Memory color effects of natural objects on color constancy

Thorsten Hansen, Maria Olkkonen, Sebastian Walter and Karl R. Gegenfurtner

Department of PsychologyUniversity of Giessen

Germany

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

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

Stimuli: Fruits and vegetables

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

DKL color space

Cone-opponent color space: two chromatic axes: L-M, S-(L+M) and a luminance axis: L+M.

0

90

180

270

Derrington Krauskopf Lennie

S-(L+M)

L-M

• Subjects adjusted the chromatic distribution in the isoluminant plane of DKL space

• The chromatic distribution was rotated and scaled towards the new location

Chromatic adjustment

banana space

Results: banana

typical color

achromatic setting

Results: more fruits

On average, the achromatic setting is made • away from the gray point• in a direction opposite to the natural color

Control condition with synthetic stimuli

No significant shift from the neutral gray point in the control condition.

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ongr

apes

oran

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carr

otzu

cchi

nile

ttuc

e

bana

na

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

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

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

Summaryis gray

looks gray

Conclusions

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

Results: individual observers

Calculation of memory color index

1) Calculate the projection of the grey setting to the typical setting

2) Divide the amplitude of the projection by the amplitude of the typical setting

Experimental chamber

Observer

Monitor

Neon lamps

Neon lamps