Second-order conditioning and sensory preconditioning are both examples of classical conditioning, which is the creation of associations between two stimuli, or a stimuli and a response, due to paired presentation of the conditioned objects. Second-order conditioning is a specific form of conditioning, where a first stimuli is conditioned to have a response, and then a second stimulus is presented and then followed by the first stimulus, which theoretically creates a situation where both stimuli have the ability to evoke the conditioned response. Sensory preconditioning is a similar idea, with the difference being that the two stimuli are paired together before one stimulus is conditioned to a response. In 1973, the lab of Rizley and Rescorla conducted research on the nature of second-order conditioning and sensory preconditioning, mainly focusing on how the two different processes differ with respect to response extinction. Using aversive, shock-based conditioning methods, they were able to conclude that, in regard to sensory preconditioning, a having one of the stimuli-response pairs weakened or destroyed will have a similar effect to the other stimulus-response pair. This provides a method that could be explored for patients with traumatic memories who try to go through extinction therapy. Since extinction therapy is a mentally taxing, but effective, therapeutic method, this means that using a sensory preconditioning could allow a traumatic memory to be reactivated and made labile without needing to actually re-experience the trauma incident.
Reference: Rizley, R.C., Rescorla, R.A. Associations in Second-Order Conditioning and Sensory Preconditioning. Journal of Comparative and Physiological Psychology (81). 1972. 1-11