Similarly Rescorla (1968) demonstrated that a CS can appear only when the US appears and yet still have the association between them be unlearnable. If a tone is arranged to bellow only when there are shocks, but there are still shocks when there are no tones (that is, the CS only appears with the US, but the US sometimes appears without the CS), no associative learning between the CS and the US will occur. Instead, subjects (in Rescorla 1968, rats) will only learn a connection between the shock and the experimental situation—e.g., the room in which the experiment is carried out.