| Abstract: | Avoidance contingencies were defined by the absolute probability of the conjunction of responding or not responding with shock or no shock. The “omission” probability (ρ00) is the probability of no response and no shock. The “punishment” probability (ρ11) is the probability of both a response and a shock. The traditional avoidance contingency never omits shock on nonresponse trials (ρ00=0) and never presents shock on response trials (ρ11=0). Rats were trained on a discrete-trial paradigm with no intertrial interval. The first lever response changed an auditory stimulus for the remainder of the trial. Shocks were delivered only at the end of each trial cycle. After initial training under the traditional avoidance contingency, one group of rats experienced changes in omission probability (ρ00>0), holding punishment probability at zero. The second group of rats were studied under different punishment probability values (ρ11>0), holding omission probability at zero. Data from subjects in the omission group looked similar, showing graded decrements in responding with increasing probability of omission. These subjects approximately “matched” their nonresponse frequencies to the programmed probability of shock omission on nonresponse trials, producing a very low and approximately constant conditional probability of shock given no response. Subjects in the punishment group showed different sensitivity to increasing absolute punishment probability. Some subjects decreased responding to low values as punishment probability increased, while others continued to respond at substantial levels even when shock was inevitable on all trials (noncontingent shock schedule). These results confirm an asymmetry between two dimensions of partial avoidance contingencies. When the consequences of not responding included occasional omission of shock, all subjects showed graded sensitivity to changes in omission frequency. When the consequences of responding included occasional shock delivery, some subjects showed graded sensitivity to punishment frequency while others showed control by overall shock frequency as well. |
