TABLE 1.

Procedures and factors that influence higher-order conditioning.

Factors	Examples	Influence on learning
Stimulus type	Auditory (e.g., tone, white noise, clicker)	Associability and similarity between S2 and S1 influence the strength of higher-order conditioning.
	Visual (e.g., flashing light, key light, context)
	Odour (e.g., almond, vanilla)
	Flavour (e.g., sucrose, saline)
	Shape (e.g., rectangle, triangle)
	Appetitive US (e.g., food pellets, sucrose pellets)
	Aversive US (e.g., footshock, illness)
Stimulus arrangement	Serial (i.e., S2 offset coincides with S1 onset) Simultaneous (i.e., S2 and S1 presented at the same time)	Simultaneous arrangement results in superior sensory preconditioning effect relative to serial arrangement (Thompson, 1972; Rescorla, 1980b; Holland and Ross, 1983). Both serial and simultaneous S2-S1 pairings produce robust second-order learning, however, the arrangement has a differential effect on the content of learning.
Stimulus similarity	S2 and S1 chosen from the same stimulus type S2 and S1 chosen from different stimulus type	Pairing of similar stimuli proceed more rapidly relative to dissimilar stimuli in second-order conditioning (Rescorla and Furrow, 1977; Rescorla and Cunningham, 1979). Spatial similarity and using same cue modality promote sensory preconditioning (Holland, 1977; Rescorla and Cunningham, 1979)
Stimulus order	Forward serial order (i.e., S2 precedes S1) Backward serial order (i.e., S1 precedes S2, US precedes S1)	Higher-order conditioning designs classically use forward serial pairings (Pavlov, 1927). However, backward serial pairings of S1 and S2; US and S1 also support learning (Barnet et al., 1997; Ward-Robinson and Hall, 1998)
Trial number	Conditioned aversion: Single S2-S1 trial Aversive: 4 serial S2-S1 trials, 8 serial S2-S1 trials (Parkes and Westbrook, 2010) Appetitive: 100 trials (Rashotte et al., 1977) 40 trials (Holland and Rescorla, 1975a) 200 trials (Reid, 1952) 2 trials (Jones et al., 2012; Sadacca et al., 2018)	Sensory preconditioning and second-order conditioning can be obtained in single S2 and S1 pairing in conditioned aversion preparation (Archer and Sjöden, 1982). Aversive higher-order learning proceeds in four trials for second-order conditioning and eight trials for sensory preconditioning (Parkes and Westbrook, 2010). In contrast, appetitive designs may require more training trials (Reid, 1952; Rashotte et al., 1977; Jones et al., 2012; Sadacca et al., 2018).
Reinforced presentations	S2-S1 pairing followed by US delivery	Second-order learning can be obtained by reinforced S2→S1 pairings following S1 training (Leidl et al., 2018; Williams-Spooner et al., 2019).