Definition extracted from their first level training manual with permission from EMDR Institute, Inc.
Second-order conditioning refers to conditioned responses that involve emotional or behavior disturbances to stimuli that are in themselves not directly threatening. This occurs in trauma survivors when there has been a frequent pairing of situations (or a highly disturbing single experience) that involves negative emotions and a neutral cue. As a result of second order condition, the formerly neutral cue is able to trigger the same disturbances as in the original incident(s). An example would be a conditioned fear of all one way streets after a person travels down one following an auto accident. Fear of automobiles is a first order conditioned response since the automobile was directly harmful. Fear of one way streets is second order conditioning since the street (a formerly neutral cue) was not the cause of the trauma.