What are the definitions of 'multi-channel coding' and 'opponent channel coding'? I am looking for the definitions of 


Multi-channel coding
Opponent-channel coding 


And specifically in the context of visual adaptation. I have searched for information on the web and in books, but I am unable to find accurate definitions.
Short answer  


Multi-channel coding in color vision refers to the different photoreceptors in the retina.  
Opponent-channel coding refers to the opposing color pairs: the red-green and yellow-blue axes. 



Background  

I think you are referring to terminologies often used in color vision. 


Multi-channel coding generally refers to different types of photoreceptors in the retina, namely rods and cones. In terms of color vision, it is the cones doing the work. In trichromatic species, such as humans, there are three types of cones in normally sighted people, namely red, green and blue cones. So humans have a 3-channel color system, the same as your computer screen you are looking at now, namely R, G, and B channels.
Opponent-channel coding refers to the opponent theory of color vision as proposed originally by Hering. It basically states that our RGB color system defines color space by 3 opponent processes, namely a black-white, a red-green and a blue-yellow opponent system (Fig. 1). Basically it is theorized that red opposes green, yellow opposes blue and white opposes black (and vice versa). This means that colors are mixed between these channels (e.g. a reddish blue equals purple), but that colors within channels are mutually exclusive (e.g. there exists no such thing as a yellowish blue).



Fig. 1. Hering model of color vivion. source: University of Calgary  

Reference
- Gouras, Color Vision. In: Kolb et al (eds), Webvision. The Organization of the Retina and Visual System. Utah University