I really dont know why they call it an eye, because its underther skin. You have to cut into the monitor to see it (i never did it to examine it)but i dont think it resembles an actual eye. The monitor doesnt really need it because some monitors had the eye destroyed by getting hurt and doesnt see to effect them. I think all monitors have the eye, but monitors arent studied well and those who study them are mostly enthusiasts rather then skilled biologists etc.
If you watch monitors under a heat source you can sometimes notice they use their top part of the head trying to sence somthing, like moving it as if they are looking or trying to look at somthing from their top part of their head. Many monitors when under the light will position their head under the hottest part, i believe this is a monitor using the sensor more then others. Many people think that the monitor regulates the heat internally through the blood or internal primative sensors rather then the eye, who really knows because when i ask my monitor how he regulates temepture, he never answers 