A peak shift on a line-tilt continuum.

Pigeons were trained to discriminate the presence or absence of a vertical line, and their performance on a subsequent generalization test was compared with that of other pigeons trained to discriminate a vertical from a 45 degrees line. On the generalization gradient after discrimination training, the presence/absence discrimination group showed a peak at 0 degrees (vertical) while the peak for the 0 degrees /45 degrees discrimination group shifted from 0 degrees in a direction away from the 45 degrees line. The results, discussed in connection with a recent suggestion about the role of color in the peak-shift effect, are interpreted as supporting the generality of the phenomenon.