Blocking.
 
Group 2 rats are trained to associate the red light and the blue stick with the cat odor.
 
Group 3 rats are first conditioned to the stick, then conditioned to joint presence of the stick and the light.
 
If Group 2 individuals react more strongly to just the light, blocking has occurred in the Group 3 individuals.
 
Thus, the initial conditioning with the stick blocks the second-order learning.