TWO TYPES OF BEHAVIORAL CONTRAST IN DISCRIMINATION LEARNING1

Abstract

Two groups of pigeons received daily discrimination training at two values on a line-tilt continuum. S+ (VI 1) and S– (EXT) intervals alternated, and a 30-sec criterion of no responding to S– was required before S+ returned. Rates of responding to S+ showed two separate contrast effects: at an intermediate stage of training a high peak rate appeared which declined, later in training, to a stable level still in excess of the VI baseline rate. The peak rate was correlated with the total number of responses to S–, while the final rate was not; suggesting that the peak rate and final rate may not be functions of the same variable. These results were compared with performance on a red-green discrimination where the two stages were not so clear. A line-tilt discrimination was repeated with fixed length S– intervals terminated by TO, and showed the same contrast magnitude in the final rate without any peak. The peak rate was interpreted as an effect of the ‘punishment’ contingency where responding to S– prolongs S– for 30 sec, while the final rate was taken to be analogous to previous demonstrations of contrast.