[tom hyle]

Allow me to propose that the big difference between the "Eastern" and "Western" helmets in question is not sharply drawn, but lies in the manner of attachment of both the main plates of the skull part, and of the dependant attachments. The masked Greek helmet shown is entirely unfamiliar to me. But the "Corinthian" or barbut type is not, and is the oldest of the types under discussion. These helmets were forged from one piece of metal, or from two joined at the crest (one piece is considered more skillfull production). Additionally, these helmets sometimes incorportated a facelike mask, supposedly a portrait of the owner, within the "Y". I don't know how such mask was joined or if it was of one piece with the helmet, but it was rigidly attached. Use of this type of helmet, little altered, continued up through the medieval period. Likewise, the Celtic and German European helmets said (falsely, IMHO) to derive from the Latin helmets (actually this seems a fairly general luxury Euro type, predating the primacy of Latins), mostly the Latin infantry helmet is creditted here; this is your "spectacled" "Viking" helmet, really a dressed-up nasal helm. The crown is made in one piece (best), or two joined at a crest: like a barbut, or, more common and lesser, is the spangenhelm; spanked helmet; rivetted or welded together with a hammer; the crown typically made of 4 or 6 triangular plates joined into a variously conical dome. The adjuncts are typically also rivetted or rigidly joined. Nasals are usually solid extensions of the crown ridge. Alternately or in combination with a nasal bar are reinforcing hatlike visors above the eyes, sometimes shaped like eyebrows (the extended spectacle-tubes seem to be limitted to the Latin gladiatorial arena until the renaissance, when they are occasionally seen, along with every other weird shape, it seems; the artisans of the time had a streak of deliberate outlandishness and/or inventiveness to much of their work; it was a time seeking transitions.....). Ear and neck plates sometimes "float" to some degree, but usually on an attachment of rotating rivets, thick leather, or sometimes metal hinges. Versions of these are known in both rawhide and coppery metal, as well as combined materials, and some early versions (seeming widespread) were covered with boars' tusks (as in the preIslamic Pacific, the pig is a very important symbol/totem in preChristian Europe). These seem to have beefed up and filled in until they became the European "great helm". The "Asian" helmets seem to find a flexible row of chain mail, usually at least two links wide (quite flexible) sufficient for attaching the plates, even often for the upper skull (not at all sure I agree that this is sufficient though a good liner helps). Nasals are often seperate and moveable. Medieval European hinged faceplates tend to rigidly lock in place; the Eastern ones seem to swing freely? What of the hinged Persian ones we're looking at? Any side attachments to stiffen up the assembly? In between geographically.......
One presumes the masks served for identification as well as to frighten enemies. Some are very fierce in expression, fanged, etc, but, and in especial reference to Gladiator helmets (actual as well as to some other helmets by extension), which are often facelike in having eye and breath holes, but not anthropomorphic, and rather robotic looking, the unhumanunemotionality of the "face" is held to be the frightening thing about it. Have you ever fought an animal that is entirely unemotional during combat? It's un-natural and, I should think, disturbing. I've never fought a large animal that only wanted to eat me; there was always anger, fear, and/or territoriality involved (I've been stalked, by pigs, a cougar, and "the follower", but didn't actually fight them. I don't know that the follower eats; I don't know that it's physical. No man knows the face of the follower. But it'll follow you.). Occasionally there is a human in whom all you can detect is determination. This seems similar to the look of intentness on the tiger when he is moving in on his prey. It is more frightening than anger.
To the side, many old helmets incorporated a padded liner, while others were meant to be worn over a (usually special) hat, and some ancient warriors were said to use their long hair coiled in specific traditional ways, as a helmet pad. The "Swabian knot"/"Swabian braid" worn by soldiers may connect to this tradition.
Japan fits in here somewhere, probably, with resemblance to the mainland Asian floating pieces/flexible joints, and then with some relation of shape, etc. to other Pacific helmets (perhaps most notably N Pacific ones).