Mind, Body, World- Foundations of Cognitive Science, 2013a

The primary visual data caused by this movement is the motion, from point A to
point B, of a point projected onto the back of the retina. The projection from the
world to the back of the eye is uniquely defined by the laws of optics and of projective
geometry.
However, the projection in the other direction, from the retina to the distal
world, is not unique. If one attempts to use the retinal information alone to identify
the distal conditions that caused it, then infinitely many possibilities are available.
Any of the different paths of motion in the world (occurring over the same duration)
that are illustrated in Figure 8-1 are consistent with the proximal information
projected onto the eye. Indeed, movement from any position along the dashed line
through the X-labelled points to any position along the other dashed line is a potential
cause of the proximal stimulus.
One reason for the poverty of the visual stimulus, as illustrated in Figure 8-1, is
that information is necessarily lost when an image from a three-dimensional space
is projected onto a two-dimensional surface.
We are so familiar with seeing, that it takes a leap of imagination to realize that
there are problems to be solved. But consider it. We are given tiny distorted upsidedown
images in the eyes, and we see separate solid objects in surrounding space.
From the patterns of stimulation on the retinas we perceive the world of objects,
and this is nothing short of a miracle. (Gregory, 1978, p. 9)
A second reason for the poverty of the visual stimulus arises because the neural
circuitry that mediates visual perception is subject to the limited order constraint
(Minsky & Papert, 1969). There is no single receptor that takes in the entire visual
stimulus in a glance. Instead, each receptor processes only a small part of the primary
visual data. This produces deficiencies in visual information. For example,
consider the aperture problem that arises in motion perception (Hildreth, 1983),
illustrated in Figure 8-2.
Moving Contour
B
T
Aperture
A
Figure 8-2. The aperture problem in motion perception.
Seeing and Visualizing 365