Bourne, L. E., Levin, J. R. et al. (1976). Picture-word differences in discrimination learning: Effects of conceptual categories. American Journal of Psychology, 89(3), 499-505.
College students were given three trials on lists of either pictures or words either under the standard condition for discrimination learning or under a condition that did not repeat the same items from trial to trial but used items that were different instances of repeated conceptual categories. Consistent with the data of an earlier experiment , and as would be predicted from the frequency theory of discrimination learning, the picture-word differences observed under the standard condition disappeared under the condition involving conceptual categories.