The effect of nitrogen varies with its pressure, i.e. with its concentration, while no changes are seen when the helium pressure is varied.
This allows the conclusion that nitrogen has anesthetic effect and helium does not have stimulant effect.
Because it appears that the anesthetic effect of inert gases depends on their molecular mass, being much stronger for heavy gases like xenon, it is possible that helium also has an anesthetic effect, but one that becomes noticeable only at higher pressures than can be safely tested.
This allows the conclusion that nitrogen has anesthetic effect and helium does not have stimulant effect.
Because it appears that the anesthetic effect of inert gases depends on their molecular mass, being much stronger for heavy gases like xenon, it is possible that helium also has an anesthetic effect, but one that becomes noticeable only at higher pressures than can be safely tested.