Color perceptionColor is a sensation (The rays are not coloured)Three types of cone photoreceptors transduce light into electrical signalsThese signals are converted into opponent colors in the retinal ganglion cellsIn the visual cortex, these excitation patterns are interpreted as colors

Trichromacy and UnivarianceLightCones

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

Evolution and ColorOne 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.

Strungen der Farbwahrnehmung

The world of the red-green blind

Cone absorption spectraL- and M-cone absorption spectra are highly similar

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

Opponent colors: AdaptationAdaptation 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

Color opponent neurons in the LGN

Gegenfarbkanle

Red-green opponencyAn L-cone center leads to an excitatory response to a bright red field and an inhibitory response to a dark green fieldAn M-cone surround leads to an inhibitory response to a bright green field and an excitatory response to a dark red fieldThe cell is maximally modulated by alternations of large red and green fieldsIt responds well to dark and light stripes--+----

Response to luminanceResponse to small bright spotResponse to bright red spotResponse to bright green spotNo response to large white field

Response to colorNo response to large white fieldResponse to bright red fieldInhibited by bright green fieldNo response to red center with green surround!

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

Retinal processing

Three channels to the brainColor opponent channels are functionally independentThey are based on different anatomical substrates, the M-, P-, and K-pathwayThey can explain many different color phenomena, but not all of themThey form the chromatic inputs to visual cortex

Color in the cortexCells in the LGN prefer the cardinal directionsCells in visual cortex exhibit preferences for intermediate directions of color space

Stages of color processingConesOpponent colorsHigher level mechanismsCategories