Further Consideration of a Paradox in the Visual Perception of Translatory Motion
J. J. Gibson, Cornell University; December 1964
The World Wide Web distribution of James Gibson's "Purple Perils" is for scholarly use with the understanding that Gibson did not intend them for publication. References to these essays must cite them explicitly as unpublished manuscripts. Copies may be circulated if this statement is included on each copy.
In 1958, a number of paradoxes were described that arise when one tries to
make experimental studies of the visual perception of motion (Second Symposium
on Physiological Psychology, ONR, p. 165-175). The classical apparatus used
in this research is a moving endless belt behind a window, with an observer
stationed in front. One of these paradoxes is that the phenomenal velocity of
the belt is substantially equivalent whether the subject observes with pursuit-fixations
of the moving surface or with a stationary fixation of the edge of the window
(Gibson et al., AJP, 1957, 70, 64-68). In one case the retinal stimulus
is a stationary inner image with a moving surrounding and in the other case
it is a moving window-image with a stationary
surrounding. The sensations, of course, are quite different, and there is said
to be a constant error in matching velocities across the two modes of observation
(the Aubert-Fleischl paradox which, however, is not easy to measure). But the
perceptions of motion are essentially the same, that is, there is an invariance
of perception despite varying sensations (Useful Dimensions of Sensitivity,
Amer. Psychol., 1963, 18, 1-15). How can this be? Can we doubt that the
stimulus for the perception of motion is retinal motion?
One resolution of this paradox has been to suggest that when a psychologist
speaks of motion he should always mean relative motion
displacement with respect to the "frame of reference". Duncker's experiments
on induced motion are in line with this explanation (Psychol. Forsch.,
1929, 12, 180-259). The implications is that the brain somehow responds to relative
motion, not to absolute motions of elements in the field of view. This formula
brings up the whole unresolved problem of the perception of relations among
sensory elements and how this could be achieved.
Another resolution of the paradox, more fundamental, is to suggest that when
the eyes shift from pursuit fixation to stationary fixation there actually exists
at the retina some stimulus that remains invariant, i.e., constant, and that
this is the information for the equivalent perceptions. This stimulus would
have to be common to the two cases of window-moving-relative-to-the-surround
and surround-moving-relative-to-the-window. What might this invariant be? It
could be something occurring in the transformations of optical structures
that correspond to the edges of the window.
One thing that occurs at an optical margin corresponding to the physical edge
separating two surfaces undergoing relative motion (if the surfaces are patterned)
is frequency. This optical event is not flicker precisely,
but it is more like flicker than it is motion. It is invariant under size-transformations,
that is, constant with different distances of the window apparatus from the
eye. Smith and Sherlock (AJP, 1957, 70, 102-105) thought of it as frequency
of "passing" and they used it to explain J. F. Brown's velocity-transformation
phenomenon without any reference to velocity whatsoever.
What are the optical transformations at the edges of windows (which have a surrounding
front ground) and at the edges of objects (which have a surrounding
background)? In figurative language, I have called the transformations
at the leading and trailing edges "winking" or "flickering" (Smith's "passing")
and I call the transformations at the lateral edges "shearing". Consider the
former. The optical events at leading and trailing edges can now be better understood
since Michotte has published his experimental studies of "screening effects"
(Studia Psychologia, Louvain, 1964). Considered as transformations of
texture there is a sort of wiping-out of elements (which then appear to be covered).
Michotte considers the forms as such in his experiments which, although screened
or occluded from view, continue their phenomenal existence. This is true even
though there are no sensory data to support the continued perceptions of form.
The family of transformations connected with occlusion, more
exactly, those accompanying a change in occlusion deserve much more study. These
kinetic edge phenomena normally result, of course, from the interception of
pencils of rays in the network of pencils filling an illuminated space (MS of
Perceptual Systems, ch. 9). They occur whenever the of the environment
changes and whenever the observer's viewpoint changes. These transformations
normally correspond to edge-depth in the world, unlike the family to transformations
that correspond to slant-depth in the world (Flock, 1964, on slant, and Gibson
et al, AJP, 1955, 68, 372-385). The transformations that correspond to edge-depth
seem to involve a kind of mathematical discontinuity whereas those that correspond
to slant depth (perspective transformations) do not.
Next consider, for example, the stimulus that occurs when an individual moves
his head from one side to the other in the presence of an "optical cliff" (Gibson
and Walk, Scient. Amer., 1960, 202, 64-71). It is essentially a lateral
shearing of the optical texture, a slicing into two parts along a horizontal
line. This stimulus is invariant. Considered as a slippage it is unchanged despite
variations in speed or direction of the upper and lower slices of texture in
the optic array. It is not itself a motion, if a motion is something defined
by a definite speed in a definite direction. The flow can be to right or to
left, fast or slow, but the invariant continues to specify the difference in
depth between the near and the far surface. Moreover the slippage of one half
of the retinal image relative to the other remains the same whether the observer
fixates on a detail above the edge or one below the edge and thereby shifts
the retinal motion from one half of the retina to the other.
It seems to be implied, therefore, that kinetic edge effects are essential for
the perception of the motion of an object in the world (including the motion
of an endless belt behind a window) and are important, if not essential, for
the perception of the depth at stationary edges in the world. The puzzles and
anomalies that arise when one tries to establish a psychophysical correspondence
between retinal notions and sensations of motion are understandable in the light
of this conclusion.
Top
If you are in need of consulting services, we recommend Atlantic Human Factors
for human factors consulting and
usability consulting.
Or Search for Human Factors and Usability Consultants
Copyright © 2003 - 2007 by HuWI.org