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Perception, 1977, volume 6, pages
Illusory line linking solid rods
Colin Ware, John M Kennedy
Division of Life Sciences, Scarborough College, University of Toronto, West
Ontario M1C 1A4,
Received 1 February 1977
Abstract. A solid object—a frame enclosing rods—can be seen as having an illusory 'line' joining
the tips of the rods.
Illusory contours are apparent lines and divisions cutting across regions of the
perceived display which are actually physically homogeneous (Osgood 1951). Figure 1
is a photograph of a three-dimensional object, which allows the viewer to perceive an
Whether viewed directly, or via a photograph, the three-dimensional object can be
seen as having a peculiar 'line' linking the tips of the rods. This 'line' is visible in
static inspection of the object, in monocular and binocular inspection, and in
inspection during head motion, motion of the object, and motion of the background.
Fixation can be on the background (near or far) or the eyes can track the object, and
the 'line' will still be seen; although, as is usual with illusory contours, some subjects
report that sections of the 'line' (between adjacent rods) disappear when they are
directly inspected, notably in the static conditions.
Previous experimenters have used two-dimensional displays to create illusory
contours and perceived depth. It has been noted that in these displays it could be
Figure 1. Photograph of a three-dimensional object, approximately 20 cm x 12 cm x 2 cm.
Inspection of the object results in an impression of an 'illusory contour' joining the ends of the
Viewed directly, rather than in a photograph, the compelling impression is of a 'line' linking
coplanar rods, though stratification in depth is readily possible given a picture of the object, as here.
602 C Ware, J M Kennedy
that the contour is a result of depth cues forcing the viewer to perceive nonexistent
stratification in depth (Kanizsa 1976; Gregory 1972). The depth-cue theory has also
been argued to hold for some binocular displays that result in illusory contours
The object shown in figure 1 has been inspected by many subjects, some of whom
have viewed it on many occasions and for prolonged periods (with considerable
interest!). The 'line' has been noted by the viewers to be present without any
perceived stratification in depth; that is, all the rods are seen as in the same depth,
and as linked by the 'line'. Presumably when looking at two-dimensional graphic
displays, the viewer is free to exercise pictorial depth-perception skills and to create
figure-ground reversals, etc. A three-dimensional display provides binocular cues,
head motion, object motion, and background motion as information for depth. The
viewer of such a display is much more constrained and thus it can be shown that
depth stratification is not necessary for perception of subjective contour.
We have made other solid objects (and also shapes on transparent surfaces) which
induce subjective figures. We have used both opaque and semitransparent materials
in their construction. It is of interest to note that the subjective figure can be seen
as 'space filling' (i.e., voluminous or 'solid') not just as linear or planar. Whether
linear, planar, or 'space-filling', the subjective figures can be seen as unambiguously
located at the same distance as the inducing strips or rods, not as overlapping the
inducing materials or as part of the background.
Evidently, the stratification-in-depth hypothesis is unsatisfactory as a general
explanation of subjective contours. Hence, alternative explanations are required, for
example the recent ones which invoke brightness contrast effects tied to appropriate
stimulus groupings (Kanizsa 1976), notably groupings of line and rod endings (Frisby
and Clatworthy 1975; Kennedy and Lee 1976).
Of course, the object of figure 1 leaves open the possibility that there are particular
conditions where the stratification-in-depth hypothesis is the most satisfactory
explanation. Just such a condition may be the one in which the form information is
binocular disparity (rather than the monocular brightness differences that are the
substrate for contrast effects). The key point that our object makes is that illusory
contours can be produced without prior stratification being the causal agent.
Coren S, 1972 "Subjective contours and apparent depth"
Frisby J P, Clatworthy J L, 1975 "Illusory contours: curious cases of simultaneous brightness
Gregory R L, 1972 "Cognitive contours"
Kanizsa G, 1976 "Subjective contours"
234 April 48-52
Kennedy J M, Lee H, 1976 "A figure-density hypothesis and illusory contour brightness"
Osgood C E, 1951 Method and Theory in Experimental Psychology (London: Oxford University
p © 1976
Pion publication printed in Great Britain