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Subjective color from apparent motion
Abstract
In an effect we call color from motion (CFM), apparent motion is accompanied by subjective color spread seen in physically achromatic regions. Here we report that physical lights can cancel the subjective color seen in CFM. As measured by cancellation, the saturation of the subjective color spread increases as the luminance of the test elements increase. Without luminance differences between test and surround elements, chromaticity differences alone can result in the perception of subjective color spread. In this case, subjective color spread is seen without seeing a subjective contour, suggesting that CFM does not require contour formation and that color — independent of contour — can be recovered in tandem with seeing motion. There are two modes in which CFM is perceived, either (1) as a localized change of illumination, a colored spotlight or shadow, moving over a textured surface or (2) as a moving, colored object seen through holes in an occluding surface. The mode in which CFM is seen depends on figural cues and on regional differences in luminance contrast between the chromatic elements and the achromatic background. Regions with distinct figural cues are always seen as moving; and CFM is seen in the first mode if the regions are of lower luminance contrast and in the second mode if the regions are of higher luminance contrast. Without figural cues, the regions of lower luminance contrast are always seen to move and CFM is seen in the first mode.
History
Received January 31, 2002; published October 7, 2002
Citation
Chen, V. J. & Cicerone, C. M. (2002). Subjective color from apparent motion.
Journal of Vision, 2(6):1, 424-437,
Keywords
color, color from motion, apparent motion, motion, modal completion, amodal completion, illusory color, luminance contrast
In static scenes, the human visual system can use
fragments of information to perceive form by constructing subjective contours
and color to fill in the missing parts of occluded objects through amodal
completion (Michotte, Thines, & Crabbe,
1964; Kanizsa,
1979;
Nakayama, Shimojo, &
Ramachandran, 1990; Nakayama and
Shimojo, 1990, 1992; Yamada, Fujita, & Masuda, 1993; Grossberg 1994). In the classic
neon color
spreading effect, the visual system uses another filling-in process: In static
stimuli, illusory color enhances the perception of an object that is already
suggested by fragments of its contour, shape, and color (Varin, 1971; van Tuijl, 1975). The perception of motion
can also effectively break camouflage (Wertheimer, 1923). When something is
partially occluded by other objects in the viewer’s line of sight,
relative motion can reconstruct the shape of the occluded object from
successive
partial views over time (kinetic occlusion), albeit with some distortion (Kaplan, 1969; Gibson, 1979; Andersen & Braunstein, 1983; Yonas, Crayton, & Thompson, 1987; Andersen & Cortese, 1989; Stappers, 1989; Shipley & Kellman, 1993, 1994). Recently, Cicerone, Hoffman, Gowdy, and Kim (1995)
introduced an effect called color from motion (CFM) for which the perception of
apparent motion is accompanied by a perception of subjective color, spreading
into achromatic regions of the stimulus (see also Cicerone & Hoffman, 1992, 1997; Shipley & Kellman,
1994;
Miyahara & Cicerone,
1997).
A typical CFM stimulus consists of multiple frames,
each composed of an achromatic background over which lies a random array of
small dots whose locations are fixed from frame to frame (Figure 1,
left).
 Figure 1. A still
view of a single frame of the stimulus is shown on the left. As shown on the
right, when frames are presented rapidly in succession, apparent motion is seen
and subject color spreads into achromatic portions of the test region.
The dots within the test region (in this case, a disk)
are assigned one color while the dots in the surround region are assigned a
different color. The test region is then translated across the field of dots by
changing only the color assignments of the dots from frame to frame while
keeping dot locations unchanged in all frames. In this example, the test dots
are green, the surround dots red, and the achromatic background is of high
luminance. In still view, a single frame is seen as a field of randomly
scattered colored dots against a uniformly achromatic background (Figure 1, left). When frames are presented rapidly in
succession, an illusory disk pops into view and appears to move with a velocity
consistent with the translation of the test region. Linked to the perception of
apparent motion, subjective color is seen to spread into the
achromatic parts of
the test region (Figure 1, right).
The effect can be seen in Movie 1
showing stimuli with green test dots of higher luminance than that of the red
surround dots and a translation of the test region equivalent to a speed of 7
deg/s. Green subjective color can be seen spreading over the physically
achromatic portions of the test region, and a clear subjective contour is seen.
(Variations in luminance and chromaticity in different displays may give a less
vivid effect in some demonstrations than were obtained in the experimental
conditions.)
Movie 1. The
color from motion effect is demonstrated with frames composed of an achromatic
background of high luminance, green dots in the test region, and red
dots in the
surround. The green dots are of higher luminance than the red dots.
Monitors and
calibrations can vary. To enhance viewing this and other demonstrations, use a
CRT display or a projector; avoid laptop LCD displays; reduce the ambient
illumination; and avoid tracking the apparent motion.
The CFM effect is distinctive in a number of ways.
First, neither contour formation nor neon color spreading is seen in still view
of a single frame of the CFM stimulus. Second, in CFM displays, the only change
from frame to frame is the color assignment of the dots; dot locations never
change. Apparent motion and subjective color spread are generated strictly by
the change in the chromaticity or luminance of the dots. In fact, if the test
region remains fixed in space while the dots in the test region are set in
random motion, no color spread is observed. Third, subjective color
spread yoked
to motion is not reported in other effects involving the reconstruction of
objects via motion; for example, it is not reported in structure from motion or
in kinetic occlusion (Kaplan, 1969; Gibson, 1979; Andersen & Braunstein, 1983).
In this work, we report that a physical light can
cancel the subjective color seen in CFM. As measured by cancellation, the
saturation of the subjective color spread increases as the luminance
of the test
dots increase, whereas the luminance of the surround dots has relatively little
effect. We also report that in the absence of luminance differences
between test
and surround elements, chromaticity differences alone can support the
perception
of subjective color spread, which, in this case, is seen in the absence of a
subjective contour. This suggests that CFM does not require contour formation
and that color — independent of shape — can be recovered in tandem
with seeing motion. When clear figure versus ground cues are lacking,
regions of
lower luminance contrast (between the chromatic elements and the achromatic
background) are the regions that are seen to move and over which subjective
color spread occurs. Spatial configurations (figure/ground) can supersede
luminance contrast relationships. Regions that appear as figure, even when they
are of higher contrast, are seen as moving, in this case, as if behind a
partially occluding surface.
Previous studies on CFM used rating methods to measure
the perceived strength of the subjective color spread (Cicerone et al., 1995; Cicerone & Hoffman, 1997). Rating
methods worked well in establishing the overall impression of the salience of
the color spreading effect. However, certain aspects of the effect, such as the
saturation of the subjective color, require a more quantitative methodology. Miyahara and 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. One drawback of the side-by-side
matching method is that it requires the observer to compare the
subjective color
spread in CFM with a physical light that is situated in a different
location and
context. The impact of two possible contaminating factors was not controlled:
(1) The matching stimulus was stationary whereas the CFM stimulus was perceived
as moving, and (2) the matching stimulus was a homogeneously colored disk
without test or surround dots as in the CFM stimulus. One option
would have been
to pursue the color matching technique using a moving physical matching light
situated in a field of dots.
Instead, in this work, we use a real light to cancel
the subjective color spread in CFM. The test dots were produced solely with the
green gun of the CRT display, the surround dots solely with the red
gun, and the
achromatic background by a combination of red, green, and blue guns in fixed
proportion. Results of previous studies show that the desaturated subjective
color spread over the test region has a chromaticity similar to that
of the test
dots (Miyahara & Cicerone, 1997).
Therefore, the cancellation stimuli were produced by subtracting, from the
background in the test region, small amounts of light of the same chromaticity
as the test dots. Of course, removing green light results in a reduction in
luminance of the background of the test region. Luminance was kept constant
across the background of both test and surround regions by adding compensating
amounts of red and blue lights – in the same fixed proportion as was used
to produce the achromatic background – to the background of the test
region. Hence, the cancellation stimulus is the amount of green light that has
to be subtracted from the background of the test region, while
holding luminance
constant, for a perception of a uniform background across the stimulus. The
observer’s task on any trial is simply to judge whether or not the
background in the test region appears to be reddish or
greenish.
Figure 2. The
cancellation stimulus, which appears
reddish, is shown on the left in still view. As shown on the right, when frames
are presented rapidly in succession, the subjective green color spread due to
color from motion is cancelled and a homogeneous background is seen throughout
the display.
The cancellation stimulus, which appears reddish, is
depicted in Figure 2 (left). When the frames are cycled
and apparent motion is seen, subjective green color spread is nulled by the
cancellation stimulus and a homogeneous achromatic background is seen, as shown
in Figure 2 (right).
Participants.
Data were collected on two observers who were highly practiced and color normal
(as assessed by color matches on the Neitz anomaloscope). Observers A
and B were
the authors.
Apparatus and
stimuli. The stimulus was a square, each of whose sides subtended 8
degrees of visual angle viewed from a distance of 57 cm. The area of the CRT
display outside the stimulus was set to the lowest luminance value and appeared
black. The 2-degree test region contained dots colored green (CIE x =
0.280, y =
0.610) produced solely by the green gun of the CRT display; the surrounding
region contained dots colored red (CIE x = 0.621 y = 0.344) produced solely by
the red gun of the CRT display or green of the same chromaticity as the dots in
the test region; the background of the surround region was achromatic (CIE x =
0.276 y = 0.286; 73 cd/m2) produced by fixed proportions of the red,
green, and blue guns of the CRT display. Individual dots were in fact squares,
3.5 minutes of arc on a side, composed of 9 adjacent pixels. The test
region was
translated up and down over a range of 5 degrees of visual angle at a
displacement rate equivalent to 7 deg/sec. In the test region, varying amounts
of green light were subtracted from the background of the test region (the
cancellation stimuli), while keeping luminance constant and equal to
that of the
background in the surround region by adding compensating amounts of
red and blue
lights in the same fixed proportion as was used to produce the achromatic
background. We plot, as cancellation value, the amount of green light (in
luminance units) subtracted from the background region. There may be
a number of
alternative units, but all sensible units would be equivalent to our chosen
unit, given that the chromaticity of the subjective color spread is
equal to the
chromaticity of the test dots. The stimuli were presented on a 21-inch Sony
Trinitron CRT monitor driven by a Silicon Graphic Indigo II computer programmed
using Open GL. The mapping between RGB values and output luminance of the three
guns was measured (Photo Research model PR-650 Spectracolorimeter) and a gamma
correction was applied to each gun to yield a linear function. The green test
dot luminance was set at 18 or 36 cd/m2. The red surround dot
luminance was set at 4.5, 6, 9, 12, and 18 cd/m2.
Procedures.
Employing a two-alternative multiple-staircase forced response procedure, CFM
displays with varying amounts of the cancellation stimulus were presented on
each trial. Sitting in a darkened room, the observer was instructed to maintain
fixation near the center of the display to judge whether the test region
appeared to be greenish or reddish and to press response keys accordingly. The
experiment was self-paced, with no fixed duration for each trial. The observer
was free to view each stimulus for as long as necessary to make a decision.
During each session, there were six conditions, with two separate staircases
(one with an initial descent and the other an ascent) per condition, randomly
presented to the observer. Staircases terminated after three reversals. This
procedure made it difficult, if not impossible, for the observer to track the
progression of any particular
staircase.  Figure 3. Plotted
are the data (± 1 standard error of the mean) for Observers A (left) and B
(right) with stimuli consisting of green test dots and red surround dots.
Cancellation values increased as the luminance of the green test dots increased
from 18 (squares) to 36 (triangles)
cd/m2.
The surround dot luminance varied between 4.5 and 18
cd/m2.
The best-fitting straight lines to Observer A’s data are shown (y =
-0.0025x + 0.4938 at top and y = -0.0008x + 0.3589 at bottom). The best-fitting
straight lines to Observer B’s data are shown (y = -0.0011x + 0.4559 at
top and y = 0.0009x + 0.3490 at bottom). The 95% confidence interval for the
slope of each line includes zero. Surround dot luminance within the
range tested
does not affect the cancellation value.
 Figure 4. Plotted
are the data (± 1 standard error of the mean) for Observers A (left) and B
(right) with stimuli consisting of green test dots and green surround dots.
Cancellation values increased as the luminance of the green test dots increased
from 18 (squares) to 36 (triangles)
cd/m2. The surround dot
luminance was varied between 4.5 and 18
cd/m2. The best-fitting
straight lines to Observer A’s data are shown (y = -0.0072x + 0.4619 at
top and y = -0.0151x + 0.3358 at bottom). The best-fitting straight lines to
Observer B’s data are shown (y = -0.0039x + 0.4083 at top and y = 0.0030x
+ 0.2192 at bottom). The 95% confidence interval for the slope of
each line does
not include zero, with the exception of the line through the data obtained with
an 18 cd/m2 test dot
luminance for Observer B. Surround dot luminance within the range tested does
affect the cancellation value. The cancellation value decreases as the
difference in luminance of the test and surround dots decrease.
There was a systematic increase in the amount of the
physical light required to cancel the subjective color spread as the luminance
of the green test dots increased from 18 to 36 cd/m2. This result
holds whether the outer dots are red (Figure 3) or green
(Figure 4). Cancellation methods produce results that
confirm those of earlier experiments (Miyahara & Cicerone, 1997) using
side-by-side color matches showing that 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 Figure 3 shows, when the test
and surround dots are of differing chromaticity, the surround dot luminance has
little impact on the subjective color spread, as measured by cancellation,
suggesting that the mechanisms underlying this effect are different from those
for color contrast.
Whereas both color and luminance distinguish the test
dots from the surround dots in the conditions represented in Figure 3, only luminance provides the difference
between test
and surround dots for the conditions of Figure
4 in which all dots share the same chromaticity. As test dot
luminance increases, there is an increase in the cancellation value,
as before. However, the results show that for fixed values of the
test dot luminance, color spread as measured by cancellation is
dependent on surround dot luminance: There is a decrease in the
cancellation value as surround dot luminance increases. We note that
without chromaticity differences, a luminance difference –
between
the test dots and the surround dots – is the only basis for the CFM
effect. Hence, as test dot luminance and surround dot luminance become similar,
the effect should dissipate, and it does.
Both chromatic differences and luminance differences
play a role in CFM (Figures 3 and 4). The results of Figure 4 show that
luminance differences alone are sufficient to produce CFM. Is a luminance
difference required for the perception of color from motion?
One of the conditions shown in Figure
3 provides an answer. When the luminance levels of both the test and the
surround dots are equal to 18 cd/m2, the cancellation value is
similar to that for all other conditions with the same test dot luminance,
regardless of the luminance of the surround dots. Color spread is seen in the
absence of a luminance difference between test and surround dots, as long as
there is a chromaticity difference. 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 as smooth” and
“slower” than the conditions in which the test and surround dots
differ in luminance. These results are consistent with the
established view that
the neural processes responsible for illusory contours rely largely
on luminance
information (Kanizsa, 1979; Marr, 1982; von der Heydt, Peterhans, &
Baumgartner,
1984). That subjective color spread is clearly seen for conditions in which
test and surround dots are equiluminant indicates that the mechanisms
underlying
CFM are likely to be distinct from those that produce the perception of
subjective contours. It should be noted that when a clear illusory contour is
seen, the color spread is contained within the boundaries of the illusory
contour, whereas when a clear contour is not seen, the subjective color appears
to change its configuration over time. (One observer described the percept as
“a moving, water-filled, colored balloon seen through a
fog.”) It is
possible that the slow and unsteady motion and lack of a clear
contour perceived
in the equiluminant condition are related to the properties of motion
processing
based on chromaticity differences alone (Cavanagh et al., 1984). It is noted that
the color spread with equiluminant test and surround dots, as measured by
cancellation and plotted in Figure 3, is just as salient
as the conditions with surround dots of lower luminance. Movie
2 is a depiction of this effect.
Movie 2. This
demonstrates that color spread is seen in the absence of a luminance difference
between test and surround dots, as long as there is a chromaticity difference.
Observers reported that there is no clear contour bounding the region of the
subjective color spread and that apparent motion is “not as smooth”
and “slower” than the conditions in which the test and
surround dots
differ in luminance.
Subjective color spread in CFM is yoked to
the perception of the motion of the test region (Cicerone et al., 1995). As such,
it might be expected that the saturation, for example, of the
illusory color in CFM, might depend on the salience of the apparent
motion. In Experiment 2, we asked whether the subjective color seen
in CFM, as measured by the cancellation method, changes as the
translation speed of the test region is varied.
Participants.
Observers A and B participated in this experiment.
Apparatus and
stimuli. The basic stimulus design for this experiment was the
same as in
Experiment 1. The green test dot luminance was fixed at 36
cd/m2. The
red surround dot luminance was set at 4.5, 9, or 18 cd/m2. The
translation speed, resulting from the rate of change of the color of
the dots in
the test region, was systematically varied from 0 to 12 degrees of visual angle
per second. As in Experiment 1, the test region was translated up and down in a
range spanning 5 degrees of visual angle.
Procedures.
The procedures in this experiment were the same as in Experiment 1.
The results (Figure 5) for each
observer can be described reasonably well by two linear functions with a
markedly higher slope for the first linear branch compared to the second. The
cancellation value for the subjective color spread increases rapidly in the
first branch as speed increases to about 1
deg/s.  Figure 5.
Cancellation values as a function of speed varying between 0 and 12 degrees of
visual angle per second are shown for Observer A (closed symbols) and
Observer B
(open symbols). The luminance of the test dots was fixed at 36
cd/m2 and that
of the red
surround dots was 4.5 (circles), 9 (diamonds), or 18 (squares)
cd/m2. The best-fitting
straight lines to Observer A’s data are y = 0.0209x + 0.0149
for x ≤
1 and y = -0.0001x + 0.0367 for x ≥ 1. The 95% confidence
interval for the slope b of the
second line describing Observer A’s results is -0.0005
≤ b ≤ 0.0003. The
best-fitting straight lines to Observer B’s
data are y = 0.0135x + 0.0278 for x ≤ 1 and y = -0.0015x + 0.0386 for x
≥ 1. The 95% confidence interval for the slope b ofthe second line describing Observer
B’s results is 0.0012 ≤ b ≤ 0.0018.
For Observer A, the data for speeds greater
than 1 deg/s are well fit by a line of slope near zero (95%
confidence interval for slope b,
-0.0005 ≤ b ≤ 0.0003).
For Observer B, although the rate of
increase in the second branch is not zero, the results indicate a
highly reduced
rate (slope decreases 15-fold) of change in color spread as speeds increase
beyond 1 deg/s. This profile of the results suggests an all-or-none
relationship
between apparent motion and subjective color spread.
Although there appears to be no color spread in still
view of single frames of the CFM stimulus, we decided to perform the
cancellation experiment with a single still frame. There is a small but
significant color spread as measured by the cancellation method
(results plotted
for 0 deg/s in Figure 5). This indicates that the
cancellation method is highly sensitive. One possibility is that the color
spread in still view can be attributed to the well-known assimilation effect
– von Bezold’s (1874)
spreading effect. It is beyond the scope of this study to determine the exact
source of the measured color spread in static view. For our purposes, it
suffices to note that each of the measurements of color from motion in Figure 5 includes a constant value of color spread inherent
in still view of a single frame, whatever the source, and, more important, that
the results cannot be accounted for by color spread attributable to static
effects alone.
As the translation speed of the test region
increases,
the perception of the illusory disk as separate from the field of dots becomes
more salient. The perception of separation is nearly absent until translation
speeds of 1 degree of visual angle per second are exceeded. For speeds greater
than 1 degree of visual angle per second, there is an increased tendency for
observers to report seeing the subjective color spread and its associated
illusory disk as a patch of light totally separated from and lying over the
field of dots. At the same time, the test dots appear to assume the
color of the
surround dots, resulting in the perception that all of the dots are of the same
color, in this case red. Thus, it appears that translation speeds
greater than 1
degree of visual angle per second produce little or no enhancement of the
subjective color spread but can influence the salience of separation
between the
figure defined by the subjective color spread and the array of dots.
The subjective color spread in CFM for the stimuli of
Experiments 1 and 2 has a neon or glowing quality with the illusory figure,
defined by the color spread, moving over the array of dots. We call
this percept
modal completion and classify it with
other effects such as transparency and the neon star (Michotte et al., 1964; Varin, 1971), for which there is color
spreading into physically achromatic regions interpreted as a spotlight,
transparent layer, or shadow. In natural scenes, objects that are screened from
full view are perceived as moving behind screening elements. A previous study
(Cicerone & Hoffman,
1997) described
CFM aligned with this natural situation. In this case, a perceptually complete,
moving object is seen through holes in an occluding surface. The
perceived color
of the object exactly matches that of the dots. We call this percept
amodal completion to signify that the
observer perceives the presence, the shape, and the color of an object behind
the occluding surface. In Experiment 3, we systematically varied the background
luminance while holding test dot luminance constant. In the static display,
lowering the luminance of the background progressively produces the impression
that the dots are sources of illumination – aperture colors rather than
focal colors. Therefore, we hypothesized that there should be an abrupt switch,
from modal to amodal completion, near the point of equiluminance between test
dots and the background region. This effect is demonstrated in Movie
3.
Movie 3.
The red and
green dot luminance levels for the display on the left match those for the
display on the right. The only difference between left and right
displays is the
luminance level of the background. When frames are animated, modal
completion is
seen on the left and amodal completion is seen on the right.
Participants.
The same two observers of Experiments 1 and 2 participated in this
experiment.
Apparatus and
stimuli. The CFM stimulus for this experiment was the same as that in
Experiment 1 with the following exceptions. Three combinations of test (green)
and surround (red) dot luminances were used: 18 cd/m2 and 18
cd/m2, respectively; 9 cd/m2 and 18 cd/m2,
respectively; 18 cd/m2 and 9 cd/m2, respectively. The
luminance of the achromatic background region ranged from 3, 6, 9, 12, 15, 18,
21, 24, 27, 30, to 33 cd/m2.
Procedures.
The observers were first asked to view a range of stimuli, while maintaining
fixation near the center of the display, and were asked to describe what they
saw. The observers spontaneously described the percept in one of two ways: In
the first, a green color spread of low saturation that is likened to a green
spot light is seen moving over a field of red dots. This was
classified as modal
completion. In the second, a colored, moving shape is seen as if through holes
in a dark surface. The object appears to lie behind the dark surface and its
color appears to match the color of the green test dots. This was classified as
amodal completion. As the luminance of the achromatic region was
randomly varied
between 0 and 54 cd/m2, the observers were asked to
identify which of
the two possible percepts was seen. Equiluminance for each observer
was based on
flicker photometric matches for the red, the green, and the achromatic stimuli
using circular fields of diameter 2 degrees of visual angle, matching the size
of the test region.
The results show an abrupt
switch from modal to amodal
color spread that occurs near the point of equiluminance between the test dots
and the achromatic region (Figure 6).
Observers reported that when
modal completion is
perceived, the colored dots appear to be textures lying on a bright
surface. The
color spread in modal completion appears to be interpreted as a spotlight that
produces a change in the luminance and color over the test region, which
otherwise is identical to those in the surround. When amodal completion was
perceived, the colored dots appear to be holes in a dark screen, lying in the
foreground, through which a bright, uniformly colored, moving object
is seen. In
either case, the color spread was seen to belong to an object separate from the
field of dots.
 Figure 6. Results
for Observer A are shown. The proportion of trials on which amodal
completion is
seen was plotted as a function of background luminance, varying
between 3 and 33
cd/m2. The results were
similar for conditions in which green dot luminance equaled red dot luminance
set at 18 cd/m2
(circles); green dot luminance set at 9
cd/m2 and red dot
luminance at 18 cd/m2
(squares); and green dot luminance set at 18
cd/m2 and red dot
luminance at 9 cd/m2
(triangles).
The appearance of the subjective color spread can be
strikingly different, depending on the luminance of the background field.
Changes in the luminance of the background appear to determine whether the dots
are perceived as texture on a reflective surface or as apertures in
an occluding
screen. In the former case, modal completion occurs and observers judge that
color spread lies on top of the field of dots; in the latter case, amodal
completion occurs and observers judge that an object is moving behind the
surface of the dots. In either case, there appears to be a clear separation of
the color spread and the surface containing the dots. These
conclusions apply to
the special case of a stimulus with clear figure/ground cues; therefore, the
next experiment was designed to consider the impact of luminance relationships
for stimuli without figure/ground cues.
In still view of the stimuli in Experiments 1, 2, and
3, the small circular test region is clearly seen as the figure and the region
of red dots as ground. In this case, the test region is always seen to move and
color spread is linked to this moving region. Suppose figure/ground
configuration is not as evident. Do the luminance relationships among green
dots, red dots, and the background become more important in
determining the area
that is seen to move and over which subjective color spread is seen? To answer
this question, the first set of stimuli for Experiment 4 were composed of bands
of equal width, alternately filled with green and red dots. Luminance contrast
between the red dots and the background compared to that between the green dots
and the background was varied. Based on the results of Experiment 3, we
predicted that apparent motion would be seen more readily in the regions of
lower contrast (Figure
7).  Figure 7. The green
dot luminance is higher for the stimulus at the top compared to that at the
bottom; in all other respects, the stimuli are identical. For the top stimulus,
green dot contrast against the background is less than red dot contrast against
the background. When frames are animated, we predict that observers will see a
uniformly colored green band moving over a field of stationary red dots. When
red dot contrast is less than green dot contrast (bottom), we predict that a
uniformly colored red band will be seen moving over a stationary field of green
dots.
If apparent motion triggers color spread, as we have
argued based on the results of Experiment 2, then color spread should
also occur
over the regions of lower contrast. In the second part of Experiment 4, we
reduced the width of the band of green dots to make this region appear more
figure like.
Participants.
Observers A and B of Experiments 1 and 2 participated in this experiment.
Apparatus and
stimuli. In the first set, the stimulus frame was divided into four
vertical strips of equal width. The dots within each strip were all
assigned one
color, alternating red and green. Multiple frames were created by a uniform
horizontal translation of the color assignment. Frames were cycled at a rate
equivalent to 7 deg/s. The luminance of the achromatic region was set at zero
for the low background luminance condition and at 81 cd/m2 for the
high background luminance condition. The luminance of the red dots in the
surround was held constant at 18 cd/m2. The green test dot luminance
ranged from 4.5, 9, 13.5, 18, 27, 36, to 54 cd/m2. In the
second set, the width of the strip in which dots were colored green
was decreased from one-fourth, one-eighth, to one-sixteenth of the
screen width.
Procedures.
Equiluminance between the red and the green stimuli was measured by the method
of minimally distinct borders (Boynton,
Hayhoe, & MacLeod, 1974). For every experimental condition, observers
were instructed to report whether they saw the red bands or the green bands
moving.
The results of Experiment 4 are shown in Figure 8 for low and high luminances of the background.
Regardless of background luminance, apparent motion and color spread are
associated with regions of lower luminance contrast. The switch in perception
between seeing red bands moving or green bands moving occurs abruptly at each
observer’s point of equiluminance between red and
green.
Movie 4. Here
the contrast between the luminance of the green dots and luminance of the
background is less than that between the red dots and the background. A
uniformly colored green band is seen moving over a stationary field of red
dots.
 Figure 8. The
horizontal axis plots the ratio of the green dot contrast against the
background
compared to red dot contrast against the background. The vertical
axis plots the
proportion of trials on which a green band is seen to move over a field of red
dots. Closed squares plot the results for a low luminance background. Open
circles plot the results for a high luminance background. The diamond plots the
equiluminance point between red and green as measured by the method
of minimally
distinct borders.
When clear figure/ground cues are lacking, luminance
contrast between the chromatic elements and the achromatic background
determines
which regions are seen as moving and filled with subjective color. When green
dot contrast against the background is less than red dot contrast against the
background, observers see a uniformly colored green band moving over a field of
stationary red dots in the CFM stimulus, and vice versa. Movie
4 demonstrates this effect for the condition in which the contrast between
luminance of the green dots compared to the luminance of the background is less
than that between the red dots and the
background.
Movie 5. This
movie is identical to the previous one, except for a lower background
luminance.
Now the red dot contrast is less than green dot contrast against the
background.
Observers see uniformly colored red bands moving over a field of green
dots.
As shown in the results of
Figure
8 and depicted in Movie 5, the same contrast
rule holds
for backgrounds of low luminance. It is always the case that apparent
motion and
subjective color spread are associated with regions of lower contrast.
As figure/ground cues are
enhanced, will regions of
lower contrast still be seen to move? The width of the band of green dots was
successively narrowed with the idea that a narrow enough band should
appear like
a figure against a larger background of red dots. The results are shown in Figure 9. As shown in these data, configurations
of the test and surround elements that support figure/ground
interpretations can supersede luminance contrast relationships so
that regions that appear as figure rather than ground are seen as
moving. When the width of the band of green dots is large, either
one-half or one-fourth of the width of the screen, the region
containing the dots with lower contrast relative to the background is
seen to move, whether the dots are colored red or green. When the
width of the band of green dots is reduced to one-eighth (Observer B)
or one-sixteenth (Observer A), the likelihood of seeing the green
band moving is enhanced, even if the red dots have lower contrast
relative to the background.
Movie 6. This movie uses a thin figure-like region
of green dots and the same luminance contrast relationships as Movie 5. Given the luminance
contrast relationships, the red band is always seen as moving in Movie 5. The figural cue enhances the likelihood of seeing a
moving green band in Movie 6.
 Figure 9.
>The horizontal axis plots the ratio of the green dot contrast
>compared to red dot contrast against the background. The vertical
>axis plots the proportion of trials on which the green band is seen
>to move over a field of red dots. The band of green dots spanned
>one-half (squares), one-fourth (circles), one-eighth (triangles), or
>one-sixteenth (diamonds) of the width of the screen. The results for
>a high luminance background are plotted. Results are similar for a
>low luminance background.
Movie 6 illustrates
that figure/ground configuration can supersede luminance
relationships. A figure-like region (in this example, a thin strip of
green dots) is seen to move even if in that region the luminance of
the dots against the luminance of the background is of higher
contrast compared to the luminance contrast of the region of red
dots.
In this study, a quantitative measure of the
subjective
color spread in CFM was established using cancellation with physical lights.
This measure served as the basis for assessing the roles of chromaticity and
luminance on the subjective color perceived in color from motion. The
saturation
of the subjective color spread appears to depend on the luminance of the test
dots but not on the luminance of the surround dots. This experimental
observation suggests that mechanisms regulating color from motion are not the
same as those for conventional color contrast, which is strongly dependent on
surround luminance. With a luminance difference between the dots in
the test and
those in the surround region, observers perceive an illusory contour that
borders the color spread. Without a luminance difference between test and
surround dots, chromaticity differences alone are sufficient to produce
subjective color spread in CFM. In this case, color spread occurs without an
illusory contour. This suggests that the neural substrate for the subjective
color spread is likely to be distinct from that generating illusory contours.
Furthermore, subjective color spread without a subjective contour is consistent
with the view that higher-level interactions may influence the perception of
seemingly primitive features like brightness and color (e.g., Nakayama et al., 1990; Merigan & Maunsell, 1993; Cicerone et al., 1995).
Although perceived motion of the test region
in the CFM
display is critical for subjective color to be seen, translation speeds greater
than 1 deg/s do little to enhance the salience of the subjective
color spread as
measured by cancellation. Thus, motion appears to act like a gating mechanism
for the perceived color spread. The perception of motion and subjective color
spread in CFM is also accompanied by a perceptual rearrangement of the visual
scene such that an object appears to be moving in front of or behind the field
of dots. It is possible that these perceptions, gleaned from
fragmented physical
information, may be due to representations of visual objects and the scene as a
whole at relatively high levels of visual processing, as noted above.
The CFM effect is distinctive in a number of ways.
First, neither contour formation nor neon color spreading is seen in still view
of a single frame of the CFM stimulus. In this way, it is clearly
different from
static neon color spreading, an effect that is already well established.
Furthermore, subjective color spread as seen in CFM is not present in
all motion
stimuli; for example, it is not reported in kinetic occlusion. Second, in CFM
displays there is no spatial dislocation of the dots; the only change
from frame
to frame is the color assignment of the dots. Apparent motion —
accompanied by subjective color spread — is generated strictly by the
change in chromaticity or luminance of the dots. To reinforce this point, we
asked naïve observers to view the following stimuli in which the test
region remains fixed in space and the test dots themselves are set in motion
either (1) independently and randomly; (2) in unison along a linear trajectory;
or (3) in unison along a random trajectory. In all of these cases, none of the
observers reported seeing color spread associated with the physical motion of
the test dots. Third, as shown in Experiment 1, the luminance of the
dots in the
region surrounding the test have no influence on the subjective color spread
when test and surround dots are of different chromaticity This is consistent
with the view that color from motion is distinct from color contrast. Fourth,
subjective color spread is seen without the perception of a subjective contour
when test and surround dot luminance levels are comparable, as long as there is
a chromaticity difference. This result presents difficulties for
explanations of
CFM requiring the prior formation of contours before the filling-in of illusory
color. Furthermore, this result supports the view that color —
independent
of contour — can be recovered in tandem with seeing motion.
Metelli (1974)
developed luminance criteria that are critical for the perception of
transparency of achromatic surfaces. The model specifies that transparency is
perceived in bounded regions of reduced contrast. This principle serves well to
identify the presence of transparent filters or shadows in the natural
environment, because both filters and shadows produce regional reductions in
luminance contrast. The conditions under which transparency is perceived have
been posed as requirements based on luminance or lightness (Metelli, 1974; Beck, 1978; Beck, Prazdny, & Ivry, 1984; Gerbino, Stultiens, Troost, & de Weert,
1990; Fukuda & Masin 1994); on
spatial configuration (Tudor-Hart,
1928; Heider, 1932; Watanabe & Cavanagh, 1993; Adelson, 1993); on motion (Adelson & Movshon, 1982; Kersten, Bülthoff, Schwartz, & Kurtz,
1992; Mulligan, 1993); on stereo
disparities of stimulus components (Nakayama
et al., 1990); and on chromatic properties (da Pos, 1989; Faul, 1996; D’Zmura, Colantoni, Knoblauch,
& Laget;
1997; Chen & D’Zmura,
1998).
Is color from
motion the same as transparency? When color from motion is seen in
modal completion, the low saturation and neon-like quality of the
subjective color spread is reminiscent of the quality of the
perception in transparency. Nonetheless, we argue that the subjective
color seen in CFM and that seen in transparency are distinct for the
following reasons. First, as noted above, the perception of
transparency occurs in classical displays due to both figural and
luminance cues already present in the stimulus. On the other hand, in
CFM, a layer of illusory color is perceived to spread into physically
achromatic regions of the stimulus to create a new colored surface,
with or without a border. In other words, when transparency is
perceived, physically differentiated surfaces are conjoined to create
a unified perceptual layer, whereas in CFM, an entirely new surface
is created by illusory color spread. Second, motion is not required
for transparency to be perceived, whereas subjective color spread in
CFM is seen only in tandem with apparent motion and is never seen in
still view of single frames. Third, for conditions in which modal
completion of a neon-like color spread is not perceived, CFM is still
achieved, in this case by amodal completion in which the moving,
colored object is perceived to lie behind — rather than in
front of — a
partially occluding surface. Examples are presented in Movie
3, right, and in Movie 6.
A number of studies have suggested that the visual
pathway is separated into independent channels of feature processing (as
reviewed in, e.g., Lennie, 1980; Merigan & Maunsell, 1993). In
particular, anatomical or physiological linkages that serve as a possible
substrate for cross talk between motion processing and color
processing have not
been identified. The perception of color from motion raises the possibility of
such interactions. Is there evidence supporting the idea that such interactions
may occur at an early stage of visual processing?
Stimuli that produce the perception of modal
completion
of illusory contours in humans produce activity consistent with such a
perception in neurons of primate V1 (Grosof,
Shapley, & Hawken, 1993) and V2 (von der Heydt et al., 1984; von der Heydt & Peterhans, 1989; Peterhans & von der Heydt, 1991). Sugita (1999) provides evidence supporting
the possibility that amodal completion may occur early in visual processing,
namely primate V1. When two line segments are separated, they are grouped
together and perceived as a single vertical bar if the region separating them
has a crossed disparity but not if the region has an uncrossed disparity. This
is consistent with the visual system’s interpretation of the region lying
in front (crossed disparity) or behind (uncrossed disparity) the line segments.
Sugita recorded the response properties of orientation-selective cells,
including both simple and complex cells, in V1 of Japanese monkeys presented
with such stimuli. Simple cells and complex cells showed comparable
responses to
physically continuous line segments and to stimuli requiring amodal completion.
Because the latency of the response for the completion of the line segments
behind the patch was not different from that for a physically defined line,
Sugita argued that it was unlikely that completion relied on feedback signals
from extra striate areas (Zipser,
Lamme, &
Schiller, 1996). Instead, Sugita argued that the likely mechanisms
underlying amodal completion are supplied by the lateral connections in V1.
Although it has often been suggested that the amodal completion of illusory
contours might be based on higher level processing, perhaps a kind of cognitive
reasoning, Sugita’s work offers support for the possibility that neuronal
mechanisms at very early stages of the visual system may be able to perform
rather sophisticated processing. There is no assurance, of course, that human
visual processing should be identical to that of other primates. Evidence using
fMRI methods suggest that in humans extrastriate areas are mainly involved in
processing illusory contours (Hirsch et al.,
1995; Mendola, Dale, Fischl, Liu, &
Tootell, 1999). More recently, fMRI measurements made with moving illusory
contours (Seghier, Dojat, Delon-Martin,
Rubin, Warnking, Segebarth, & Bullier, 2000) suggest that V5 and V1 are
also activated.
If we assume that, as Sugita (1999) suggests, the lateral
connections of V1 neurons are capable of sophisticated tasks like modal and
amodal types of illusory contour completion, is it likely that color
from motion
also can be explained by V1 mechanisms? Some of our own results are compatible
with this scheme. For example, results based on ratings show that the
sequential
frames of the CFM display can be presented alternately to one eye and then the
other — odd numbered frames to one eye and even numbered frames to the
other — to produce an effect that is equal to full presentation to one or
both eyes (Cicerone &
Hoffman, 1997;
Experiment 1). However, there are a number of reasons why we think that
explanations based on V1 mechanisms alone may not be sufficient to
explain color
from motion. One difficulty of extending findings based on static displays to
color from motion stimuli is that color from motion requires the perception of
apparent motion. To our knowledge, based on evidence from primate recordings,
only cells in MT respond to apparent motion stimuli as they would to
real motion
stimuli (Newsome, Mikami, & Wurtz,
1986; Kaneoke, Bundou, Koyama, Suzuki,
& Kakigi, 1997). Unlike the static illusory contour or neon color
spreading stimuli used in previous studies, color from motion does not occur at
all in static stimuli. Furthermore, the second set of results in Experiment 4
show that figure/ground configuration can override luminance relationships as
the determinant of which areas appear to move and to be filled with subjective
color. As it is currently understood, figure/ground organization is likely to
depend on higher level visual processing.
In natural scenes, objects or surfaces may
not be seen in the jumble of color, luminance, or texture of nearby
surfaces. In other cases, objects may be hidden from full view by
occluding surfaces. Although restricted parts of the object can be
seen through gaps in the screening elements, often neither the
object, nor its color, nor its location is perceived. In still view,
the physical representation in such scenes gives only an equivocal or
fragmented definition of the object and its surround. When the hidden
object moves relative to the rest of the scene, subjective color
spread helps to reveal the object as if in plain view. The results of
this study show that color from motion can be seen in two modes. In
modal completion, the subjective color spread is of low saturation
and has a neon-like quality. This mode evokes the perception of a
colored light or shadow moving over the scene. In amodal completion,
an object filled with color is perceived to be moving behind a
punctuated surface. We propose that color from motion works in
natural scenes as an organizing mechanism that reveals localized
regions of illumination or partially occluded objects.
This work was made possible by a grant from the
National Institutes of Health (EY-11132). Commercial Relationships: None.
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