| Abstract: | For four pigeons key-peck responding was reinforced on a variable-interval reinforcement schedule in the presence of a vertical white line. When response rates had stabilized a horizontal white line was introduced, in the presence of which reinforcement was not available (extinction). The horizontal line was presented once per session, immediately before the vertical line was presented. The duration of the horizontal line varied randomly from session to session, being either 0 sec (i.e., no presentation), 10 sec, 30 sec, 2 min, 10 min, 40 min, or 120 min. When the horizontal line was present for more than 0 sec, behavioral contrast was obtained in the presence of the following vertical line. Contrast increased with increasing durations of the horizontal line, asymptoting when the horizontal line was present for 40 min. |
