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Colour causes a depth illusion in human visual perception
Abstract
UNING FUNCTIONS). REPLICATED. EXTENDED KINGDOM 2003.
SUCCESSFUL REPLICATION. FOUND THAT SOME THINGS CHANGED WITH
ORIENTATION / SF/ PHASE

Introduction
Scientists are interested in how colour sensitive and luminance sensitive mechanisms interact
when a subject is presented with stimuli that embody the particular relationships that exist
between colour and luminance in the human vision system.
Colours plays a highly crucial role in the vision characteristics of the human sight, because of
its immense sensitivity the subject holds great importance among vision scientists throughout
the globe (e.g. review by Regan, 2000). The main strategy to gain knowledge in this subject
matter is to study the performance, set against a certain criteria, using only iso-luminant
(single colour characteristics) and iso-chromatic (multiple colour characteristics) stimuli. The
In this study, we aim to study the colour and luminance characteristics of the human vision
with an attempt to establish a proper relationship between the colour, depth and luminance in
the human vision. Kingdom (2003) proposed that a significant amount of knowledge could be
gain by analysing the behaviour of colour and luminance in the human vision perception, and
how this phenomenon can emulate the spatio-temporal relationships between the colour and
luminance found in human vision. Kingdom et al (2005) attempted the first successful
approach to understand this subject and investigate the proper relationship in such
phenomenon. They studied that when a chromatic grating is added at a certain level to
luminance grating; one of them gains the impression of groovy structure, this process of

transference from colour to shape is called the Depth Enhancement. On the contrary, if
second grating of chromatic grating is further added to this process at a different level, the
impression of depth is either reduces or completely eliminated, and this process of
elimination or reduction is called Depth Suppression. These kinds of phenomena are
generally experienced in achromatic kinds of studies that are highly influenced by different
colour contrasts (Lehky & Sejnowski, 1988; Ramchandran, 1988; Attick et al., 1996; Sun &
Perona, 1997).
The depth enhancing processes, formation of a shape from shade due to the grating between
chromatic and luminance patterns proposed that natural human visual system has certain
inbuilt capabilities;
1. The main cause of variation in chromatic and luminance behaviour that are spatially
aligned against each other is due to the variation in surface reflectance.
2. The main cause of pure or impure variations in the luminance behaviour is due to the
non-uniform illumination, such as shading and shadows.
These physical relationships between the chromatic and luminance grating holds
dominant importance in the field of human vision and the scientist are contented upon
the agreement that such relationships give rise to acknowledge these in-built system in
the human vision (Rubin & Richards, 1982; Cavanagh, 1991; Mullen & Kingdom,
1991; Olmos & Kingdom, 2004) along with the colour shading affect which is quite
evident to appreciate that these are embedded into the human visionary configuration.
There are several other factors that may signal the perceptions of surface shapes. There is
interest in whether colour contrast on the perception of shapes influences the perceptions of
shading, such as texture; and whether the colour contrast influence the contribution of
shading to surface curvature when it is present alongside other cues. It is possible that the
influence of colour contrast on shape-from-shading is reduced, or even eliminated, when

surface information other than colour is present, because in such circumstances the surface
versus illumination interpretative role of colour contrast becomes redundant. The aim of this
study, as we have already mentioned, is to establish a relationship between the colour,
luminance and depth of human vision with a focus to investigate the influences of the colour
contrast on perceived shapes in pattern that produces shape from shading with shape from
texture. Mamassian and Landy (2001) also noticed that the orientation defined textures have
been shown to combine synergistically with shading to create strong impressions of depth.

Numerous questions have so far been aroused concerning the chromatic properties of the
colour shading. It is often bring into consideration to investigate that the combination of two
phenomena, depth enhancement or depth suppression, in colour directions is more important.
There is high possibility the colour shading effect is weaker when the directions of depth
enhancement and depth suppression phenomena is same, as in such phenomenon the human
vision system might bound together both colouring patterns into a single object, releasing the
luminance variations from being designated as changes in reflectance, and designating them
instead as shading, even though they are spatially aligned with one of colour patterns.

In this study, we have attempted to answer the questions regarding mixed colour and
luminance plaids with an aim to manipulate the direction and colour texturing of both the
depth enhancement and depth suppression. The results of this study furnishes further about
the information and understanding of the chromatic properties in terms of colour shading
textures and formation of shape from shading by the natural human vision system, and
therefore tries to acknowledge the assumptions related to the relationships between the
colour, luminance and depth of the vision system. In order to grab proper and precise
knowledge about the relationship between the colour, luminance and depth in the perception

of vision, we have utilize an adjustable stimulus that have original and real corrugations and
bumps in its structure, defined stereoscopically.

The findings of this study can be summarised by suggesting that the impression of depth is
presented when variations in colour were appeared at different orientation to plaid gratings
and at the same orientation but out of phase, therefore, the colour variations at different
orientation and out of phase will yield the depth enhancement. Additionally, the addition of
colour variations of the same orientation and in phase will suppress the grating that will yield
the depth suppression.

Method
Participants

The participants who took part in the chromatic-achromatic experiment were 8 psychology
students and 1 professor in the University of York: A, E, S, J, R, JS, JR, H and AW. For the
other 5 experiments, there were 8 participants in total except subject A. Details such as age,
gender and handedness were not necessarily collected for this experiment. Personal
identifying information were used anonymous.
Materials

The materials used in this experiment were stimuli viewed though a CRT screen and a
keyboard. First of all, the screen was a NEC Multisync 200 screen and the diagonal size of
screen was 20 inch. The refresh rates of how quickly the screen updates were 100Hz. Figure
1 shows the viewing distance between eyes and the screen. The viewing distance was the
total distance (1+2+3+4+5) about 700mm, but not the straight distance between eyes and
screen. The field of view (w x h) was 29.50 x 22.34 deg. The structure of depth (disparity)
was the distance between fovea and the place of images. Each eye was separated to see the

stimuli, it was the way that how stereo images achieved. The reason for that was to make
each eye construct different figures to achieve the disparity not to change the overall disparity
of objects.

5

Screen

4

3

Mirror

2
1

Eyes
Figure 1. Lab settings in this experiment
Secondly, participants’ adjustments of the amplitude of the depth corrugations in the stereoimages were made by pressing the up and down arrow keys on a standard keyboard. The
mean starting value of adjustments were -6.00 and the range of that was between -5.00 and
+5.00. Subjects’ responses were accompanied with auditory indicators.
Thirdly, stimuli were displayed on a grey background. The perform calibration of all
phosphors for chromatic data showed the location in colour space for 3 guns. It was

calibrated by using the Spyder 3 colorimeter. The R (red), G (green) and B (blue) gun outputs
were gamma-corrected after calibration. The CIE coordinates of the monitors’ phosphors
were R: x=0.640, y=0.330; G: x=0.300, y=0.600; B: x=0.150, y=0.060. The stimuli were
constructed from three component gratings: luminance modulated gratings, colour modulated
gratings and drift modulated gratings. All achromatic gratings with contrast of 100% had a
spatial frequency of 2 cpd and an orientation of 90 deg. The stimuli were presented in a
circular, hard-edged window. The achromatic gratings were ‘black and white’, and were
produced by modulating all three RGB phosphors in (1,1,1). The colour gratings were ‘redgreen’, and was designed to dissociate the post-receptor chromatic mechanism that
differences the L (long-wavelength-sensitive) and M (middle-wavelength-sensitive) cones.
The colour space of LMS was (1, -1,0). The drift gratings with contrast of 10% had a spatial
frequency of 1 cpd and were alternated to avoid movement after-effects. Alternation had a
temporal frequency of 1Hz. In this condition, horizontal gratings could not be used because
the shift bars from left to right was not seems to be moved. So, vertical gratings were made to
get larger disparity. All gratings were formed from sinusoidal modulations of cone contrast.
In more details, stimuli were made and divided into 6 conditions in this experiment: 1.
chromatic and non-chromatic, 2. in phase (0°) and out of phase (90°), 3. Orientation (0°, 30°,
60°, 90°), 4. Phase (0°, 30°,60°,90°), 5. Spatial frequency (1, 2, 4, 8 x original), and 6.
Drifting and static.
Design

The IVs (Independent Variables) in this experiment were 6 stimuli conditions. The DVs
(Dependent Variables) was the subjects’ adjustments for each condition. The main design
was a between subjects design.

Procedure

The type of procedure used here was called ‘psychophysics. Participants were asked to
estimate the apparent depth of the corrugations in the stereo-gratings on a CRT screen and
adjusted the amplitude of the depth corrugations in the stereo-gratings until they matched the
apparent depth of the corrugations in the test stimuli by pressing the up and down arrow keys
on a keyboard. There was no time limit. Each testing session took approximately 1 hour and
there were 6 individual conditions. During each session, stimuli were presented in a random
order with several practice trails and test trails. In the experiment 1, 2 and 5, participants were
tested 8 test trails for each condition. In the experiment 3 and 4, participants were tested 5
trails for each condition. Some participants experienced fading of images or other possible
adverse effects such as headache or dry eyes during along time of staring at a computer
screen. So, participants were encouraged to let their eyes roam around the stimuli to avoid the
negative influence on the adjustments. Finally, written consent forms were obtained from all
participants.

Results
As illustrated in figure 2, average disparity threshold showed that the chromatic condition
(M= 5.11, SD= 3.32) tends to be higher than the achromatic condition (M=2.93, SD= 2.46).
The mean difference between two conditions was 2.17 and the 95% confidence interval for
the estimated population mean difference is between 0.44 and 3.91. A paired sample test was
carried out to show that the difference between conditions was significant (t= 2.885, df= 8,
p< 0.020, 2-tailed).

Mean disparity threshold (arc
mins)

5.5
5

4.5

4

3.5

3

2.5

2
Chromatic

Achromatic

Figure 2. Mean disparity threshold for chromatic and achromatic stimuli.

As illustrated in figure 3, average disparity threshold of the in phase stimuli (M= 2.90, SD=
2.14) was lower than the out of phase stimuli (M= 4.91, SD= 3.60). The mean difference
between two conditions was -2.02 and the 95% confidence interval for the estimated
population mean difference is between -4.96 and 0.93. A paired sample test showed that the
difference between conditions was non-significant (t= -1.62, df= 7, p= 0.149, 2-tailed).

Mean Disparity Threshold
6
5
4
3
2
1
0

In Phase
Out Of Phase

Figure 3. Mean disparity threshold for in phase and out of phase. Figure 4 showed the
average disparity threshold for four separated orientations: 0 (M= 4.37, SD= 2.88), 30 (M=
3.74, SD= 2.17), 60 (M= 4.08, SD= 2.45) and 90 (M= 2.23, SD= 2.14) degrees. There was a
significant effect (ANOVA?) of the degree of orientation, F(3,21) = 3.207, p= 0.044. Then a
pairwise comparison was carried out to show the difference between each individual degree
of orientation. It indicated that there was no significant difference between 0, 30, 60 and 90
degrees.

mins)

5
4.5
4
3.5
3
2.5
2
0

30

60

90

Orientation (degrees)

Figure 4. Mean disparity threshold for four different degrees of orientation.
Figure 5 showed the average disparity threshold for four individual phase variables: 0 degree
(M= 2.34, SD= 1.69), 30 degree (M= 3.26, SD= 2.16), 60 degree (M= 4.48, SD= 2.72) and
90 degree (M= 4.46, SD= 2.67). A non-significant effect of phase shift was found, F (3,21)=
2.997, p= 0.054. Then a pairwise comparison was conducted which indicated that four
individual phase degrees were not significantly differ from one another.

Mean disparity threshold
(arc mins)

5
4.5
4
3.5
3
2.5
2
0

30

60

90

Phase shift (degrees)

Figure 5. Mean disparity threshold for four different degrees of phase shift. As illustrated in
figure 6, average disparity threshold for spatial frequency of 1(M= 4.48, SD= 2.26), 2 (M=
3.00, SD= 1.86), 4 (M= 2.56, SD= 1.91) and 8 (M= 2.48, SD= 1.89) showed a significant
effects, F (3, 21) = 5.232, p= 0.007. After which, pairwise comparisons was employed and a
significant difference was found between spatial frequency 1 and spatial frequency 2 (p=
0.042). There were no significant differences between the spatial frequency 1 and 3, 1 and 4

mins)

and 3 and 4.

5
4.5
4
3.5
3
2.5
2
1

2

4

Spatial Frequency

Figure 6. Mean disparity threshold for four types of spatial frequency.

8

Figure 7 indicated the average disparity threshold for drifting (M= 3.77, SD= 3.06) and static
(M= 2.04, SD= 1.64) images. There was a non-significant effect (t= 1.75, df= 7, p= 0.124, 2tailed) between these two conditions by using a paired sample test.

Mean Disparity Setting
4
3.5
3
2.5
Drifting
2

Static

1.5
1
0.5
0

Figure 7: Mean disparity threshold for drifting and static stimuli.

Discussion
The present study for the Colour causes a depth illusion in human visual perception describes
based on the number of experiments and detailed analysis as done in the study that color
sensitive and luminance sensitive mechanism can be observed from it. The results of the
above study can be summarised based on the analysis as follows:
Based on the capacity to enhance and suppress depth as detected by the interference in the
mixed color plus luminance plaids it can be observed that the L – M and S gratings during
interference are similar. The observed capacity of the mentioned grating as above for getting
the desired depth does not depend on the enhancing grating it is either L – M or S. The depth
illusion is generally depending on the color for the human eye as observed from the analysis
above. The blue yellow capacity of the grating to get the desired depth is not affected by the

blue color falling orientation either in the dark or in the bright part of the observed shading
during the interference. The results indicates that for the out of phase condition the blue
yellow grating obtained while doing experiment, the blue yellow grating seem to be less
affective depth suppressors in respect to the blue yellow grating when they are in phase. The
color grating that have been defined along and between the cardinal directions of the obtained
color space, shows that its results seem to match with the general results obtained by
Kingdom (2003) as for the new color directions.
The obtained cardinal directions for the human visual perceptions do not seem to be different
in case of the capacity that enhance or suppress the desired depth that leads to the final result
of the study done for the human visual perceptions. The color shading effect in the study is
moreover dependent on the contrast of the color. In this study the complete gamut of color
directions are not studied therefore it can be said that the possibility of the color directions
can be rule out for the depth enhancement. It can be observed from the results that the two
orthogonal luminance which are in orientation have the contrast and equal nature the desired
depth is less than the actual depth. The luminance gratings which have the low contrast
behave like as potent depth enhancers for the higher contrast orthogonal in orientation
ratings. The results explain that the grating effect or interference effect in deciding the depth
illusion for the human visual perceptions depends on the color contrast because it made the
difference in the depth. The results are as per the expectation and positive to the data input for
the analysis.

Relation of discussion to the hypothesis
In this study the hypothesis has the strong relation with the discussion as the discussion is
based on the analysis and the analysis is based on the obtained result where using the

approximation to the observations. The hypothesis that is used for analysing the depth of the
color directions is that the magnitude of the desired depth would be lower in case when the
depth - enhancing and depth suppressing color directions seem to be similar in respect to the
color directions looks like different. The discussion also describes that the color depth for the
human vision perceptions is completely based on the color and grating that are obtained
during the interference. L – M and S grating seem to be similar based on the capacity of
depth - enhancing, this suggests that the color depth is important. The suggestion was for the
visual system that it might be possible as plaid components that have the similar color
composition in to a single surface, and interpret that for any residual luminance the
components should be shading.
There is no much evidence available for this hypothesis therefore; there is no evidence or
support present for the color – binding idea during the experiment. Another hypothesis in
this study is that the blue yellow chromatic gratings would not be that much effective for the
suppressors’ depth because in the chromatic grating the blue phase of the grating fell in the
dark instead of felling in the bright part of the shading grating. The shaded regions on the
ground will be bluer than the non-shaded regions for the same grating of blue yellow
chromatic grating. This hypothesis is not either have much evidence available for support of
this statement. The discussion implies that even if there is no evidence available the obtained
result from the analysis through the data describes that the hypothesis somewhat matches
with the result obtained. The hypothesis explain that for the blue yellow grating which are in
out of phase the desired depth suppression will be less in respect to the blue yellow grating in
phase. In this discussion it has been found that there should not be any depth suppression if
the blue yellow gratings are in out of phase but it is there because of the relative phase’s
shifts and so this happens. The sinusoidal modulations of blue yellow gratings shifts so
because of that the color and luminance do not produce in every subject a categorical shift

form aligned to non - aligned. The another hypothesis is that there might be a possibility of
depth – enhancing for L –M grating that has been used for the blue yellow experiment in
order to have the ceiling effect to get the desired depth. Thus it can be said that the hypothesis
made during the experiment or analysis was quite good and matches with the obtained result
as discussed above.

Implications of the results
The obtained results in the study are much significant to explain that colour causes a depth
illusion in human visual perception. Color always makes illusion to the human visual
perceptions as discussed in this study that blue color and yellow color fell in the different
regions because of the grating and the depth of the color shade. In this study we find that the
all hypothesis and the conditions for the interference and grating that the depth suppressions,
was higher or can say the depth enhancement was lower in case of 90 degree than in
compared to 270 degrees grating on the ground for the depth of color share. The effect as
seen from the obtained result for both the case as in 90 degree or in 270 degree don’t have
much significant effect. There are some no parametric and parametric statistical results that
might be significant in assessing the statement for the human vision perceptions. The
parametric tests in this study are more significant and powerful then the non – parametric and
there are less chances of having the error on the side instead of getting the number of errors
on the non-parametric side. Using the errors while testing as parametric will provide the more
enhance view for the cautions that can be consider improving the human vision perceptions.
It can be seen that the obtained results in figure -7 that shows the Mean disparity threshold
for drifting and static stimuli explains that in case of drifting the setting level was higher than
the static. The present study for the Colour causes a depth illusion in human visual perception

describes based on the number of experiments and detailed analysis as done in the study that
color sensitive and luminance sensitive mechanism can be observed from it. It has been seen
that the obtained results from the study will make a good sense to the visual system to
suppress luminance boarders that will make a favour to the chromatic ones because chromatic
boarders as used in this study show the more reliable indications to the surface of the
boundaries. The results of the present study add an important caveat to this idea, by showing
that there are circumstances in which color contrast promotes luminance contrast for visual
form judgments. Given that shadows and shading can be used by the visual system for object
recognition, shape perception and motion perception, it would make sense for the visual
system to recruit color vision to help differentiate those luminance variations that are due to
shadows and shading from those that are due to changes in surface reflectance. The
suppression of shape-from-shading by aligned chromatic variations is just the other side of
the coin; the visual system makes the reasonable assumption that such luminance variations
most likely originate from changes in surface reflectance. So the implications of the obtained
result from the experiment about the colour causes a depth illusion in human visual
perception are related to the statement and signifies the importance of the color in the illusion
of depth for the human visual perceptions.

Strengths and limitations of the experiment
We replicated Kingdom’s 2003 results and got the same results for chromatic and achromatic
gratings. We got non-significant results for the phase. But this result was nearly significant.
Perhaps if we did more subjects it would be better. This shows that the illusion of the depth
for the visual perceptions for the human is right as because of the color. The intensity and
rays of the different colors are different which might result in the different depths that people
observe. Tested a hypothesis that you could get the same effect with achromatic grating in the
experiment number six but we could reject this hypothesis. This effect >does< seem to

depend on color. Some subject had noisy results -perhaps because the experiments took too
long. We could reduce the time for the experiments - perhaps by automating it more or
breaking the experiments into shorter blocks. Warn people not to wear contact lenses - their
eyes get dry. Yes the people should not go for these contact lenses as it causes problem
because it resists the moisture to the eyes so create issue. The color also gets problem in these
contact lenses and the actual thing look differently. The findings of this study are relevant to
the vision perceptions so completely argumentative and useful. The only issue is that this
result can’t be observe as 100 percent perfect because the number of experiments and
iterations are not so long as because of that the actual trend can’t be observe.
It can said that the future generation vision can be improved based on the obtained results but
as observed the things are based on the hypothesis so it cannot be assumed that this study is
100 percent perfect. There are some limitations of the study as this can’t be useful for the
accurate result but yes the trend can be observed easily. For getting the exact pattern and
perfect results there is need to have the number of consistent experiments or simulations that
will give the accurate and reliable result in comparison to this experiment. These have
generally been restricted to an artificial world where the only luminance variations present
are those arising from shading. Such models will tend to fail with more naturalistic scenes
where luminance changes due to surface reflectance are confounded with those due to
inhomogeneous illumination. These models might be made more successful if they included a
stage in which color contrasts were detected and used as local weighting functions to
strengthen uncorrelated luminance inputs (and weaken correlated inputs) to the shapeanalysis stage.

Appropriate future directions for research

For future the study is very helpful in deciding the future vision issues and the illusion that
the human have. Further depth enhancement can be made based on the number of given data
and analysis. For getting the accurate result and perfection there should be used the number
of experiments and analysis or if possible should go for the real analysis through surveys,
testing and simulations. The further research area can be done for shade effects using the
mono-chromatic gratings and should use more colors for testing.

Conclusion
This study is very helpful in deciding about the human vision perceptions for the depth
illusions based on the color grating. The people should be careful for their vision and not to
sue the contact lenses to avoid any problem to the eyes. The conclusion of this study is that
Colour causes a depth illusion in human visual perception.

References:
1. MacLeod, D. I. A., & Boynton, R. M. (1979). Chromaticity diagram showing cone
excitation stimuli by stimuli of equal luminance. Journal of the Optical Society of
America, 69, 1183–1186.
2. Mollon, J. (2000). Cherries among the leaves: The evolutionary origins of color vision.
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computational perspectives (pp. 10–30). New York: Oxford Univesity Press.
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and origins of primate colour vision. Journal of Experimental Biology, 146, 21–38.
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Gouras (Ed.), The Perception of colour. In & J. Cronly-Dillon (Eds.). Vision and
visual dysfunction (Vol. 6, pp. 198–217). Oxford: Macmillan.

5. Sankeralli, M. J., & Mullen, K. T. (1996). Estimation of the L-, M-, and S-cone
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7. Smith, V. C., & Pokorny, J. (1975). Spectral sensitivity of the foveal cone
photopigments between 400 and 700 nm. Vision Research, 15, 161–171.
8. Stromeyer, C. F., III, Cole, G. R., & Kronauer, R. E. (1985). Secondsite adaptation in
the red-green chromatic pathways. Vision Research, 25, 219–237.
9. Sumner, P., & Mollon, J. D. (2000). Catarrhine photopigments are optimised for
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10. Sun, J., & Perona, P. (1997). Shading and stereo in early perception of shape and
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images. Vision Research, 37, 3283–3298.

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