1 theory is that the aliens see with echo location or something simolar to that .
an orgam in the back of the aliens head produces sound waves wich are amplafide buy a huge resonating chamber spaning the lenctho iof the aliens head explaining why it loocs like it dose. the sound waves are projected infrunt of the alien through the dome on its fore head the sound waves span out from the alien and reflect of anye solid mater back to the alien wair its pict up buy a sensativ organ in the aliens dome wich changes the sound waves into an image wich is then sent to the brain.
so it wudant mace a diforance wether the pred is cloced or not its still solid mater