What are the privileges required to execute a trigger function in PostgreSQL?

What are the privileges required to execute a trigger function in PostgreSQL 8.4? It seems that privileges set to a role do not matter when executing a trigger function. I think I have seen some day that the privileges required to execute a trigger function is the EXECUTE privilege but for the owner of the table, not the actual role that performs the action which fires the trigger that calls the trigger function. I cannot find the documentation part that explains that point, any help?