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					                                                                                                                                           The role of eye movements in color matching of real objects
                                                                                                                                                                                          Matteo Toscani, Matteo Valsecchi, & Karl R. Gegenfurtner
                                                                                                                                                                                                     Justus-Liebig-University Giessen
                                                                                                                                                                                                                                           Contact: Matteo.Toscani@psychol.uni-giessen.de



                                                                     INTRODUCTION                                                                                                                                                                                                                                                                                                                                                                        EXPERIMENT 2
                       Real objects can have non homogeneously colored surfaces. It is not yet clear how we can form a global                                                                                                                                                       In Experiment 1, the right side of the objects was brighter because of the position of the light
                       impression of their colorimetric properties.                                                                                                                                                                                                                 source. The tendency to fixate the brightest pixels could in principle be due to a tendency to fixate
                       Brenner, Granzier, and Smeets (2007) in a classification imaging study found that observers tended to                                                                                                                                                        the right portion of the object (i.e. the one which was nearer to the matching box). In order to test
                       report the colors of points close to the fixation point, and suggested that eye movements could determine                                                                                                                                                    this hypothesis, in Experiment 2 (N Observers = 6, N repeated matches = 6) we inverted the position
                       which part of a surface was given most weight. There is also evidence that restricting eye movements can                                                                                                                                                     of the light source, thus obtaining a similar illumination with the opposite spatial distribution. For
                       make a large difference in a color-matching task (Cornelissen & Brenner, 1991). Kuriki (2004) hypothesized                                                                                                                                                   each fixation we calculated a normalized eye position from the left border of the object to the right
                       that the color matches to multicolored stimuli do not involve any kind of averaging but rely on the most                                                                                                                                                     border of the object. The mean eye position is significantly higher in the condition with the light
                       salient color cues, e.g. the brightest elements of a stimulus and Giesel & Gegenfurtner, in a color                                                                                                                                                          from the right side (first experiment) than in the condition with light from the left side (second
                       matching task with real objects (under review) found that lightness matches were closest to the brighter                                                                                                                                                     experiment) for all the objects. This indicates that the tendency to fixate the brightest part of the
                       colors of the objects.                                                                                                                                                                                                                                       objects in Experiment 1 was not spurious.
                       The aim of the present study is to investigate whether eye movements could influence the color                                                                                                                                                                                      Yellow Candle                                                                                                                                       Orange Cone                                                                Red Candle
                       appearance of real objects, for example by sampling the points of the object which show salient color                                                                                                                                                             1                                                                                                                                                 1                                                                          1

                       cues. In particular, we conducted three experiments in order to determine whether there is a relationship                                                                                                                                                        0.8                                                                                                                                          0.8                                                                             0.8




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                       between fixations and matched color and whether this relationship has a causal nature.                                                                                                                                                                           0.6                                                                                                                                          0.6                                                                             0.6

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                                                                                                                                                                                                                                                                                                   Horizzontal axis of the object (pixel)                                                                                                              Horizzontal axis of the object (pixel)                                  Horizzontal axis of the object (pixel)
                                                                                                                                                                                                                                                                                                           Green Candle                                                                                                                                  Green Wool Bommel                                                       Green Wool Cylinder
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                                                                                                                                                                                                                                                                                                   Horizzontal axis of the object (pixel)                                                                                                              Horizzontal axis of the object (pixel)                                  Horizzontal axis of the object (pixel)

                                                                                                                                                                                                                                                                                                   Green Wool                                                                              Green Wool Cylinder                                     Green Candle                     Red Candle                             Orange Cone                  Yellow Candle
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                                                                                                                                                                                                                                                                                         0.6                                     0.6                                                                                                 0.6                                    0.6                       0.6                                        0.6

                                                                                                                                                                                                                                                                         Eye Position   0.55                                0.55                                                                                                0.55                                       0.55                      0.55                                       0.55

                                                                                                                                                                                                                                                                                         0.5                                     0.5                                                                                                 0.5                                    0.5                       0.5                                        0.5

                                                                                                                                                                                                                                                                                        0.45                                0.45                                                                                                0.45                                       0.45                      0.45                                       0.45

                                                                                                                                                                                                                                                                                         0.4                                     0.4                                                                                                 0.4                                    0.4                       0.4                                        0.4

                                                                                                                                                                                                                                                                                        0.35                                0.35                                                                                                0.35                                       0.35                      0.35                                       0.35

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                     Methods. Using a PR650 spectroradiometer, we obtained the spectral images of a set of objects with a                                                                                                                                                                                                                                                                                                                                      Light Source
                     space resolution of 1.5 degree of visual angle.
                     We recorded eye movements during a color matching task in which the observers (N Observers =7, N                                                                                                                                                                                                                                                                                                                                  EXPERIMENT 3
                     repeated matches = 6 ) could change lightness, hue and saturation of a 5 degree circular matching
                     box in CIE L*C*h color space.                                                                                                                                                                                                                                  The third experiment (N observers = 4, N repeated matches = 6) investigated the nature of the
                                                                                                                                                                                                                                                                                    relationship between fixations and matched color. We used photos on three representative
                     Analysis. For all the subject and all the objects the mean match for the luminance was higher than the
                                                                                                                                                                                                                                                                                    objects (Orange Cone, Green Wool Bommel, and Yellow Candle) of the same objects used in the
                     mean of the distribution, as expected from Giesel & Gegenfurtner‘s results.
                                                                                                                                                                                                                                                                                    former experiments shown on the screen. The participants were forced to fixate a specific fixation
                     We computed the absolute differences between the measured value (L*, h* and C*) of each point of the
                                                                                                                                                                                                                                                                                    point on the image (a gaze-contingent display ensured that the image disappeared if they
                     object and the mean of 6 color matches. A linear regression of these differences on the number of
                                                                                                                                                                                                                                                                                    attempted to fixate other parts of the image).
                     fixations for each point consistently produced negative slopes. This indicates that the color of the most
                                                                                                                                                                                                                                                                                    Because lightness was the most distributed light property in the objects, we chose 85th percentile
                     fixated pixels is more similar to the match.
                                                                                                                                                                                                                                                                                    (Bright Fixation Condition—BF) and the 15th percentile (Dark Fixation Condition—DF) of the
                                                                                                                                                                                                                                                                                    lightness distribution as fixation points. The matches were significantly brighter in the Bright
                                                                                                                                                                                                                                        C* Differences                              Fixation Condition. This points to a direct causal link between the way the image is fixated and the
                                  Overall Differences                                         L* Differences                                                           h* Differences
                                                                                                                                                                                                                                                                                    perceived overall color.
                                                                                                                                                                                                                                                                                                                                                                                                                 NW                                                   JR                             PT                                                SM
                                                                                                                                                                                                                                                                                                  Dark                                                                                     20
                                                       Pixel value                                                 Pixel value                                               Pixel value                                                            Pixel value                                                                                                                                  Light source from the left side
                                                       Mean Value                                                  Mean Value                                                Mean Value                                                             Mean Value                                                                                                                                   Light source from the right side

                                                                                                                                                                                                                                                                                                                                                                                           15
                                                                                                                                                                                                                                                                                                                                  Match for brigh points(L*) - Match for dark points(L*)
                                                                                                                                                                                                                                                                                                                         L* (BF) - L* (DF)




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                                                                                                                                                                                                                                                                                                                                                                                            5
                                                                                                                                                                                                                                                                                                  Bright

The participants tended to fixate the brightest parts of the objects, coherently with their tendency to                                                                                                                                                                                                                                                                                     0


match the brightest colored parts. This was confirmed by a binomial test showing that the proportion of
fixations in the brightest half of the pixels was higher than expected by chance.                                                                                                                                                                                                                                                                                                           -5




                                             Green Wool Bommel                                                                     Green Wool Cylinder                                                                      Green Candle                                                                                                                                                   -10

                                           Binomial Test: p value ~= 0                                                           Binomial Test: p value ~= 0                                                           Binomial Test: p value ~= 0
                      0.5
                                      Distribution of Fixations
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                                                                                                                              Distribution of Fixations
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                                                                                                                                                                                                                   Distribution of Fixations
                                                                                                                                                                                                                                                                                                                                                                                                                                                       CONCLUSIONS
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                                      Distribution of Luminance
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                                                                                                                              Distribution of Luminance
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                                                                                                                                                                                                                   Distribution of Luminance
                                                                                                                                                                                                                                                                                     There is a correlation between fixations and match: the most fixated points
                                                                                                                                                                                                                                                                                    are the nearest to the matched color, especially in the brightness dimension.
Relative Frequency




                                                                                       Relative Frequency




                                                                                                                                                                                   Relative Frequency




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                                                                                                                                                                                                                                                                                     Observers tend to fixate the brightest points of the objects (and to match the
                      0.2                                                                                    0.2                                                                                         0.2                                                                        brightest colors).
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                                                                                                                                                                                                                                                                                     This correlation is due to a causal link between the light properties of the
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                                                                                                                                                                                                                                                                                    fixated points and the matched color, at least for the luminance.
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                                  2          4         6       8        10   12                                           2            4          6           8        10                                      2   4      6     8     10       12    14   16   18
                                                        CIE Y                                                                                 CIE Y                                                                                 CIE Y
                                                                                                                                                                                                                                                                                    The observers tend to fixate the brighter points and their matches depend
                                                 Red Candle                                                                           Orange Cone                                                                          Yellow Candle                                            on the light properties of the points they fixate. This is a suitable strategy to
                                           Binomial Test: p value ~= 0                                                           Binomial Test: p value ~= 0                                                           Binomial Test: p value ~= 0
                      0.5
                                      Distribution of Fixations
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                                                                                                                              Distribution of Fixations
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                                                                                                                                                                                                                   Distribution of Fixations
                                                                                                                                                                                                                                                                                    compensate for variations in surface geometry and illumination (Giesel and
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                                      Distribution of Luminance
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                                                                                                                              Distribution of Luminance
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                                                                                                                                                                                                                   Distribution of Luminance
                                                                                                                                                                                                                                                                                    Gegenfurtner, under review ).
Relative Frequency




                                                                                       Relative Frequency




                                                                                                                                                                                   Relative Frequency




                     0.35                                                                                   0.35                                                                                        0.35
                                                                                                                                                                                                                                                                         References:
                      0.3                                                                                    0.3                                                                                         0.3

                     0.25                                                                                   0.25                                                                                        0.25                                                             Kuriki, I. (2004). Testing the possibility of average-color perception from multi-colored patterns. Optical Review, 11 (4), 249 –257.
                      0.2                                                                                    0.2                                                                                         0.2                                                             Sunaga, S., & Yamashita, Y. (2007). Global color impressions of multicolored textured patterns with equal unique hue elements.
                     0.15                                                                                   0.15                                                                                        0.15                                                             Color Research & Application, 32 (4), 267-277.
                      0.1                                                                                    0.1                                                                                         0.1
                                                                                                                                                                                                                                                                         Brenner, E., Granzier, J. J. M., & Smeets, J. B. J. (2007). Combining local and global contributions to perceived colour: an analysis of
                                                                                                                                                                                                                                                                         the variability in symmetric and asymmetric colour matching. Vision Research, 47 (1), 114-125.
                     0.05                                                                                   0.05                                                                                        0.05
                                                                                                                                                                                                                                                                         Giesel & Gegenfurtner (under review) Color appearance of real objects made of different materials. Journal of Vision.
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                            0.5        1         1.5       2      2.5    3   3.5   4                                  2            4         6            8       10    12                                         5           10              15         20
                                                        CIE Y                                                                                 CIE Y                                                                                 CIE Y