Human beings and other animals are constantly confronted with an extraordinarily
complex array of external stimuli. Yet, sense is somehow made of these varied stimuli. One
way of reducing the demands on an organism's sensory and information-processing systems is
for it to treat similar stimuli as members of a single class; by so doing, substantial
cognitive economy can be achieved, thus freeing the animal's adaptive machinery to deal
with other competing exigencies of survival. In addition, categorical processing permits
an organism to identify novel stimuli as members of a particular class and to generalize
knowledge about that class to these new members. Thus, an organism need not be bound to
respond to only those stimuli with which it has had prior experience, thereby enhancing
its ability to cope with a continually changing world.
Although theorists often extol the adaptive virtues of categorization and
conceptualization, we remain far from understanding exactly how organisms process stimuli
so as to partition the world into classes of related objects and events. Indeed, given the
early writings on the subject, we should wonder whether nonhuman animals are even capable
of conceptual behavior. Over a century ago, Charles Darwin noted, "If one may judge
from various articles which have been published lately, the greatest stress seems to be
laid on the supposed entire absence in animals of the power of abstraction, or of forming
general concepts" (Darwin, 1871/1920, p. 84). Twenty-five years later, Lloyd Morgan
denied animals the ability to behave conceptually. To do so, he said, requires that we
"neglect all that is variable and focus the attention on the uniform relation. [Then]
we have reached a conception, and this conception is not concrete, particular, and
individual, but abstract, general, and of universal application" (1896, p. 263).
Morgan believed that only adult humans (not even children) are capable of
conceptualization. The vestiges of this belief still persist in the field of experimental
Several recent lines of research are radically changing that initial opinion
(Wasserman, 1995). One of those new lines of research concerns the acquisition of the
same-different concept by animals. The classification of two or more items as the same as
or different from one another requires a level of abstract conceptualization previously
thought to be unique to human beings.
Evidence of this ability in nonhuman primates is now substantial (e.g., Premack, 1976,
1983; Wright, Santiago, Urcuioli, & Sands, 1983); however, initial evidence of
same-different conceptualization in the pigeon was promising, but weak. Indeed, early
attempts to train pigeons to discriminate same from different stimuli led to rather
pessimistic conclusions: "Because [a Same-Different concept] transcends the items
themselves there is a possibility that it may be a difficult concept to learn. There is
the possibility that monkeys can abstract the task and learn the Same-Different concept
whereas pigeons cannot" (Wright et al., 1983, p. 316).
In one of the earliest studies of pigeon same-different categorization, Wright et al.
(1983) trained pigeons to peck one key when two slides of real objects were the same as
one another and to peck a second key when the two slides were different from one another.
After nearly 20,000 trials of training using 210 different pictures, the pigeons averaged
over 80% correct to the familiar pictures, but accuracy dropped to only 62% correct to an
unfamilar set of pictures.
Edwards, Jagielo, and Zentall (1983) trained pigeons to peck one key when two
line-drawn circles or pluses were presented and to peck a second key when a plus and
circle were presented. After 2,208 trials, the pigeons averaged 87% correct; but, upon
transfer to differentially reinforced novel displays, accuracy dropped to only 59% during
the first 48-trial transfer session in Experiment 1 (where the novel stimuli were red and
green colors) and to only 57% during the first 48-trial transfer session in Experiment 2
(where the novel stimuli were flashing or steady white lights). These test results suggest
that the pigeon's original learning was primarily item specific.
Most of the prior research on abstract conceptualization in animals has involved the
classification of just two visual items as the same as or different from one another
(e.g., Edwards et al., 1983; Santiago & Wright, 1984; Wright, Santiago, Sands,
Kendrick, & Cook, 1985; Wright et al., 1983). Two items represent the smallest number
necessary for a same-different classification; but, it is also possible, and perhaps even
easier, to make same-different judgments with displays involving a greater number of
items. Wasserman, Hugart, and Kirkpatrick-Steger (1995) recently reported an experiment in
which pigeons were taught to classify displays involving 16 rather than 2 items that were
either all the same as one another or all different from one another
(Figure 1). The displays were 4
x 4 arrays of 16 computer icons that were chosen from a set of 16 possible icons; the Same
displays involved a single randomly selected icon repeated 16 times, whereas the Different
displays involved all 16 icons in one of 16 different spatial configurations. Pigeons
received food pellet reinforcement for pecking one of two buttons ("same") in
the presence of Same displays and for pecking the second button ("different") in
the presence of Different displays. The same-different discrimination was rapidly acquired
and it supported strong transfer to novel 16-item displays; accuracy on trained displays
averaged 83% correct after approximately 13,000 trials and 71% correct on new displays
created from a set of 16 untrained icons.
Recent work in our laboratory has made significant strides toward understanding how
nonhuman animals solve these simultaneous same-different discriminations (Young &
Wasserman, 1997; Young, Wasserman, & Garner, 1997). We have also extended our research
to investigate the pigeon's discrimination of lists of successively presented identical
items from lists of successively presented different items (Young, Wasserman, &
Dalrymple, 1997; Young, Wasserman, Hilfers, & Dalrymple, 1999). In this chapter, we will present a summary of our published work on the
same-different concept, introduce some of our latest findings, and consider comparative
issues by sharing the results of our recent studies on human discrimination of
simultaneous same-different displays.
Next Section Simultaneous Same-Different