Roman Cavalry parade ceremonial helmets

[Nathan Ross]

It may be a bit later than the period you're interested in,. but this lengthy discussion of the use (or not!) of masked helmets by later Roman cataphracts might be interesting:

Face mask or face guard in late Roman period

Particularly, maybe, the idea often expressed in literature that masked men resembled 'living statues', and that they inspired 'terror' and were 'dreadful to behold', which might fit with your psychological angle.