Monitoring same/different discrimination behavior in time and space: finding differences and anticipatory discrimination behavior.

Discrimination behavior in a standard, two-alternative forced choice same/different task is usually measured by the pigeon's pecking one or the other of two arbitrary report areas. We found that pigeons make anticipatory, discriminative responses to the visual display during the stimulus observing period prior to the availability of the report areas; the spatial distribution of these anticipatory discriminative responses strongly correlated with the upcoming choice response. These anticipatory pecks provide evidence that the process of discrimination occurs well before the moment of choice and that key aspects of this process can be revealed by looking at the distribution of observing responses. We also manipulated the variability of the displayed items to study the nature of these anticipatory responses; again, the spatial distribution of responding during the stimulus observing period strongly correlated with the upcoming choice response. The distribution of these prechoice pecks supports the theory that pigeons search for differences in the displayed items. If differences are found, then pigeons prepare to report "different"; if not, then they report "same."