An article attribted to Ctein in Mar/Apr 02 Photo Techniques is reported to say "We see more than line pair resolution....That little discontinuity where one line butts up against another really jumps out at you when the lines aren't perfectly matched.This is called vernier acuity."
I mislaid his article but IIRC, Roger Hicks put the lpm resolution on a print viewed from 10ins as 8 lppm and the vernier acuity at 30 lppm.