hc - 2 : Head direction

Hello,
In the description of the hc-2 data it states that it contains "The coordinate and direction of the rats head during the experiment". I looked at the data and it contained (x,y) values for the two LEDS. However, I don't understand how this can give me head direction relative to the body without having a third LED. Am I missing something?
Thank you,
Dan Howarth
Powered by
Ploneboard