Check gamefaqs.com, they should have a guide or map.
well idk but i found the blind skull on co-op
it pretty much makes the aliens mad and they try harder to kill you. like the grunt that tries to kill you whe you get the skull.
all of the skulls can be picked up by holding down the "X" button.
It boosts the AI of enemies and allies, meaning that your allies have much greater accuracy, and are less likely to hit you accidentally. Enemies can also hear and react to footsteps and the noise of...