In the latter case, when the CS's association with the US evokes some trace of the US in the memory of an animal, this trace, in turn evokes the UR reflexively.
Evidence exists to support the involvement of both types of association in classical conditioning.
The only explanation can be that the animal learned an association between CS1 and CS2 during the sensory preconditioning phase of the experiment, so clearly these stimulus-stimulus associations are learnable. One cannot, however, clearly infer from this result is that the animal must have learned a stimulus-stimulus association between CS1 and the US during the conditioning phase. Thereafter CS2 and CS1 are paired and once again test the subject's response to CS2 alone. Adding a final twist to the experiment, however, provides more conclusive evidence for stimulus-response associations. When CS1 is paired with the new US2 this should not effect the response produced to CS2.