This paper distinguishes two models specifying the application of impl
icit knowledge. According to one model, originally suggested by Reber
(1967), subjects either apply sufficient knowledge to always produce a
correct response or else they guess randomly (High Threshold Theory;
subjects only apply knowledge when there is sufficient knowledge to ex
ceed a threshold ensuring a correct response); according to the other
model, suggested by Dienes (1992), subjects respond with a certain pro
bability towards each item, where the probability is determined by the
match between the items structure and the induced constraints about t
he structure (probability Matching Theory; subjects match their probab
ility of responding against their personal probability that the item b
elongs to a certain category). One parameter versions of both models w
ere specified and then tested against the data generated from three ar
tificial grammar learning experiments. Neither theory could account fo
r all features of the data, and extensions of the theories are suggest
ed.