Perception of Visual Direction in Human Infants
Richard N. Aslin University of Rochester
One of the most obvious characteristics of the visual world is its stability. The world does not rotate as you turn around (you would become badly disoriented if it did) nor does it shoot from side to side or up and down as you shift your fixation from one object to another. This fact is so obvious that most of us take it as a matter of course and do not realize that there is any need for explanation. And yet it is really a very astonishing fact. Things possess a direction-from-here not with respect to the margins of the visual field but with respect to a fixed visual world--an external frame of reference which seems unexplainable on the basis of the retinal picture.
-- J. J. Gibson ( 1950, p. 31)
This quotation from Gibson describes the puzzle of position constancy; that is, how do objects maintain an invariant perceived direction despite variations over time in the projection of the object's image onto different retinal locations? This is the question that I will turn to in the second half of this chapter. First, however, it seems appropriate to ask a more fundamental question, which Gibson did not address explicitly: How does the visual system correctly assign discrete visual directions to each location on the retina? In other words, given a single immobile eye centered in the orbit, how does the visual system assign a directional value of zero or straight ahead to that portion of the retinal image that falls in the center of the fovea? Why is the foveal stimulus not incorrectly assigned a directional value of "20° to the right"? Similarly, why are the topographic relations of the objects around us isomorphic with the topographic relations of our perceptions? Why do points in the retinal image maintain the property of adjacency when transformed into the perception of visual direction?