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the Protocol for Devaluation in an Instrumental

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									                   Protocol for Devaluation in an Instrumental Task


General Overview
       We have developed a protocol for assessment of devaluation in an instrumental task
that is well-adapted for testing mice. Typically mice that are trained to respond on two
levers, each associated with a distinct reward, will learn to associate each response with
its predicted rewarding outcome. Subsequent prefeeding with one of the rewards (i.e.,
sensory-specific satiety) will result in a reduction that reward‟s motivational and
incentive value. This devaluation is manifest in a test, by suppression of the response
associated with the „devalued‟ reward, but not the other response associated with the non-
devalued reward. Note that this change in behavior can occur spontaneously when mice
are tested without any reward in the devaluation test. Mice that are unable to adapt to
changes in outcome value, or to guide behavior based on the current value of predicted
outcomes (such that they act in a habitual manner) will fail to show this devaluation
effect. A direct test of reward consumption is also included in the protocol (rewarded
choice test) to assess the motivational effect of satiation and the ability of mice to
suppress responses to a devalued reward itself, rather than to the events that predict
reward.
Apparatus
        All mice are trained in identical individual chambers with aluminum front and
back walls, clear polycarbonate sides, and a floor made of stainless steel rods (Med
Associates, St Albans, VT). Each chamber has been modified to include a liquid
dispenser into which 0.1ml of liquid can be delivered, in addition to a vacuum attached to
the bottom of the food cup (to suction off residual solution at the end of a trial). An
infrared photocell placed inside the food cup monitors time spent and number of entries
into the food cup. Retractable ultrasensitive mouse levers (Med Associates, St Albans,
VT) are available on the left and right sides of the food cup. Ambient illumination for the
chamber is provided by a 28V, 100 mA house light mounted on the inside wall of the
sound attenuating chamber. An IBM-compatible computer equipped with Med-PC
software (Med Associates, St Albans, VT) controls and records all stimuli and responses.


Procedures:
Food Cup Training.
       Initially mice are given food cup training, which consists of two 45-min sessions
per day (one in the A.M. and one in the P.M) for a total of two days. In the A.M. session
mice are given random presentations (Random Time-30 second schedule) of 0.1-ml
orange-flavored sucrose solution (“orange”) and in the other, P.M. session, a 0.1-ml
grape-flavored sucrose solution (“grape”) is delivered. The order of the two sessions is
fully counterbalanced. The solutions are 0.1% Koolaid brand flavorings in 10% sucrose
(w/v).

Lever-press training
         Mice then receive two instrumental training sessions per day (i.e., A.M. and
P.M.), one with only the left lever present and one with only the right lever present, with
the order of the two sessions alternating daily. For half the mice in each group, left lever
responses result in delivery of grape and responses on the right lever produce delivery of
orange. The remaining mice are assigned the opposite response-outcome contingencies.
For the first two days mice are given 60-min sessions in which each response is
reinforced. For the remaining 8 days the session duration is reduced to 20-min and
reward delivered on a random ratio (RR) 5 schedule (i.e., on average every 5 responses
results in reward delivery) on days 3 and 4, and RR10 on days 5-8, and an RR15 on days
9 and 10. Thus, mice are given a total of 10 sessions of instrumental training on each
lever.

Devaluation Treatment (i.e., sensory-specific satiety)
         The next day, mice receive a sensory-specific devaluation treatment by prefeeding
each mouse with one of the two outcomes for a 2-hour period. Each mouse is placed in a
separate holding cage, with a cube filled with 2-ml of either grape or orange, fully
counterbalanced across the prior response-outcome contingencies. The experimenter
monitors and records reward consumption during this phase. As the solution is consumed,
it is replaced in 1-ml increments to ensure continuous reward availability for each mouse.

Devaluation Test
        Immediately following the devaluation treatment, the mice are given a 30-min
extinction test session in the experimental chamber during which responses are not
reinforced with reward delivery. Unlike in training, both levers are available in this test
session. To the extent that responding is controlled by the current value of the reward
anticipated after each of the two responses (left and right lever presses), mice should
preferentially perform the response associated with the reward that was not prefed (i.e.,
the non-devalued response).

Rewarded Choice Test
       Finally, the effectiveness of the prefeeding devaluation treatment in altering the
mice‟s preference for the rewards themselves is assessed. On a separate day mice are
given prefeeding identical to that used in the previous devaluation treatment. On
completion, the mice are given access to two cubes, one containing 2-ml of the prefed
reward and other containing 2-ml of the other reward. Mice are then given 30-min to
consume each reward, with the expectation that consumption will be greater for the non-
devalued reward.

								
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