Thus far, we have discussed the pigeon's classification of
variability for simultaneously presented array of icons. It was
possible that pigeons may be successful at extracting variability
within an array of icons, but be incapable of extracting
the variability present within a list of icons. Perceiving
array variability makes no memory demands and leverages the perceptual
system's ability to determine whether two or more items are the
same or different from one another. Perceiving list variability
makes significant memory demands (only one of the items is present
at any moment) and relies on the cognitive system to compare
a viewed item with one or more items that are stored in memory.
Thus, the classification of list variability may be far more
difficult than the classification of array variability.
We initially trained pigeons to discriminate lists of 16 identical
icons from lists of 16 different icons, where Same lists involve
minimal variability and Different lists involve maximal variability
(Young, Wasserman, & Dalrymple, 1997). Pigeons were subsequently tested with
novel lists consisting of all new icons (a transfer test) and
novel lists consisting of (a) mixtures of same and different
icons in various temporal locations within the list or (b) different
numbers of same and different items (Young et al., 1999). Through
the systematic exploration of the pigeon's responding to these
novel lists, we sought to determine the effective stimulus that
was controlling the pigeons' same-different report responses.
The successive version of the same-different task required
that the pigeon peck once at each of the 16 icons in the list
before the next icon was displayed. The simultaneous version
of the task required that the pigeon peck 16 times anywhere on
the 7 cm x 7 cm display. Forty-eight highly distinguishable Macintosh
icons were chosen as the total item pool; these icons were randomly
sorted into three sets of 16 icons each (Set 1, Set 2, and Set
3), and from these three 16-icon sets, 16-icon lists were constructed.
For any given Same list, a single icon from the appropriate set
was randomly chosen and used to make up a list of 16 identical
icons. For the Different arrays, all 16 of the icons of a set
were used with no repetitions. The 16 same or 16 different icons
were randomly distributed over 25 locations in a 5 x 5 grid.
During training, pecks to the green button on Same trials
or to the red button on Different trials were correct and were
reinforced with food; pecks to the red button on Same trials
or to the green button on Different trials were incorrect and
were punished by repetition of the trial until the correct response
was made. Button color was reversed for half of the subjects.
applet is available on-line that demonstrates the task (with
you playing the part of the pigeon). There are some differences
between the on-line task and the actual task (e.g., the Java
script alternates Same and Different trials for demonstration
purposes, whereas the original program used block randomization);
consult Young, Wasserman, and Dalrymple (1997) for procedural details.
Four pigeons were initially trained to discriminate 16-icon
lists constructed from either Set A or Set B (reaching an 85%
performance criterion) and were later tested with lists constructed
from a second set (Set B or Set A, respectively). In our group
of four pigeons, initial acquisition of same-different responding
was rapid (averaging 35 days), but we did not observe strong
transfer to novel 16-item lists; accuracy on training lists averaged
92% correct, but only 57% correct on new testing lists that were
created from 16 untrained icons. The pigeons were given further
training with lists comprising icons from either Set A or Set
B in an attempt to produce a more generalizable concept and were
then tested with lists constructed from a third set (Set C).
Acquisition of the same-different discrimination for the new
set (Set A or Set B) was very rapid (averaging 11 days), and
we now observed strong transfer to the novel 16-item lists constructed
from Set C; accuracy averaged 94% correct on training lists and
73% correct on the new testing lists. This finding closely corresponds
with other research on natural and abstract conceptualization
by both humans and nonhumans showing that larger sets of training
stimuli promote stronger generalization performance (see Wasserman,
1993 for a review).
This experiment therefore contributed an unprecedented finding:
namely, memory-based conceptualization by a nonhuman animal.
Unlike all previously reported results, successive same-different
discrimination had to be based on the bird's remembering 1 or
more of the 16 icons that had been presented in a list. Although
this result was remarkable, we were interested in whether the
addition of memory demands had produced a fundamental change
in how the pigeon solved this successive same-different discrimination:
would entropy explain these results or was the pigeon relying
on one ore more discriminative processes?
In Experiment 1 of Young et al. (1999),
we tested 16-item lists comprising either 1, 2, 4, 8, or 16 different
types of icons. Furthermore, these icon types were temporally
organized in different ways to produce differences in the number
of transitions between icons. For example, a list comprising
4 icon types could be one of the following: aaaabbbbccccdddd,
aabbccddaabbccdd, or abcdabcdabcdabcd with 3, 7,
or 15 transitions, respectively (each letter in the list represents
a randomly chosen icon type). The results revealed that increasing
the number of icon types led to more "different" responses
and that temporally distributing those different types of items
(i.e., increasing the number of transitions) increased the likelihood
of a "different" response (Figure 10).
Because entropy predicts that variability is a function of the
number and distribution of icon types, but that the temporal
distribution of the icons should have no effect on performance,
these results suggest that entropy (as previously applied to
simultaneous arrays) is not a complete account of successive
same-different discrimination performance.
In Experiment 2A, we more fully investigated
the role of time on our pigeons' same-different discrimination
behavior. We anticipated that items at the end of the list would
have a greater influence on pigeons' behavior than items at the
beginning of the list (a recency effect). To test this idea,
we used lists containing a mixture of same and different items
wherein the same items all occurred at the beginning of the list
(with the different items occurring at the end of the list) or
the same items all occurred at the end of the list (with the
different items occurring at the beginning of the list). For
example, a "same-first" mixture list comprising a mixture
of 8 same items and 8 different items would be presented as aaaaaaaabcdefghi.
A pigeon more influenced by later items than by earlier items
would be expected to respond different more often to that list
(where the different items are all at the end of the list) than
to abcdefghiiiiiiii (where the same items are all at the
end of the list). This is precisely what we observed (Figure 11).
Experiment 2A thus documented a strong
recency effect on our pigeons' behavior. It was possible that
a primacy effect could also have been in operation, but that
this effect was much weaker than the observed recency effect.
We examined this possibility in Experiment 4 by using tests in
which a set of 4 different items appeared either at the beginning,
in the middle, or at the end of a list otherwise containing same
items (we also used lists in which a set of same items occurred
at various positions within a list of different items, but the
effect of 4 same items had no significant effect on choice behavior
in any of the temporal positions).
Our analyses revealed a strong recency
effect, but no primacy effect in our task.
It is apparent that memory was affecting
our pigeons' same-different list discrimination. In Experiment
3, we manipulated the amount of time between each icon in the
list. We observed a decrease in performance accuracy for Same
and Different lists as the interstimulus interval increased.
This result is consistent with pigeons' failing to remember all
of the list items.
If the pigeons were not remembering all of the list items,
then the entropy of the entire list should not (and did
not) account for the birds' discriminative behavior. Extending
our behavioral account by including the presence of memory processes
thus had the potential of explaining the data; perhaps the pigeons
were still using entropy as the discriminative stimulus, but
entropy was computed only on those items that were recalled
at choice time. Further tests revealed that this simple extension
In Experiments 2A, we tested the birds
with same and different lists involving either 2, 4, 8, 12, or
14 items. In Experiments 2B, we tested the birds with same and
different lists involving either 2, 4, 8, 12, 20, or 24 items.
We found that increasing the number of list items raised discrimination
accuracy on both same and different trials (Figure 12).
This result has an important implication. The systematic effect
of list length on same lists indicates that entropy is an insufficient
account of our pigeons' same-different list discrimination. Any
recalled subset of a same list will always have an entropy of
0.0. If choice behavior were solely a function of the entropy
of recalled items, then the birds should have pecked at the "same"
key for all of the same lists regardless of their length; this
result would parallel that observed in pigeons trained with the
simultaneous same-different discrimination. We did not, however,
observe an asymmetry in the effects of list length on same and
different trials; accuracy approached chance for both same and
different lists when list length was decreased. We believe that
these results implicate an evidence accumulation process.
vs. Three Mechanisms
Our analysis of the pigeons' discrimination
of variability in simultaneously presented arrays of items provided
strong evidence that the variability in list items had
a strong effect on the choice behavior of our subjects. Similar
examinations of the pigeons' discrimination of variability in
successively presented lists of items provided strong evidence
that the entropy of the list's items was an insufficient explanation
of the choice behavior of our subjects. An account of the successive
same-different discrimination appears to require two additional
processes: a memory mechanism (wherein the likelihood
of an item being recalled is a direct function of its recency)
and an evidence accumulation mechanism (wherein more list
items produce more definitive choice behavior). A computational
account involving all three mechanisms was successful in accounting
for a much higher proportion of the variance in our pigeons'
behavior (89% variance accounted for) than an account that included
only entropy (68% variance accounted for).
We next considered whether the account
offered here for lists of successively presented icons could
be applied to the previous experiments involving arrays of simultaneously
presented icons; such an integration would provide a parsimonious
account of the pigeon's behavior in two very different tasks.
Pigeons' behavior in the simultaneous
same-different discrimination involved a series of pecks at the
presented icons (pigeons in the simultaneous task are routinely
required to peck at the array between 20 and 30 times). This
pecking behavior could be functionally equivalent to viewing
a list of icons, with each peck at an icon being equivalent to
viewing a single icon in a list. The sampling of icons in the
simultaneous array is under the control of the pigeon, however,
not the experimenter; thus, nothing prevents the pigeon from
pecking at 1 icon 30 times rather than at 15 icons 2 times. The
pigeons' successful discrimination of simultaneous same arrays
from simultaneous different arrays (Young & Wasserman, 1997)
suggests, however, that the pigeons are sampling more than a
single icon in the display.
Under the assumption that each peck
samples a single icon, each "list" of icons that is
produced by sequential pecking at a simultaneous array would
constitute 30 items (under an FR 30 schedule) and thus be equated
for accumulated evidence. In addition, item memory would be factored
out because there would be no control of when or how long a pigeon
viewed each item (thus making item memory a random factor). These
methodological differences leave list variability as the only
factor of our three-factor model that could account for discriminative
performance in the simultaneous same-different task. And, indeed,
a metric of list variability, entropy, was documented to be an
excellent predictor of such performance.
This integration of the two discrimination
tasks assumes that pigeons sequentially process the icons in
simultaneous displays; but, this assumption may not be completely
correct. It is possible that pigeons view and process collections
of array items in a simultaneous display. To best determine the
extent to which our account of successive same-different discrimination
can be extended to simultaneous same-different discrimination,
it would be necessary to conduct studies involving the manipulation
of evidence (e.g., by decreasing the FR requirement for some
test trials) and the manipulation of item memory (e.g., by removing
items from the display as they are pecked) in a simultaneous
same-different task. The integration of pigeons' performance
on these two discriminations is an intriguing possibility that
we will likely pursue.
Next section: Conclusions