4 color "sensors" of the eye?

[Vay]

My teacher told me that he read some news about some women having 4 types of color "sensors". I hadn't bothered to ask him to define this phenomenon more specifically. So basically, the average person has 3 color sensors, which are the three primary colors of additive color theory. So what is this 4th color sensor?

 

I couldn't find the news on this, so hopefully someone here knows the source of this news.

Edited December 2, 2011 by Vay

[John Cuthber]

Does this help?

http://en.wikipedia.org/wiki/Tetrachromacy#Possibility_of_human_tetrachromats