Color perception

1. Color perception • Color is a sensation („The rays are not coloured“) • Three types of cone photoreceptors transduce light into electrical signals • These signals are converted into opponent colors in the retinal ganglion cells • In the visual cortex, these excitation patterns are interpreted as colors

2. Trichromacy and Univariance Light Cones

3. Cone absorption spectra • L- and M-cone absorption spectra are highly similar • They evolved from a common ancestor about 35 Million years ago • L- and M-cone pigment genes are highly similar

4. Evolution and Color One idea about the evolution of color vision is that it helped us to detect ripe red fruit amongst green leaves. The trees growing that type of fruit evolved at around the same time. The monkeys eating the fruit helped distribute the seeds (together with some fertilizer). This process is called coevolution.

5. Störungen der Farbwahrnehmung

6. The world of the red-green blind

7. Cone absorption spectra L- and M-cone absorption spectra are highly similar

8. Color contrasts of natural objects There is a high correlation between L- and M-cone signals. This is not very efficient!

9. Opponent colors: Adaptation Adaptation along one „cardinal direction“ leads to an increase of detection thresholds along that direction Detection thresholds along the other cardinal direction are not affected Adaptation along an intermediate direction leads to an overall increase in thresholds

10. Color opponent neurons in the LGN

11. Gegenfarbkanäle

12. - Red-green opponency • An L-cone center leads to an excitatory response to a bright red field and an inhibitory response to a dark green field • An M-cone surround leads to an inhibitory response to a bright green field and an excitatory response to a dark red field • The cell is maximally modulated by alternations of large red and green fields • It responds well to dark and light stripes - - - - - + -

13. Response to luminance • Response to small bright spot • Response to bright red spot • Response to bright green spot • No response to large white field

14. Response to color • No response to large white field • Response to bright red field • Inhibited by bright green field • No response to red center with green surround!

15. Color categories • The „cardinal directions“ of color space do not correspond to unique hues • Humans have the ability to group colors into a small number of categories (7-11) • Different subjects have different category borders • For each subject, the borders are quite sharp and consistent over time

16. Retinal processing

17. Three channels to the brain • Color opponent channels are functionally independent • They are based on different anatomical substrates, the M-, P-, and K-pathway • They can explain many different color phenomena, but not all of them • They form the chromatic inputs to visual cortex

18. Color in the cortex Cells in the LGN prefer the cardinal directions Cells in visual cortex exhibit preferences for intermediate directions of color space

19. Stages of color processing Opponent colors Higher level mechanisms Cones Categories