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DELAYING THE TERMINAL LINK STIMULUS ONSET IN CONCURRENT CHAINS SCHEDULES Choice
(or preference) is
operationally defined as responding more for one option than for another.
In the animal lab, the most common method for studying choice
involves concurrent schedules or concurrent chains schedules.
Concurrent
schedules involve two or more schedules simultaneously available on
different response keys. A Concurrent
Chains involves two or
more chain schedules simultaneously available. The present
experiment used a concurrent chains procedure to study preference
for immediate or delayed stimulus onsets preceding food reinforcement
(these stimuli are thought to be conditioned reinforcers for responding
that produces them). The initial link of the chain is the
"choice" phase and consists of two concurrently available
response keys. Variable
interval 30s schedules operated on these keys. When an interval timed out
on one of the keys, the next response on that key produced the next
schedule (and stimulus) and darkened the other key.
The pigeon then completed the schedule on the lighted key and
received food reinforcement. After
reinforcement, both keys were lighted and the initial links started again.
Terminal link schedules were a constant FT 15s. The amount of food was equal in all compared schedules; only
the delay parameters were manipulated. The experimental question: Would delaying the onset of the
terminal link stimulus shift responding to the side without the delay? |
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METHOD Participant An
adult, white, King Hubbard/Carneaux pigeon (Columba livia) was used in
this study. The pigeon was
housed in a home cage with water available at all times.
Access to food was limited to the experimental chamber five days
per week, but the pigeon was fed in the home cage on weekends.
The subject was maintained at approximately 90% of its free feeding
weight and had previously participated in other experiments. Apparatus A
BRS/LVE operant chamber that contained three horizontally arranged
response keys; 25.4 cm from the floor and 6 cm apart was used in this
study. The chamber also
contained an automated food hopper and a house light.
An IBM compatible computer with MED PC interfacing and Turbo Pascal
software was used to arrange the production of stimuli and to record
responses in the chamber. Visual
stimuli were projected on the left and right circular response keys above
the food hopper, one key was to the left and one key was to the right of
the food hopper. Pecks to
these keys closed a micro-switch and were recorded as responses by the
computer. Operant
chamber #1 used the colors RED, GREEN or a white X on a black background
projected on the keys as arranged by the experimental conditions.
Operant chamber #2 used RED, GREEN or YELLOW colors.
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Procedure The Choice
period (Initial link) A
daily session began with the onset of the left and right response keys
(two Xs for chamber #1 and two RED stimuli for chamber #2).
The pigeon could peck either key. After an average of 30s elapsed,
a key peck turned off the two stimuli and turned on a new stimulus
associated with the outcome for the pecked key. The
Outcome period (terminal link) Chamber
1's outcome stimuli were RED on the left key and GREEN on the right key
and chamber 2's outcome stimuli were GREEN on the left key and YELLOW on
the right key. Outcomes
consisted of a 15s fixed time period before food was presented (the
reinforcement period). During
the reinforcement period, all key lights were off (but the chamber was
still illuminated by the house light) and food was available for 3
seconds. Sessions ended after
40 reinforcement periods or 28 minutes (which ever came first). Independent
Variables = Immediate vs.
Delayed Outcome stimuli presentation Immediate
Outcome Stimulus Presentation:
Following the offset of the initial link stimulus, the onset
of the terminal link stimulus occurred immediately. Delayed
Outcome Stimulus Presentation:
Following the offset of the initial link stimulus the key light was off for
5 seconds. After 5 seconds
the terminal link stimulus was lighted for the remaining 10 seconds of the
terminal link. Dependent
Variable= Choice period
Responding The
dependent variable in this experiment was the proportion of responses to
the left key during the choice phase (in the initial concurrent VI30"
schedules). A proportion of
.5 indicated indifference to the two outcomes.
A proportion of less than .5 indicated a preference for the outcome
on the right key and a proportion of greater than .5 indicated a
preference for the outcome on the left key. |