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."
The ability to categorize a set of items as "same" or "different" is often deemed to be foundational to human cognition (Katz, Wright, & Bodily, 2007; Wasserman & Young, 2010). These abstract relations have been intensively studied because they are vital to adaptation in a complex and changing world; sensitivity to same and different relations allows us to draw important comparisons among the many objects and environments that we encounter.
Despite the key roles that sameness and differentness play in cognition and adaptive action (Wasserman, Young, & Cook, 2004), the capacity to detect abstract relations was once believed to be a purely human competence, "which the Faculties of Brutes do by no means attain to" (Locke, 1690/1975, pp. 159-160). Nevertheless, research with nonhuman animals has shown that other primates (Katz, Wright, & Bachevalier, 2002; Wasserman, Fagot, & Young, 2001) and even pigeons (Wasserman, Hugart, & Kirkpatrick-Steger, 1995; Wright & Katz, 2006) exhibit sensitivity to abstract relations.
Although same and different are often assumed to be equally important conceptual "twins" (Delius, 1994), recent research has revealed that these two concepts may not be equally salient. Mounting evidence suggests that same and different relations are not equivalently discriminated.
The first piece of evidence concerns asymmetrical rates of learning in same/different tasks in which animals must peck at a particular set of items rather than at an arbitrary report area. For example, Blaisdell and Cook (2005) found that different1 pigeons taught to peck at a pair of different items, but not to peck at a pair of same items, learned faster than same1 pigeons trained to peck at a pair of same items, but not to peck at a pair of different items. Similarly, Castro, Kennedy, and Wasserman (2010) taught a single group of pigeons to peck at a same array or a different array of items depending on a superordinate color cue; the birds learned to peck "different" more quickly than they learned to peck "same." Finally, faster learning of different than of same relations was reported by Wasserman, Frank, and Young (2002), who gave pigeons a go/ no-go task: Different1 pigeons taught "peck at different/ no peck at same" learned faster than did same1 pigeons taught "peck at same/no peck at different." This set of experiments documents that, with some training procedures, same and different trials are not learned at equivalent rates.
The second piece of evidence is the asymmetrical behavioral effect of reducing the number of items. When pigeons are first trained with arrays containing 16 items and they are later tested with displays containing fewer items, discrimination falls (Brooks & Wasserman, 2010; Young, Wasserman, & Garner, 1997). …