Pigeons were trained to choose between colored lights (A, B, C, and D)
, first in a two-pair ambiguous-cue problem (A+B-, B+C-), and then, wi
th all colors nondifferentially reinforced, in a three-pair loop probl
em (A+B-, B+C-, C+A-) followed by a four-pair loop problem (A+B-, B+D-
, D+C-, C+A-). Systematic efforts were made to simulate the data with
a variety of models incorporating one or another of three conceptions
of stimulus compounding prominent in the literature on compound condit
ioning. One conception is that the components of a compound stimulus a
re altered by interaction; the second is that they are not altered, bu
t only supplemented with a new (compound-unique) component generated i
n the interaction; and the third is that the components entirely disap
pear in a configurational transformation. The ambiguous-cue data could
be simulated accurately with each of the models, but the loop data wi
th none of them. A convincing explanation of performance in loop probl
ems remains to be found.