Ms Talk Pt. 1

Delay Discounting of Delay Discounting of Qualitatively Different Food Qualitatively Different Food

Rewards in Rats Rewards in Rats

Amanda L. CalvertAmanda L. CalvertWashington University in St. Washington University in St.

LouisLouis

Delay DiscountingDelay Discounting refers to the decrease in refers to the decrease in subjective value of a reward as the delay to its subjective value of a reward as the delay to its receipt increases.receipt increases.

A discounting function describes the relation A discounting function describes the relation between the value of a reward as a function of between the value of a reward as a function of delay. Both human and non-human discounting is delay. Both human and non-human discounting is well described by the simple hyperbola (Mazur, well described by the simple hyperbola (Mazur, 1987):1987):

where where V V is the subjective value of the reward, is the subjective value of the reward, A A is is the actual amount of the delayed reward, and the actual amount of the delayed reward, and D D is is the delay to receipt. The parameter the delay to receipt. The parameter k k describes the describes the discounting rate.discounting rate.

V = A / V = A / (1 + (1 + kk

DD),),

The slope of the discounting function is The slope of the discounting function is expressed by the free parameter expressed by the free parameter kk : :

V = A / V = A / (1 + (1 + kk DD).).

As As kk increases, the slope of the discounting increases, the slope of the discounting function becomes steeper. In other words, function becomes steeper. In other words, as as k k increases, the reward looses value increases, the reward looses value more quickly with increasing delay.more quickly with increasing delay.

It appears that the rate at which delayed It appears that the rate at which delayed rewards are discounted is not invariant, rewards are discounted is not invariant, even within individualseven within individuals. .

In humans, the rate of discounting depends on the In humans, the rate of discounting depends on the amountamount of the delayed reward. Specifically, larger amounts are of the delayed reward. Specifically, larger amounts are discounted less steeply (smaller values of discounted less steeply (smaller values of kk) than smaller ) than smaller amounts. This is known as the amounts. This is known as the magnitude effect.magnitude effect.

Green, Myerson, & Green, Myerson, & Ostaszewski, 1999Ostaszewski, 1999

Neither rats nor pigeons Neither rats nor pigeons show the show the magnitude magnitude effecteffect when rewards when rewards differ in amount.differ in amount.

Richards, Mitchell, De Witt, & Richards, Mitchell, De Witt, &

Seiden, 1997Seiden, 1997

Green, Myerson, Holt, Slevin, & Green, Myerson, Holt, Slevin, & Estle, 2004Estle, 2004

Rat

The lack of the magnitude effect in The lack of the magnitude effect in non-humans may be due to underlying non-humans may be due to underlying species differences.species differences.

‘‘Mental account’ explanation applies only Mental account’ explanation applies only to to

humans, but implausible humans, but implausible (Kahneman & Tversky, 1979)(Kahneman & Tversky, 1979). .

Language (i.e., framing effects) could play Language (i.e., framing effects) could play a a

mediating role mediating role (Grace & McLean, 2005)(Grace & McLean, 2005). .

Evolutionary explanation?Evolutionary explanation?

It may also be that the processes It may also be that the processes governing discounting of delayed money governing discounting of delayed money and consumable rewards are different.and consumable rewards are different.

The well-supported finding is that delayed hypothetical The well-supported finding is that delayed hypothetical money is discounted less steeply than consumable money is discounted less steeply than consumable rewards,rewards, be they food be they food (Odum & Rainaud, 2003)(Odum & Rainaud, 2003), or substances of abuse , or substances of abuse (e.g., (e.g.,

cigarettes, Bickel et al., 1999; heroin, Madden et al., 1997)cigarettes, Bickel et al., 1999; heroin, Madden et al., 1997) . .

But, the magnitude effect has been found with But, the magnitude effect has been found with differentdifferent amounts of delayed hypothetical beer, soda, and amounts of delayed hypothetical beer, soda, and candy bars candy bars (Estle et al., 2006)(Estle et al., 2006)..

It may be because the range of It may be because the range of amounts used are typically smaller for amounts used are typically smaller for non-humans than for humans.non-humans than for humans.

In Green et al. In Green et al. (2004)(2004), the range of amounts for , the range of amounts for pigeons pigeons was from 5 to 32 pellets; the range for rats was was from 5 to 32 pellets; the range for rats was from 5 to 20 from 5 to 20 pellets.pellets.

In Richards et al. In Richards et al. (1997)(1997), the range of amounts for rats , the range of amounts for rats waswas from 100 to 200 from 100 to 200 µL of water.µL of water.

For humans, the range in Green et al. For humans, the range in Green et al. (1997)(1997) was from was from $100 to $100,000, as one example.$100 to $100,000, as one example.

It may be due to differences in the quality It may be due to differences in the quality of the rewards.of the rewards.

It could be argued that large amounts of money can buy It could be argued that large amounts of money can buy

qualitatively different goods than small amounts of qualitatively different goods than small amounts of money,money,

and that preference among these goods may also be and that preference among these goods may also be

disparate. disparate.

If a reward is highly preferred, then one might be more If a reward is highly preferred, then one might be more

willing to wait for the larger reward (that is, the willing to wait for the larger reward (that is, the subjective subjective

value would be greater).value would be greater). Adults, children, and pigeons show a greater proportion of choice of the Adults, children, and pigeons show a greater proportion of choice of the

delayed reward when only that alternative yields the more highly delayed reward when only that alternative yields the more highly

preferred reward preferred reward (Forzano & Logue, 1995; King & Logue, 1990)(Forzano & Logue, 1995; King & Logue, 1990)..

There is some empirical support that reinforcer There is some empirical support that reinforcer quality is a dimension of reinforcer value.quality is a dimension of reinforcer value.

Miller Miller (1976)(1976) found matching on found matching on conc conc VI VI schedules in VI VI schedules in pigeons when the alternatives yielded different grains. pigeons when the alternatives yielded different grains. Estimates of bias indexed preference among the grains.Estimates of bias indexed preference among the grains.

Behavioral contrast has been found when components of a Behavioral contrast has been found when components of a multiple schedule differ in reinforcer quality multiple schedule differ in reinforcer quality (in rats, Beninger & (in rats, Beninger &

Kendall, 1975; in pigeons, Ettinger, McSweeney, & Norman,1981)Kendall, 1975; in pigeons, Ettinger, McSweeney, & Norman,1981)..

This leads to an alternative method for This leads to an alternative method for investigating magnitude effectsinvestigating magnitude effects in non-in non-

humans; using consumable rewards that humans; using consumable rewards that differ differ in qualityin quality..

organisms will choose a high-quality organisms will choose a high-quality reward over a low-quality reward.reward over a low-quality reward.

the discounted value of the high-the discounted value of the high-quality reward should be greater quality reward should be greater than the discounted value of the low-than the discounted value of the low-quality reward, all else being equal.quality reward, all else being equal.

If reward quality affects reward If reward quality affects reward value, then:value, then:

Two goals of the present Two goals of the present experiment:experiment:

Replicate the finding of the lack of a Replicate the finding of the lack of a magnitude effect in rats with rewards magnitude effect in rats with rewards that differ in that differ in amountamount..

Extend the investigation of the Extend the investigation of the magnitude effect in discounting to magnitude effect in discounting to include rewards that differ in include rewards that differ in qualityquality..

Preference TestsPreference Tests 5 rats5 rats 2 test types: Operant choice and 2 test types: Operant choice and

Home cage free accessHome cage free access 3 occasions: before discounting, after 3 occasions: before discounting, after

11stst 4 conditions, and after discounting 4 conditions, and after discounting 3 reward types: sucrose, precision, 3 reward types: sucrose, precision,

cellulosecellulose Reward types presented in pairsReward types presented in pairs Percent choice or consumed measuredPercent choice or consumed measured

suc vs.

cellprec vs.

cellsuc vs.

prec

Choice Preference TestOccasion 1

% C

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Home Cage Preference TestOccasion 3

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suc vs.

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suc vs.

cellprec vs.

cellsuc vs.

precsuc vs.

cellprec vs.

cellsuc vs.

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suc vs.

cellsuc vs.

cellprec vs.

cellprec vs.

cellsuc vs.

precsuc vs.

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Preference TestsPreference Tests

All rats showed an overwhelming All rats showed an overwhelming preference for precision and sucrose preference for precision and sucrose pellets over cellulose pellets on all pellets over cellulose pellets on all occasions for both test types.occasions for both test types.

Inconsistent preference between Inconsistent preference between precision and sucrose pellets.precision and sucrose pellets. Could be due to differential exposure to the Could be due to differential exposure to the

pellet types at the time of testing.pellet types at the time of testing.

Discounting ProcedureDiscounting Procedure

5 rats5 rats 3 reward qualities: precision, 3 reward qualities: precision,

cellulose, sucrose pelletscellulose, sucrose pellets 2 amounts: 10 and 30 pellets2 amounts: 10 and 30 pellets 4 delays at each reward quality and 4 delays at each reward quality and

amount: 2, 4, 8, and 24 secondsamount: 2, 4, 8, and 24 seconds 6 total conditions (3 qualities x 2 6 total conditions (3 qualities x 2

amounts); each rat experienced at amounts); each rat experienced at least 5 conditionsleast 5 conditions


Trials consisted of Trials consisted of choice between an choice between an immediate, smaller immediate, smaller amount of food amount of food reward, and a reward, and a delayed, larger delayed, larger amount of food amount of food reward.reward.

Standard amount10, 30Delay

2, 4, 8, 24”

Adjusting amount

immediateIT

I ITI

Free SS.MOV Free LL.MOV

C:\Documents and Settings\Amanda Calvert\Desktop\Quality study\Rat Videos\Free SS.MOV

C:\Documents and Settings\Amanda Calvert\Desktop\Quality study\Rat Videos\Free LL.MOV


Daily sessions composed of blocks of Daily sessions composed of blocks of trialstrials

4 trials per block4 trials per block Trials 1 & 2 were forced choiceTrials 1 & 2 were forced choice

These trials ensured that the animal These trials ensured that the animal experienced each outcome.experienced each outcome.

Trials 3 & 4 were free choiceTrials 3 & 4 were free choice These trials were used to asses preference These trials were used to asses preference

and to determine the adjustment to be and to determine the adjustment to be made to the immediate outcome. made to the immediate outcome.


During the free choice trials within a block, if During the free choice trials within a block, if the rat chose the immediate alternative twice, the rat chose the immediate alternative twice, then the amount of the immediate alternative then the amount of the immediate alternative decreased by one pellet for the next block of decreased by one pellet for the next block of trials.trials.

If the rat chose the delayed alternative twice, If the rat chose the delayed alternative twice, then the amount of the immediate alternative then the amount of the immediate alternative increased by one pellet for the next block of increased by one pellet for the next block of trials.trials.

If the rat chose each once, then the immediate If the rat chose each once, then the immediate amount remained the same for the next block.amount remained the same for the next block.


When the rat chose each alternative When the rat chose each alternative equally often, it was indifferent between equally often, it was indifferent between the two alternatives.the two alternatives.

The amount received immediately that is The amount received immediately that is equal in value to the amount received after equal in value to the amount received after a delay is an estimate of the a delay is an estimate of the Subjective Subjective ValueValue of the delayed alternative. of the delayed alternative.

The simple hyperbola was fit to the The simple hyperbola was fit to the Subjective ValuesSubjective Values for each condition to for each condition to arrive at an estimate of arrive at an estimate of k.k.

Health & Medicine

Ms Talk Pt. 1