's ecological psychology is weird, if you are coming from a more traditional information-processing background. The two approaches make radically different assumptions about the nature of the world to be perceived; they have radically different ontologies. This means that there is little if any useful overlap in the way they do things, and communicating across the gap is very hard. I have a recent paper - preprint here - where I go into detail about the two ontologies as I defend ecological psychology from interface theory. It's essentially Turvey et al, 1981, but that's a bear of a read if you aren't already ecologically minded. Do mine first :)

Anyway, concrete examples help. My go-to is the outfielder problem but people are tired of that one. My other favourite is progressive occlusion (Gibson, Kaplan, Reynolds & Wheeler, 1969; Kaplan, 1969). Gibson worked this example up himself in great detail and so it stands as a nice concrete example to illustrate some elements of the ecological ontology. Given the recent total solar eclipse, it seems like the right time to blog it!

This post will review occlusion, talk about how it works and work with some demos. These are all linked from here; there is Matlab/Psychtoolbox code to run a demo, a video of that running and a Powerpoint with some slides. I'll refer to these throughout - occlusion is a dynamic process and so you need to see it moving for it to make sense.