Color appearance of natural objects Thorsten Hansen, Sebastian Walter and Karl R. Gegenfurtner Department of Psychology University of Giessen Germany

Stages of color vision 1.cones 2. cone-opponent 3. higher-order 4. color mechanisms categories retina retina/LGN visual cortex S M L black white red green yellow blue orange pink gray brown

Stadien der Farbverarbeitung

Motivation Previous studies have investigated the color appearance of homogeneous disks in the isoluminant plane of DKL color space (e.g., De Valois et al. 1997, Webster & Miyahara 2000, De Valois et al. 2000). These studies investigated hue scaling for the unique hues RED, GREEN, YELLOW, and BLUE in DKL color space. These studies have shown that the cardinal axes do not coincide with the unique hues. In particular, the S-(L+M) axis varies from violet to yellow-green. Stimuli were always homogenous patches.

Motivation In contrast to standard disk stimuli, natural objects are characterized by a distribution of different chromatic hues.

Motivation Here we investigated two question: 1.Does the presentation of colors at different luminance values alters the location of the unique hue? 2.Does the use of natural objects with a distinctive object color (e.g., a banana) influence the color appearance ?

DKL color space Cone-opponent color space: two chromatic axes (L-M and S-(L+M)), one luminance axis (L+M) Derrington Krauskopf Lennie

DKL cone-opponent axes vs. Hering's opponent colors Ewald H. Hering 1834–1918

L-M S-(L+M) L-M Methods Stimulus creation: translate color distribution of the objects in the isoluminant plane of DKL space.

banana space

Methods Observers rated the appearance of the unique hues (RGYB) for stimuli at 36 equally spaced chromatic direction in the isoluminant plane of DKL color space.

Cone contrast in DKL space

Methods Observers rated the appearance of the unique hues (RGYB) on a scale from 0 to 8. R B G Y Stimuli at each color directions were shown twice in randomized order in each block. Five blocks were run for each observer, resulting in a total of 10 hue scaling for each chromatic direction.

Methods Natural fruit stimuli (banana, salad, carrot) are compared against 1.colored disks of homogeneous color 2.outline shapes of homogeneous color at 5 different luminance values. N=3 naïve observers participated in the study, with normal color vision as assessed by Ishihara pseudoisochromatic plates.

Results Color appearance of a standard disk stimulus.

Results Variation of brightness.

Results Natural objects

Results Single subject data

Results Comparison with outline shapes

Results Other fruit: salad.

Summary In agreement with earlier studies we found that The cardinal axes do not coincide with the unique hues This is true for stimuli at a large range of different brightness values. ≠

Summary Using natural fruit objects as stimuli, we have found an increased rating of the unique hue that most closely corresponds to the object color. The present study complements other work of our group using natural objects where we have investigated color discrimination (VSS 2005, ECVP 2005) and color memory (ECVP 2004). The effect is smaller for outline shapes compared to real images with a variety of hues.

Conclusion The distribution of hues in natural objects influences color appearance and needs to be taken into account for a full characterization of color appearance.