How would the fungus control the ant where he goes, attaching to a major vein on the underside of a leaf seems to either imply that the ant (brain) is affected

How would the fungus control the ant where he goes, attaching to a major vein on the underside of a leaf seems to either imply that the ant (brain) is affected to make the ant move to that location, or that fungus is capable of interpreting environment which seems quite unlikely. I find it far more likely that the full effect is quite complex, perhaps the fungus is able to create chemical "urges" in the ant which force him to leave the colony and go for undersides of the leaves, only to overwhelm the ant completely by controlling the muscles directly once it is in position to lock his mandibles on the leave until death.