as frequently as did sighted subjects
..., but as a group (the blind) they generally came up with the same meaning at least as frequently as did sighted subjects.
I'm okay with the meaning.
But why an inversion structure is used at the end of the sentence?
Is it necessary or optional? I guess it should be optional.
... as adv. as someone did/does , this is the normal case I'm familiar with, but not the example sentence I've given.
Could you help me with that?