An invariant is a critical component of Gibson's (1979) theory of ecological perception. In this theory, visual "stimulation" is viewed as a dynamic flux, which is produced by changes in the world or by changes in viewing position due to locomotion through the world. This dynamic flux is highly variable. However, in all of this variability some characteristics will remain constant. These are the invariants of the optic array, and Gibson claims that these invariants are picked up by direct perception, and specify affordances to the perceiving agent.
Importantly, Gibson's invariants are second-order properties. That is, what is invariant in an optic array is the relationship between two of its varying components -- the components vary, but the relationship does not.
- Gibson, J. J. (1979). The Ecological Approach To Visual Perception. Boston, MA: Houghton Mifflin
(Added October 2009)