Range effects in transient and maintained generalization gradients

In three experiments pigeons were trained to discriminate visual flicker rate stimuli. The stimulus set was varied so that the effects of overall stimulus range and border separation between positive and negative stimuli could be assessed. Experiments 1 and 2 showed that transient generalization gradients were lower in height and flatter with increases in either border separation or overall stimulus range. Border separation and overall range interacted in that the effect of range was greatest with relatively greater border stimulus separation. Experiment 3 showed that increased overall stimulus range reduced the magnitude of positive dimensional contrast in maintained gradients. Flattening of the maintained gradient with increases in stimulus range was similar to flattening of post-discrimination gradients found in Experiments 1 and 2. The results from all experiments indicate that increases in overall stimulus range do not decrease discriminability of unchanged stimulus values. Instead, increases in stimulus range may increase the variability of response or criterial factors in discrimination.