What is second-order conditioning (SOC)
Second-order conditioning is a form of associative learning in which after a stimulus becomes conditioned through an initial step of association (first-order) becomes the basis for a subsequent stimulus to become conditioned (second-order). It is higher-order conditioning.
There are three phases in the second-order learning mechanisms1.
Phase 1 (1st order conditioning) – the neutral stimulus is followed by an unconditioned stimulus (US) and becomes a first-order conditioned stimulus (CS1) that can elicit a conditioned response (CR).
Phase 2 (2nd order conditioning) – another neutral stimulus is followed by CS1 and becomes the second-order conditioned stimulus (CS2).
Phase 3 – CS2 alone can elicit the same CR that CS1 can even though CS2 has never been directly paired with the US.
Second-order conditioning examples
Example of second-order conditioning in animals
Here are the SOC training phases Pavlov (1927) conducted on his dog2.
Phase 1 – the sound of a metronome (CS1) was paired with the presentation of food (US) to trigger salivation in the dog (CR).
Phase 2 – the dog saw a black square (CS2) paired with the sound of a metronome.
Phase 3 – then the dog salivated at the sight of the black square.
Example of second-order conditioning in humans
It is easier to find second-order learning in animals than in humans although SOC in humans does exist.
Here is a SOC experiment conducted on humans3.
Phase 1 – a geometric shape (CS1) was paired with an aversive loud noise (US) to generate an electrodermal response.
Phase 2 – a picture (CS2) was paired with the geometric shape.
Phase 3 – CS2 alone could elicit an electrodermal response.
Models of SOC
There are many theoretical models attempting to explain SOC. They can be categorized into four types based on the different kinds of associations formed during SOC.
S-R learning model
CS2 and CR develop a direct connection when CS1 is present during the second-order training.
In a chain of association fashion, CS2 will evoke a CS1 representation, and CS1 will evoke the US representation, which in turn will evoke CR.
A direct link between CS2 and US is established during the second-order training trial. Therefore, CS2 could activate the CR.
There is an association between CS1 and CS2 where CR emission is only due to the CS1 representation in CS2.
One special character of conditioning in the second order is that extinction in a first-order stimulus does not necessarily lead to extinction in the second-order stimulus4.
After SOC has been established, extinction occurs if first-order CS is repeatedly presented without being followed by the US. However, even after the first-order association has been extinguished, the second-order CS persists more or less at the same magnitude.
The second-order association has not been affected by the first-order extinction5.
SOC in posttraumatic stress disorder (PTSD)
The difficulty in extinguishing second-order pavlovian conditioning may contribute to PTSD.
In PTSD, the trauma is the US and patients’ fear response is the UR. During the trauma, environmental cues become a conditioned stimulus (CS1) through first-order aversive conditioning. Later, CS1 becomes associated with another environmental cue through the conditioning of S2.
Later, even though the trauma is no longer present and CS1 is extinguished, CS2 remains associated with it and can trigger continued fear responses for many years6.
Also See: Higher Order Conditioning
- 1.Jara E, Vila J, Maldonado A. Second-order conditioning of human causal learning. Learning and Motivation. Published online August 2006:230-246. doi:10.1016/j.lmot.2005.12.001
- 2.Sahley C, Rudy JW, Gelperin A. An analysis of associative learning in a terrestrial mollusc. J Comp Physiol. Published online 1981:1-8. doi:10.1007/bf00612791
- 3.Lee JC. Second-Order Conditioning in Humans. Front Behav Neurosci. Published online July 8, 2021. doi:10.3389/fnbeh.2021.672628
- 4.Davey GCL, McKenna I. The Effects of Postconditioning Revaluation of CS1 and UCS Following Pavlovian Second-Order Electrodermal Conditioning in Humans. The Quarterly Journal of Experimental Psychology Section B. Published online May 1983:125-133. doi:10.1080/14640748308400899
- 5.Rescorla RA. Simultaneous second-order conditioning produces S-S learning in conditioned suppression. Journal of Experimental Psychology: Animal Behavior Processes. Published online 1982:23-32. doi:10.1037/0097-7403.8.1.23
- 6.Wessa M PhD, Flor H PhD. Failure of Extinction of Fear Responses in Posttraumatic Stress Disorder: Evidence From Second-Order Conditioning. AJP. Published online November 2007:1684-1692. doi:10.1176/appi.ajp.2007.07030525