Alien: Physicality of the Alien humanoid's head.

Leading from

physicality of the humanoid Alien creature

a) Seeing organ
 

Giger's first design for the alien had large slanted black eyes that resembled motocyclist goggle. 

 

They came to a decision that while the monster had no eyes, it could see find exactly what it wanted. 

 

This meant that somehow the translucent shell upon the head somehow served as the creature's eye.  

 

There wasn't an exact answer as to whether it could actually see or not with this as an eye or whether it could just as it were sense like an insect, Ridley didn't have the answer (See: "No, no eyes")

 

b) The Brain area
Ridley also would imagine the transparent dome of head as a gelatinous mass, a sort of thick aspect jelly and it is all brain. 

 

He thought about how the head in a way was almost like a horse's head but if it was touched, it would be soft like a firm jellyfish and this would be its brain.

Quote sources

1. Ridley Scott: I always thought that the head was remarkable. I always thought it was a gelatinous mass, thick aspic like jelly and it is all brain. It is almost like a horse's head but if you touched it, it would be soft, like a firm jelly-fish and that was its brain. (Ridley Scott Alien: The Director's Cut - Interview released on the internet on a site now long gone to promote the release of the director's cut)
2. Ridley Scott: The first thing I wanted to see was something you didn't understand, so when Harry Dean Stanton goes after the cat, I figured I'd just bring him in from upside down, which was basically just a tail coming down behind his back, then a kind of jellified almost like aspic forehead. Then he turns around , then it comes up and you see its face, and then you knew you were in real trouble. (Alien Saga documentary)
3. Ridley Scott: I loved the, what I call the jelly, the jelly bag, I always thought the head should be look, should be like aspic, you know what I mean by aspic, it should be like an aspic, erm, which, in , in itself was an eye, whether the alien could see or whether the alien could sense like an insect I didn't even ever have to answer that question. (Alien Legacy documentary)
4. HR Giger: My first design had a large black eye, but then we thought it would be more frightening to have a monster that was blind, but could still find exactly what it wanted. So the eye became instead a translucent shell that covered the top of its head.  (Alien: The archive p71)