Four pigeons were trained with VI reinforcement to peck a key which was briefly illuminated by a flickering light. Generalization gradients were then obtained with nine different rates of flicker, four faster than S+ and four slower. Two birds were then trained to discriminate between S+ and the fastest stimulus (S−). These birds were then trained to discriminate between S+ and the two fastest stimuli, alternated as S−'s. This procedure was continued, adding one new S− at a time, until all four stimuli faster than S+ were S−'s. The remaining two birds were trained on this latter discrimination without intervening training. In a final stage, using the first two birds, the slowest stimulus was added as a fifth S−. Generalization gradients in extinction were obtained from each bird after each stage of training. As more stimuli from one end of the continuum served as S−'s, responding increased in the presence of stimuli from the other end of the continuum, and the gradient tended to become flattened at this end.
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