Abstract
The performances of three rats were stabilized on a multiple schedule that maintained responding by a free-operant avoidance schedule during independent presentations of tone and light. The simultaneous absence of these stimuli signalled shock-free periods and controlled response cessation. Subsequently, test sessions were administered consisting of independent presentations of each stimulus and these stimuli compounded (tone-plus-light). During an extinction test, additive summation was observed to the compounded stimuli, i.e., more responses were emitted to the compound than to either tone or light. During a series of 28 maintenance-test sessions in which the shock schedule remained operative, the compounded stimuli produced a generally enhanced response rate and fewer pauses terminating with shock than either single stimulus condition. These results extend the generality of free-operant additive summation to responding maintained by aversive control. In addition, a comparison of the present study with previous experiments reporting additive summation of positively reinforced responding indicates that similar variables-rate and aversive differences between training stimulus conditions-should be considered in accounting for response distributions during stimulus compounding when responding is controlled by either positive or negative contingencies.