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

In this situation, a motion detector’s task is to detect the movement of a contour,
shown in grey. However, the motion detector is of limited order: its window on
the moving contour is the circular aperture in the figure, an aperture that is much
smaller than the contour it observes.
Because of its small aperture, the motion detector in Figure 8-2 can only be
sensitive to the component of the contour’s motion that is perpendicular to the edge
of the contour, vector A. It is completely blind to any motion parallel to the contour,
the dashed vector B. This is because movement in this direction will not change
the appearance of anything within the aperture. As a result, the motion detector is
unable to detect the true movement of the contour, vector T.
The limited order constraint leads to a further source of visual underdetermination.
If visual detectors are of limited order, then our interpretation of the proximal
stimulus must be the result of combining many different (and deficient) local
measurements together. However, many different global interpretations exist that
are consistent with a single set of such measurements. The local measurements by
themselves cannot uniquely determine the global perception that we experience.
Consider the aperture problem of Figure 8-2 again. Imagine one, or many, local
motion detectors that deliver vector A at many points along that contour. How
many true motions of the contour could produce this situation? In principle, one
can create an infinite number of different possible vector Ts by choosing any desired
length of vector B—to which any of the detectors are completely blind—and adding
it to the motion that is actually detected, i.e., vector A.
Pylyshyn (2003b, 2007) provided many arguments against the theory that vision
constructs a representation of the world, which is depictive in nature. However, the
theory that Pylyshyn opposed is deeply entrenched in accounts of visual processing.
For years the common view has been that a large-scope inner image is built up by
superimposing information from individual glances at the appropriate coordinates
of the master image: as the eye moves over a scene, the information on the retina
is transmitted to the perceptual system, which then projects it onto an inner screen
in the appropriate location, thus painting the larger scene for the mind side to
observe. (Pylyshyn, 2003b, pp. 16–17)
Proponents of this view face another source of the poverty of the visual stimulus. It
is analogous to the limited order constraint, in the sense that it arises because vision
proceeds by accessing small amounts of information in a sequence of fragmentary
glimpses.
Although we experience our visual world as a rich, stable panorama that is present
in its entirety, this experience is illusory (Dennett, 1991; Pylyshyn, 2003c, 2007).
Evidence suggests that we only experience fragments of the distal world a glance
at a time. For instance, we are prone to change blindness, where we fail to notice a
substantial visual change even though it occurs in plain sight (O’Regan et al., 2000).
366 Chapter 8