DEFINITION OF AN OMISSION PROCEDURE   [p. 17 of handout]
Normal classical conditioning

CS UCS

CS+CR UCS

e.g

light FOOD

light+saliva FOOD

Classical conditioning with omission

CS UCS

CS+CR NO UCS

e.g. Williams and Williams, 1969

light FOOD

light+peck NO FOOD

  With the omission procedure, the Pavlovian effect of anticipatory conditioned responding is pitted against the instrumental influence which would suppress responding in order to maximise goal achievement — Williams & Williams (1969) found that pigeons would miss up to 50% of possible food rewards.

With an aversive UCS, the instrumental influence should enhance responding in an omission procedure. This is avoidance training, where it is clearly more difficult to separate Pavlovian and Instrumental effects, but where both undoubtedly can occur. However Sheffield (1965) found no evidence that avoidance training works with salivation: if acid injected into the mouth is used as the US, conditioned salivation occurs, but with an omission procedure, in which salivation to the CS prevents the occurence of the acid, dogs do not learn to salivate consistenly to avoid the acid, but salivate on about 50% of the trials. Sheffield (1965) had also found that a dog which normally displayed anticipatory salviation to a CS which signalled food did not learn to suppress this under an omission procedure, but again continued to show anticipatory salivation on 50% of the trials. He suggested that a possible conclusion was that "the law of effect does not apply to involuntary responses", especially responses whose execution involves little sensory feedback (Sheffield, 1965; p. 313: see Lieberman, 2000, p. 443-4)