Method
Subjects
Three adult pigeons (Columba livia), one White King (#858) and two Silver Kings (#14
and #447), obtained from Palmetto Pigeon Plant (Sumter, SC) served as experimental subjects.
Supplementary mixed grain was provided after daily sessions as necessary to maintain 80-85%
of their free feeding body weights. The pigeons were housed individually in standard cages with
free access to water and grit in an environmentally controlled animal facility maintained under
natural lighting conditions. All pigeons had prior experience with operant conditioning tasks, but
not with the tasks required in this study.
Apparatus
Two identical standard BRS-140 pigeon chambers were used, each measuring 35.5 cm x
29.5 cm x 24 cm. The front panel was equipped with three 2.5-cm response keys arranged
horizontally and a food hopper, 12.5 cm below the center key, provided access to grains of whole
wheat. Each key could be illuminated white, red, or green and was located 22 cm above the
floor. A force of approximately 0.15 N was necessary to operate each key. Each experimental
chamber was enclosed in a sound-attenuating box equipped with a 7-W 120-V nightlight, located
on the upper back wall of the box (behind the operant chamber), and a fan that provided air
circulation and masked extraneous noise. Miniature color TV cameras allowed the pigeons and
the front panel to be observed in an adjacent room. Both operant chambers were controlled by a
single Dell personal computer (Pentium 4) programmed in MED-PC IV, which controlled all
experimental conditions and recorded every event and its time of occurrence with 10-ms
resolution.
Procedure
All subjects were exposed to the same series of conditions in a within-subject design. All
conditions, except the last one (the Reversed-Sequence condition), provided food for the same
Left-Right (L-R) key-peck sequence. Each of the conditions differed only by the colors of the
lights behind the keys, which we manipulated as cues to influence the accuracy of the L-R keypeck sequence. In this section, we first describe the features common to all of the conditions,
followed by identification of the features unique to each condition. Finally, we provide the
rationale for each condition and the order in which the subjects were exposed to this series of
conditions.
Common Features: All conditions implemented a discrete-trials procedure that provided
food or timeout for response sequences containing two pecks. All conditions except one (the
Reversed-Sequence condition) implemented the identical response-reinforcer contingency: A
single peck on the left key followed a single peck on the right key produced 3-s food delivery
and terminated the trial. Food delivery was followed by a 3-s intertrial interval (ITI) in which the
keys were not illuminated and pecks had no programmed consequences. Any other two-peck
sequence produced a 6-s timeout (identical to the ITI) before the next trial began. No feedback
about sequence accuracy was provided until a 2-response sequence was completed. Pecks to the
(dark) middle key during a trial were recorded but not treated as an error. Sessions terminated
after the earlier of 45 min or 60 reinforcements.
Some of the transitions from one condition to another required the subject to satisfy
accuracy and stability criteria. The subject must maintain 90% L-R sequence accuracy r above
for fine consecutive sessions with no increasing or decreasing trend. The unique features of each
condition are described below.
Guiding-Cues condition: The Guiding Cues condition was generally our baseline
condition as the pigeons completed the L-R sequence. At the beginning of each trial, the left key
was illuminated by a red light and the right key was illuminated by a green light. After a peck to
either the left or the right key, the lights reversed position, resulting in a green left key and a red
right key immediately prior to the second response. Correctly completing the L-R sequence
entailed pecking the red key (S+) and avoiding the green key (S-) for both responses.
Reversed-Cues condition: The Reversed Cues condition reversed the order of
illumination of the key lights compared to the Guiding-Cues condition. At the beginning of each
trial, the left key was illuminated by a green light and the right key was illuminated by a red
light. After a peck to either the left or the right key, the lights reversed positions to provide a red
left key and a green right key. Even though the positions of the key colors were reversed,
reinforcement continued to require the L-R response sequence. Thus, correctly completing the LR sequence entailed pecking the green key (S+) and avoiding the red key (S-) for both responses.
Both-Green condition: At the beginning of each trial the left and right keys were both
green. After a peck to either key, both keys briefly blinked off for .1 s, and then both returned to
green. Because both keys were green, the pigeons were given no guiding cues as to how to
complete the L-R key-peck sequence.
Both-Red condition: The Both-Red condition duplicated the conditions of the BothGreen condition except that both keys were illuminated red instead of green.
Reversed-Sequence condition: The pattern of key colors in the Reversed-Sequence
conditions was the same as that in the Reversed-Cues condition. At the beginning of each trial,
the left key was green and the right key was red. After a peck to either key, the lights reversed
position, displaying a red left key and a green right key. This condition differed from all others,
however, in that reinforcement required a reversed, Right-Left (R-L) key-peck sequence, and all
others produced timeout. Just as in the Guiding-Cues condition, the red key was S+ and the
green key was S-, but the positions of these key colors were reversed, requiring the R-L response
sequence.
Experimental Design and Rationale
The sequence of conditions for each pigeon and the rationale for each transition
generally followed that in the between-group series of experiments with rats described in Reid,
Nill, and Getz (2010) and Reid, Rapport, and Le (in preparation). In each case, once L-R
sequence accuracy reached our accuracy and stability criteria, subjects were exposed to a new
condition for at least five sessions. Transient changes in L-R sequence accuracy measured the
effects of stimulus control by the key lights across the two conditions. Changes in stimulus
control were assessed in six transitions between conditions, listed in order of occurrence.
1. Guiding-Cues → Both-Green. The Guiding-Cues condition was designed to allow
subjects to master the L-R sequence when key lights provided explicit guiding cues to influence
response selection. Mastery of this sequence could be due to stimulus control provided by the
guiding cues or by the cues resulting from practicing the L-R sequence (presumably influenced
by location cues). Once sequence accuracy was high and stable, each subject was exposed to the
Both-Green condition, which eliminated the potential influence of the key lights as explicit
guiding cues affecting response selection. The Both-Green condition assessed the degree to
which the subjects could complete the L-R sequence without the explicit guiding cues (i. e.,
behavioral autonomy). [Note: this transition between conditions is comparable to Experiment 1
in Reid et al. (2010.)]
2. Guiding-Cues → Reversed-Cues. Subsequently, we returned each subject to the
Guiding-Cues condition to reestablish baseline sequence accuracy. Once accuracy was high and
stable, the pigeons were exposed to the Reversed-Cues condition, which assessed the degree to
which response selection would be controlled by the reversed order of the key lights or by the
cues resulting from practicing the L-R sequence. In this condition, green was S+ (and red was S-)
in both response positions of the sequence – opposite to the prior condition. [Note: this
transition between conditions is comparable to Experiment 2 in Reid et al. (2010.)]
3. Reversed-Cues → Guiding-Cues. Because the Reversed-Cues condition in Reid et al.
(in prep.) was used as their baseline for Experiment 3, we exposed our pigeons to this condition
until accuracy was high and stable. This condition required the pigeons to complete the same LR sequence even though the key lights were presented in R-L order. As a result, green was S+
(first on the left, and then on the right) in both ordinal positions of the sequence, whereas red (the
previous S+) became S- at both positions. Thus, this condition pitted stimulus control by explicit
R-L key-light cues against control by cues resulting from practicing the L-R sequence. Once
response sequence accuracy was high and stable, each subject was re-exposed to the prior
Guided-Cues condition. This transition assessed the degree to which response selection was
controlled by the cues resulting from practicing the L-R sequence, versus stimulus control by
green keys as S+.
4. Guiding-Cues → Reversed-Cues. Reid et al. (in prep) reported that the effects of
reversing the order of transitions from Guiding-Cues → Reversed-Cues to Reversed-Cues →
Guiding-Cues were surprisingly different. Their Guiding-Cues → Reversed-Cues transition
produced a 90% drop in accuracy, whereas the transition in the opposite order, Reversed-Cues →
Guiding Cues, produced a minimal effect (<10%) in a different group of rats. Reid et al. (2010)
explained these differences as an overshadowing effect, but Reid et al. (in prep) claimed they
resulted from feature-positive discrimination bias. We explored the effects of returning subjects
to the Reversed-Cues condition (replicating the earlier Guiding-Cues → Reversed-Cues
transition) to ascertain whether the large drop in sequence accuracy observed in Reid et al.
(2010) would be replicated, or whether response selection would be controlled by the order of
the key lights or by cues resulting from practicing the L-R sequence.
5. Reversed-Cues → Both-Red. Each subject was exposed to the Both-Red condition,
which eliminated the potential influence of the key lights as explicit guiding cues affecting
response selection. The Both-Red condition assessed the degree to which the subjects could
complete the now well-rehearsed L-R sequence without the explicit guiding cues. Thus, this
assessment was similar to the Guiding-Cues → Both-Green assessment early in this study.
6. Guiding-Cues → Reversed-Sequence. The pigeons were re-exposed to the GuidingCues condition until they met our stability criteria. The novel Reversed-Sequence condition
reversed the order of the key lights, but it also reversed the required order of responding. It
required a reversed, Right-Left (R-L) key-peck sequence, and all other sequences produced
timeout. All previous conditions had provided reinforcement for the same L-R key-peck
sequence, so pigeons had rehearsed this response pattern for about 50 sessions. Thus, the practice
cues resulting from repeating the L-R sequence may have come to control behavior (i.e.,
behavioral autonomy). By switching to a condition requiring a different response pattern, we
were able to assess the strength of this source of stimulus control.
Table 1. This table shows the order of conditions for each pigeon and the number of sessions
exposed to each.
1
2
3
4
5
6
7
8
9
Conditions
P-858 P-447 P-14
Guiding-Cues
7
5
7
Both-Green
5
5
5
Guiding-Cues
5
6
6
Reversed-Cues
7
9
8
Guiding-Cues
7
6
7
Reversed-Cues
6
6
11
Both-Red
5
5
5
Guiding-Cues
6
7
8
Reversed-Sequence
5
5
5