Podgorny, P., & Garner, W. (1979). Reaction time as a measure of inter- and intraobject visual similarity: Letters of the alphabet.
Perception & Psychophysics, 26, 37-52.
Experimenters used two different procedures. In one
condition, participants were asked to make a same-different
judgment between two letter stimuli. In another condition,
participants were asked to rate on a Likert scale how
similar two letters were.