Long term effects of chromatic experience on human behavior and neural function

Visual information shapes our behavior in ways which we are not consciously aware. Perhaps, the most evolutionarily ancient sensation is the ability to detect changes in wavelength content of the environment. Thus, it is not surprising that our nervous system has adapted to respond to color on many levels. Explored here are a range of subconscious influences of color on behavior and neural function. Behavior patterns of the most primitive organisms, the archaea, are regulated by changes in sky color throughout the day. This persists in reptiles that set their biological clocks by monitoring spectral changes in the sky using an ancient parietal eye. Given the evolutionary importance of color in circadian behavior it is not surprising that retinal inputs to the circadian pacemaker in primates have chromatically opponent responses. Thus, in a first series of experiments, color changes, like those that occur at dusk and dawn were tested for their effects on the human circadian pacemaker. The results show that pure color changes have a powerful effect on the human circadian clock. At a very different level of the visual system, organisms use color to extract information about qualities of objects; for example the external color of fruit which indicates ripeness. To do this, circuits for color vision must be sensitive to the characteristics of the environment. The effects of altered chromatic environments on hue experience were measured to determine the extent to which visual information can instructively tune color circuitry. Remarkably, short periods of altered chromatic experience caused long-term perceptual changes. Finally, it was determined to what extent the nervous system can use chromatic information in forming neural circuits to provide a new visual capacity when a new cone type is added. A new dimension of color vision can be added in response to an increase in the number of cone types from two to three; however, no gain in color vision capacity was demonstrated in women who have four distinct cone types. One possible explanation is that environmental information is not sufficiently rich to instruct the color circuitry to extract new information from the fourth cone type.