The unpaired group was chosen as an associative control, because these animals receive the same number of CSs and USs but in a non-overlapping manner. Indeed, unpaired controls in fear conditioning experiments commonly show no signs of CS-elicited fear following training, indicating that no positive association was formed between the CS and US (e.g., Cain and LeDoux, 2007). However, it is important to note that unpaired controls can learn. Like their paired counterparts, they can undergo nonassociative learning from exposure to the aversive US (i.e., sensitization), associative context conditioning, or even conditioned inhibition (safety signal learning).
