Oxford Compendium of Visual Illusions

Arthur Shapiro and Dejan Todorovic, Editors

Chapter xx

Illusory Color Spread from Apparent Motion

Carol M. Cicerone1, Professor Emeritus, and Donald D. Hoffman, Professor

Department of Cognitive Sciences

University of California, Irvine

Irvine, CA 92697

1Corresponding author’s address: 2931 University Terrace, NW

Washington, DC 20016 USA

carolcicerone@nas.edu

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Abstract and Keywords

Color from motion describes the perception of subjective color that spreads over physically

achromatic regions that are seen in apparent motion. Multiple frames are shown in quick

succession, each frame composed of a random placement of differently colored dots on an

achromatic background. From frame to frame, the locations of all dots are fixed, whereas the

color assignments of dots in the test region change. Subjective color can be measured by color

matches to and cancellation by real lights; can be seen with chromaticity differences alone in test

and surround dots; and is independent of contour formation. In stereoscopic view, the perception

of depth, as well as color and form, can be recovered in tandem with seeing motion. We suggest

that in natural scenes, mechanisms triggered by motion may reconstruct the depth, color, and

form of partially obscured objects so that they can be seen as if in plain view.

Keywords: color from motion, subjective color, apparent motion, depth perception, camouflage

xx.1 Introduction

It is known that the visual system is capable of constructing illusory colors and contours that may

be absent in the physical stimulus (e.g., Grossberg, 1994; Kanisza, 1979; Michotte, Thines, &

Crabbe, 1964; Nakayama & Shimojo, 1990, 1992; Nakayama, Shimojo, & Ramachandran, 1990;

Peterhans & von der Heydt, 1991; van Tuijl, 1975; Varin, 1971; Yamada. Fujita, & Masuda,

1993). In particular, motion is effective in allowing the visual system to use multiple fragmented

views of an object over time to reconstruct its shape as a whole (e.g., Anderson & Braunstein,

1983; Andersen & Cortese, 1989; Gibson, 1979; Kaplan, 1969; Lappin, Doner, & Kottas, 1980;

Shipley & Kellman, 1983, 1984; Stappers, 1989; Wallach & O’Connell, 1953; Wertheimer,

1923; Yonas, Craton, & Thompson, 1987). We introduced (Cicerone & Hoffman, 1992) an effect

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called color from motion for which, the perception of apparent motion is accompanied by the

perception of illusory color that is seen in physically achromatic regions of the stimulus. We

(Cicerone, Hoffman, Gowdy, & Kim. 1995; Cicerone & Hoffman, 1997; Miyahara & Cicerone,

1997; Chen & Cicerone, 2002a, b) explored this phenomenon to define the conditions under

which it occurs, to understand how it might be useful to organize the visual scene, and to link it

to the perception of motion, contour, color, and depth.

A typical display of color from motion is shown in Figure 1. Each frame (8 deg of visual angle

on a side) consisted of an achromatic background field (CIE x = 0.276, y = 0.286, luminance 73

cd/m2) over which was randomly arrayed 800-1200 dots (each 3.5 min of arc in diameter).

Within a circular test region (typically 1 to 2 deg of visual angle in diameter), the dots were

colored green (CIE x = 0.280, y = 0.610). All other dots were red (CIE x = 0.621, y = 0.344). To

create successive frames, no dots changed their locations within the frame; only the color

assignments of the dots were changed, according to a uniform vertical displacement (0.12 deg of

visual angle) of the test region in successive frames. When frames are cycled, typically with an

effective displacement rate of the test region equivalent to 7 deg/sec, up and down over a vertical

region spanning 5 deg of visual angle, an illusory green disk moving up and down pops into view

and a spread of illusory green color is seen in the physically achromatic test region. This effect

and others described below can be viewed in an on-line publication of the Journal of Vision

(Chen & Cicerone, 2002a).

< Figure 1 here >

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The rating methods used in the early studies (Cicerone & Hoffman, 1992; Cicerone et al., 1995)

established the salience of the color spreading effect and its link to the perception of apparent

motion. Certain aspects of the illusory color, however, such as its saturation, require more

sensitive methodologies. Miyahara & Cicerone (1997) used a side-by-side matching method and

found that the hue of the subjective color spread approximates that of the test dots. In their

methodology, the matching stimulus was stationary and homogeneously colored, whereas the

color from motion stimulus was perceived as moving and included test and surround dots. Chen

& Cicerone (2002a) used real lights to cancel the subjective color spread in color from motion,

while keeping luminance constant, to measure the hue of the illusory color spread. An increase

in the luminance of the test dots produces an increase in the saturation of the physical lights

required to match the subjective color spread as measured by the side-by-side method (Miyahara

& Cicerone, 1997) and the cancellation method (Chen & Cicerone, 2002a).

xx.2 Perception of motion is essential in color from motion

Illusory color spread as measured by a rating method is linked to apparent motion of the test

region. Therefore, we asked if the salience of the color spread as measured with cancellation

was linked to the salience of the apparent motion (Chen & Cicerone, 2002a). We varied the

effective rate of vertical translation of the test region between zero and 12 deg of visual angle per

sec. The results for two observers (Figure 2) can be well described by two linear functions with

a steeply sloping first branch (from zero to 1 deg/sec) and a second branch with a much reduced,

near zero slope (from 1 deg/sec to 12 deg/sec). The general profile of the results suggests an all-

or-none relationship between apparent motion and illusory color spread. Observers report that as

the translation speed of the test region increases, the perception of the illusory, green-colored

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disk as a separate form that moves over the field of dots is enhanced. This perception of

separation is reported reliably for speeds exceeding 1 deg/sec. Concurrently, the test dots appear

to assume the color of the surround dots so that all dots appear to have the same color, in this

case red, even if the test dots are physically colored green. Thus, it appears that for speeds of

effective translation of the test region that are greater than 1 deg/sec there is little or no

enhancement of the illusory color spread whereas the salience of the perceived separation of the

figure – defined by the illusory color spread – from the array of dots is enhanced.

<Figure 2 here>

xx.3 Color from motion without contour formation

Is color from motion linked to contour formation as well as to the perception of motion?

Luminance contrast is known to be necessary for the formation of static subjective contours (e.g.,

Frisby & Clatworthy), for the perception of apparent motion in achromatic stimuli (e.g.,

Ramachandran & Gregory, 1978; Cavanagh, Boegelin, & Favreau, 1985), and for the perception

of achromatic neon spreading (e.g., Bressan, 1983). If, near equiluminance, color spread occurs

without the formation of a subjective contour, then color from motion is likely to be regulated by

mechanisms distinct from those regulating contour formation. Miyahara and Cicerone (1997)

tested this idea with green (test) and red (surround) dots that were matched in luminance as

determined for each observer individually by means of heterochromatic flicker photometry.

Chromaticity differences between the test dots and the surround dots in the absence of luminance

differences produced the perception of spread of color from motion. As the luminances of the

test and surround dots approach equality, the strength of the subjective contour surrounding the

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test region is reduced while the subjective color spread is still perceived. Observers reported that

in such equiluminant conditions, there is no clear contour bounding the region of the subjective

color spread and that apparent motion is “not smooth” and “slower” than the conditions in which

the test and surround dots differ in luminance (Chen & Cicerone, 2002a). These results are

consistent with findings that suggest that the neural mechanisms responsible for contour

formation rely on luminance information (e.g., Kanizsa, 1979; Marr, 1982; von der Heydt et al.,

1984). These findings support the idea that the mechanisms regulating color from motion are

separable from those regulating contour formation and that color spread can occur without

contour formation.

Another way to study the role of luminance differences is to use red-green dichromatic observers

who are incapable of making red-green discriminations on the basis of chromaticity

discriminations alone. Miyahara and Cicerone (1997) presented these stimuli to a deuteranope.

The deuteranope lacks the middle-wavelength-sensitive pigment and is therefore insensitive to

chromaticity differences in the middle- to long-wavelength range of the visible spectrum. In this

range, where our red and green stimuli lie, the deuteranope sees only luminosity differences,

based on the activity of the long-wavelength-sensitive cones. Thus, the results of a red-green

dichromat should allow us to assess the impact of luminosity differences alone for these stimuli.

When the luminosity of the green dots in the test region was high relative to that of the dots in

the surround, the deuteranope saw a bright disk moving over the test region. As expected, near

equiluminance between the test and surround dots, the deuteranope in our study saw no apparent

motion nor did he see brightness spread in the test region. This is in contrast to color normal

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observers who saw apparent motion and color spread without a clearly defined contour when test

and surround dots were matched in luminance.

xx.4 Modal versus amodal completion in color from motion

There are two modes in which color from motion is perceived, either (1) as a localized change of

illumination, a colored spotlight or shadow, moving over a textured surface (Cicerone &

Hoffman, 1992; Cicerone et al., 1995; Miyahara & Cicerone, 1997) or (2) as a moving, colored

object seen through apertures in an occluding surface (Cicerone & Hoffman, 1997; Chen &

Cicerone, 2002a). The mode in which color from motion is seen depends on figural cues and

regional differences in the luminance contrast between the chromatic elements and the

achromatic background. Regions with figural cues tend to be seen as moving. These regions are

seen in the first mode (modal completion, Figure 3, left) if their defining figural elements, dots in

this case, are of lower luminance as compared to the background and in the second mode

(amodal completion, Figure 3, right) if the defining figural elements are of higher luminance as

compared to the background. As compared with color from motion seen in modal completion,

the perceived color in amodal completion is markedly higher in saturation and the organization

of the scene is different in that objects are perceived to lie behind a partially occluding screen.

Nonetheless, in both cases, the perception of motion is linked to a spread of color over regions

defined by motion. Hence, the mechanisms driving the color spread, whether a desaturated

veiling color (modal completion) appearing to glide over the display or a highly saturated disk

(amodal completion) moving behind a partially-occluding screen, are likely to be the same.

<Figure 3 here>

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xx.5 Luminance relationships shape visual scene organization in color from motion

In still view of our standard stimulus, the small test region of green dots is clearly seen as the

figure, and the surrounding region of red dots is seen as the ground. Regardless of the luminance

of the achromatic background, the test region is seen to move and color spread is linked to this

moving region. To explore the importance of figural cues, Chen and Cicerone (2002a) used

stimuli composed of alternating bands of equal widths of red and green dots whose luminance

contrasts, as compared to the achromatic background, could be manipulated. For such stimuli

without clear figure/ground configurations, we asked whether color from motion remains a

salient effect and how luminance relationships help to organize the visual scene. Regardless of

the luminance of the achromatic background, apparent motion and color spread are associated

with regions of lower luminance contrast. A switch in perception of seeing red bands moving or

green bands moving occurs at each observer’s point of equiluminance between the red and green

stimuli. The relative widths of the bands of red and green dots were varied to test whether

figure/ground cues could supersede luminance cues. Indeed, for bands that are thin enough

(roughly an 8 to 1 ratio for our observers), the narrower bands (more figure-like) are seen as

moving regardless of luminance contrast relationships (Chen & Cicerone, 2002a).

xx.6 Color from motion is regulated at a point beyond binocular combination

The dependence of color spread on the perception of apparent motion in this phenomenon and

the spread of color in the absence of contour formation suggest that the locus of the mechanisms

underlying color from motion may be at a point beyond binocular combination. Evidence

supporting this idea was obtained by the dichoptic presentation of every other frame of the full

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stimulus sequence to one eye and, out of phase, the alternate frames to the other eye. To

overcome binocular rivalry, the eye that did not receive the standard stimulus was presented with

a stimulus that was identical in every way, except that the test dots were absent. A compelling

perception of color from motion, as measured by a rating method, is seen that is equal to that

obtained when the full stimulus sequence is presented to each eye alone (Cicerone & Hoffman,

1997). This is consistent with the regulation of color from motion at sites beyond the

convergence of monocular pathways.

Perhaps even more persuasively, Chen and Cicerone (2002b) showed that depth, as well as form

and color, is recovered from apparent motion. Stimuli were presented dichoptically with left

and right eye views identical as to the locations of all dots. Binocular disparity was introduced

by means of translations in the color assignments for corresponding image elements in the two

eyes. Horizontal crossed or uncrossed disparities of 0.5 deg of visual angle were created by

differences in color assignments alone. In random presentation of the crossed or uncrossed

horizontal displacements, observers were asked to judge if the illusory figure defined by the

color spread was behind or in front of the field of dots. In still view of the stimulus, binocular

rivalry occurs and neither apparent motion, nor color spread, nor depth is seen. In this case,

observers performed at chance level when required to judge depth. When the frames are cycled

at an effective rate of 7 deg/sec over a vertical distance upward and downward spanning 10 deg

of visual angle, apparent motion, color spread, and depth are perceived. In random presentations

of stimuli for crossed and uncrossed disparities in motion view, observers performed well above

chance (95% confidence interval) with some observers performing with 100 per cent accuracy.

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xx.7 General discussion

The distinctive features of color from motion

The effect we call color from motion is distinctive in a number of ways. First, neither contour

formation nor color spread is seen in still view of our stimuli. In this way it is distinct from static

neon color spreading, an effect that is well known, as we have reviewed above. Furthermore,

illusory color spread as seen in color from motion is not a general feature in motion stimuli; for

example, it is not reported in kinetic occlusion. Second, in color from motion, there are no

spatial displacements of the dots; only the color assignments of the dots change from frame to

frame. Apparent motion and the attendant illusory color spread are generated only by the change

in chromaticity or luminance of the dots. To buttress this second point, we created stimuli in

which the test region remains fixed in space and the test dots were set in motion either 1)

independently and randomly; 2) in unison along the same trajectory; or 3) in unison along a

random trajectory. Naïve observers were tested with all of these stimuli, and none reported

seeing color spread (Chen & Cicerone, 2002a). Third, although the saturation of the illusory

color spread increases with increases in the luminance of the test dots, the luminance of the dots

in the surround region has no impact on the illusory color spread when the chromaticities of the

test and surround dots differ (Chen & Cicerone, 2002a). This differentiates color from motion

from color contrast, wherein the luminance of surround elements has considerable impact.

Fourth, subjective color spread is seen without the perception of a subjective contour near the

point of equiluminance between test and surround dots, as long as there is a chromaticity

difference between the dots (Miyahara & Cicerone, 1997). This result suggests that the spread of

illusory color in color from motion does not require the prior formation of a contour and that

color, independent of contour, can be recovered in tandem with seeing motion.

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When color from motion is seen in modal completion, the low saturation and neon-like quality of

the illusory color spread is reminiscent of the quality of the perception in displays of

transparency. Is color from motion the same as transparency? We argue that it is not for the

following reasons. First, the perception of transparency occurs in displays due to both figural

and luminance cues that are already present in the stimulus (e.g., Adelson, 1983; Beck, 1978; da

Pos, 1989; D’Zmura, Colantani, Knoblauch, & Laget, 1997; Metelli, 1974), whereas in color

from motion a new colored surface, with or without a border, is created by the visual system in

physically achromatic regions. In other words, when transparency is perceived, physically

present but differentiated regions are unified into a single perceptual layer, whereas in color from

motion, an entirely new, and illusory layer is constructed by the visual system. Second, motion

is not required for transparency to be perceived, whereas color from motion requires the

perception of apparent motion and is never seen in still view. Third, color from motion can be

seen in amodal completion (Cicerone & Hoffman, 1997; Chen & Cicerone, 2002a), a perception

that differs markedly from any of the characteristics of transparency.

Our results indicate that color from motion is regulated at a point beyond binocular combination

(Cicerone and Hoffman, 1997), that it requires the concurrent, if not prior, perception of motion

(Cicerone et al., 1997; Chen & Cicerone, 2002a); that in addition to form and color, depth can be

recovered in color from motion (Chen & Cicerone 2002b); and that figure/ground configuration

can override luminance relationships as the determinant of which areas appear to move and to be

filled with illusory color (Chen & Cicerone, 2002a). Considering our current understanding of

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visual processing in the primate brain, these findings suggest that the mechanisms supporting the

perception of color from motion include neural processing at higher levels.

The functional significance of color from motion

Can our results be related to the visual system’s ability to break visual camouflage? In natural

scenes, objects or surfaces may not be perceived because of the color, luminance, or texture of

nearby surfaces. We reasoned that if color from motion is a robust camouflage-breaking

mechanism, then it should be able to render the test object visible even when color is not an

obvious cue. The stimulus was modified so that a proportion (0 to 0.8) of the dots in the

surround region were green instead of all red. In still view, the test region with green dots was

not reliably seen; thus, in still view, the test region was effectively camouflaged. Nonetheless,

when the stimulus sequence was cycled as before and apparent motion was perceived, a moving,

illusory green disk was seen (Cicerone & Hoffman, 1997).

In other natural scenes, objects may be hidden from full view by occlusion. To mimic this

situation, we reduced the illumination of the achromatic background in our stimuli to show that

color from motion can be seen in amodal completion as a highly saturated green disk that moves

over a highly saturated red background, all seen through random perforations in a dark screen

(Cicerone & Hoffman, 1997; Chen & Cicerone, 2002a). The mode in which color from motion

is seen depends on figural cues and on regional differences in luminance contrast between the

chromatic elements and the achromatic background (Chen & Cicerone 2002a).

In still view, the physical representation of the scene may give an equivocal interpretation of

objects and surfaces. When the test is seen in apparent motion, subjective color spread helps to

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reveal the hidden object in modal or in amodal completion. Furthermore, not only form and

color, but also depth can be recovered in tandem with seeing motion. We propose that the neural

mechanisms that support perceptions in color from motion may be the same as those that work in

natural scenes to reveal form, color, and depth to the visual system, even when it is confronted

with fragmented physical information, as occurs in camouflage.

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Figure Captions

Figure 1. A typical display of color from motion is shown. Each frame (8 deg of visual angle on

a side) consists of an achromatic background field (CIE x = 0.276, y = 0.286, luminance 73

cd/m2) over which was randomly arrayed 800-1200 dots (each 3.5 min of arc in diameter).

Within a circular test region (typically 1 to 2 deg of visual angle in diameter), the dots were

colored green (CIE x = 0.280, y = 0.610). All other dots were red (CIE x = 0.621, y = 0.344).

The luminance of the red and green dots could be independently varied. To create successive

frames, no dots changed their locations within the frame; only the color assignments of the dots

were changed, according to a uniform vertical displacement (0.12 deg of visual angle) of the test

region in successive frames. On the left: Still views of successive frames are depicted. On the

right: When frames are cycled, typically with an effective displacement rate of the test region

equivalent to 7 deg/sec, up and down over a vertical region spanning 5 deg of visual angle, an

illusory green disk moving up and down pops into view and a spread of illusory green color is

seen in the physically achromatic test region.

Figure 2. (Adapted from Chen & Cicerone, 2002a) The salience of the color spread as measured

with cancellation is linked to the salience of the apparent motion. We varied the effective rate of

vertical translation of the test region between zero and 12 deg of visual angle per sec. The

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results for two observers can be well described by two linear functions with a steeply sloping

first branch (from an effective speed of translation from zero to 1 deg/sec) and a second branch

of much reduced or zero slope (from 1 deg/sec to 12 deg/sec).

Figure 3. There are two modes in which color from motion can be perceived, either in modal

completion (left) as a localized change of illumination, a colored spotlight or shadow, moving

over a textured surface or in amodal completion (right) as a moving, colored object seen through

apertures in an occluding surface. The mode in which color from motion is seen depends on

figural cues and regional differences in the luminance contrast between the chromatic elements

and the achromatic background. Regions with figural cues tend to be seen as moving. These

regions are seen in modal completion if their defining figural elements (dots in this case) are of

lower luminance as compared to the background and in amodal completion if the defining figural

elements are of higher luminance as compared to the background.

 

Still View of Single Frames Apparent Motion

Speed (deg/sec)

Canc

ella

tion

Valu

e (c

d/m

)2

0 5 10 15

0.2

0.4

0.6

ModalAmodal

Apparent Motion

Modal Amodal

Apparent Motion