International Journal of Comparative Psychology

Though differential reinforcement, a discriminative stimulus (S ^D ) acquires two properties.  The operant contingency is responsible for the S ^D s response-discriminative property.  However, as stimulus control develops an S ^D also acquires incentive-motivational properties through its association with reinforcement changes.  A systematic series of experiments are described that breaks the usual co-variation of response and reinforcement rates in most discriminative operant situations.  In three groups, S ^D s (a tone and a light) occasioned steady moderate lever pressing in rats that ceased when neither S ^D was present.  Probably of reinforcement in these S ^D s, relative to when both were off, was systematically manipulated to make them incentive-motivationally excitatory, neutral or inhibitory.  In each S ^D , for the “excitatory” group reinforcement (food) probability increased from 0 to 100%, for the “neutral” group it was unchanged and for the “inhibitory” group it decreased from 100 to 0%.  Although behaviorally indistinguishable in training, a stimulus-compounding assay revealed that tone-plus-light tripled response rate in the incentive-excitatory group, doubled rate in the incentive-neutral group and didn’t increase rate in the incentive-inhibitory group – producing the instrumentally derived incentive-motivational function for the first time.  This is discussed context of two-process learning theory, a functional analysis of transfer-of-control research plus how the response-discriminative and incentive-motivational properties acquired by an S ^D contribute to the stimulus control of behavior.