Here is my reasoning. In Condition 1,2 and 3 he is doing well with all 3 systems available in cond 1 and somato and vest available in 2 and 3. But in 4,5,6 somato is unavailable and though visual is available in cond 4 he is still losing his balance. Then 5,6 is of course mainly vestibular. But since losing in 4 too it implies his visual is ineffective to help him.
Hope this helps.