Memory color effects of natural objects on color constancy

1. Memory color effects of natural objects on color constancy Thorsten Hansen, Maria Olkkonen, Sebastian Walter and Karl R. Gegenfurtner Department of Psychology University of Giessen Germany

2. Color constancy Color constancy is the constant perception of object color (reflectance) under changing illumination.

4. Mechanisms of color constancy • Local image contrasts: cone ratios across edges are largely illumination invariant • Global image color: assuming a “gray world” can discount many illumination changes • Memory color: the characteristic color of familiar objects can be recalled from long-term memory • Earlier studies found a positive effect of memory color, but it is unclear how large the effect was and whether it would also work with natural objects

5. Stimuli: Fruits and vegetables

6. Paradigm: Achromatic setting • The subjects had to adjust the images until the isolated objects appeared neutral gray • Moreover, the subjects had to adjust the images to the typical object color of the fruit or vegetable • In a control experiment, achromatic settings were made with homogeneous discs and pink noise • Settings were made in a color chamber. Illumination was calibrated to match the monitor background color. • N = 17 observers

7. DKL color space Derrington Krauskopf Lennie Cone-opponent color space: two chromatic axes: L-M, S-(L+M) and a luminance axis: L+M. S-(L+M) 90 L-M 0 180 270

8. Chromatic adjustment • Subjects adjusted the chromatic distribution in the isoluminant plane of DKL space • The chromatic distribution was rotated and scaled towards the new location

9. banana space

10. typical color achromatic setting Results: banana

11. Results: more fruits • On average, the achromatic setting is made • away from the gray point • in a direction opposite to the natural color

12. Control condition with synthetic stimuli No significant shift from the neutral gray point in the control condition.

13. carrot banana lemon lettuce grapes orange zucchini Quantifying the effect • The memory color effect was quantified by relating the excursion of the gray setting to the typical setting • The index was on average 10% • … and significantly different from zero for all natural stimuli

14. Evidence from hue scaling Hansen & Gegenfurtner (2006), Visual Neurosci., in press.

15. Summary • Images of fruits or vegetables are perceived in the characteristic color of the object if all pixels in the image are gray • For the objects to appear gray, subjects adjusted the average color 7–16% in the color direction opposite to the memory color. For example, the banana that appears gray was adjusted to a bluish color • No such effect was found for the homogeneous discs or the pink noise stimuli • Memory colors can support color constancy

16. Summary is gray looks gray

17. Conclusions The perceived color of natural objects is biased towards their natural color.

18. Results: individual observers

19. Calculation of memory color index • Calculate the projection of the grey setting to the typical setting • Divide the amplitude of the projection by the amplitude of the typical setting

20. Experimental chamber Monitor Neon lamps Neon lamps Observer