SIGNAL PROBABILITY, REINFORCEMENT AND SIGNAL-DETECTION

Five pigeons were trained to detect differences in light intensity. Two stimuli, S1 and S2, differing in intensity, were arranged on the center key of a 3-key chamber according to set probabilities. A peck on the center key turned on the 2 side keys. When S1 was presented on the center key, a peck on the left key was correct and when S2 was presented, a peck on the right key was correct. Correct responses produced reinforcement and incorrect responses produced 3 s blackout. Detection performance was measured under 3 procedures. The 1st was a standard signal-detection design in which the probability of S1 was varied and the number of reinforcements obtained for correct responses to S1 was allowed to covary. In the 2nd procedure, the probability of S1 was varied but the distribution of reinforcements between the 2 choices was kept equal. In the 3rd procedure, probability of S1 was held constant while the distribution of reinforcements was varied between the 2 choices. Changes in response bias were a function of variations in the relative reinforcement ratio for the choice responses and not a function of variations in the probability of stimulus presentation. Discriminability remained constant across the 3 procedures.