Ms Talk Pt. 1


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This is Part 1 of some of my recent research.

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Ms Talk Pt. 1

  1. 1. Delay Discounting of Qualitatively Different Food Rewards in Rats Amanda L. Calvert Washington University in St. Louis
  2. 2. <ul><li>Delay Discounting refers to the decrease in subjective value of a reward as the delay to its receipt increases. </li></ul><ul><li>A discounting function describes the relation between the value of a reward as a function of delay. Both human and non-human discounting is well described by the simple hyperbola (Mazur, 1987): </li></ul><ul><li>where V is the subjective value of the reward, A is the actual amount of the delayed reward, and D is the delay to receipt. The parameter k describes the discounting rate. </li></ul>V = A / (1 + k D ),
  3. 3. The slope of the discounting function is expressed by the free parameter k : V = A / (1 + k D ). As k increases, the slope of the discounting function becomes steeper. In other words, as k increases, the reward looses value more quickly with increasing delay. It appears that the rate at which delayed rewards are discounted is not invariant, even within individuals .
  4. 4. <ul><li>In humans, the rate of discounting depends on the amount of the delayed reward. Specifically, larger amounts are discounted less steeply (smaller values of k ) than smaller amounts. This is known as the magnitude effect. </li></ul>Green, Myerson, & Ostaszewski, 1999
  5. 5. <ul><li>Neither rats nor pigeons show the magnitude effect when rewards differ in amount. </li></ul>Richards, Mitchell, De Witt, & Seiden, 1997 Green, Myerson, Holt, Slevin, & Estle, 2004 Rat
  6. 6. <ul><li>The lack of the magnitude effect in non-humans may be due to underlying species differences. </li></ul><ul><li>‘ Mental account’ explanation applies only to </li></ul><ul><li>humans, but implausible (Kahneman & Tversky, 1979) . </li></ul><ul><li>Language (i.e., framing effects) could play a </li></ul><ul><li>mediating role (Grace & McLean, 2005) . </li></ul><ul><li>Evolutionary explanation? </li></ul>
  7. 7. <ul><li>It may also be that the processes governing discounting of delayed money and consumable rewards are different. </li></ul><ul><li>The well-supported finding is that delayed hypothetical </li></ul><ul><li>money is discounted less steeply than consumable rewards, </li></ul><ul><li>be they food (Odum & Rainaud, 2003) , or substances of abuse (e.g., </li></ul><ul><li>cigarettes, Bickel et al., 1999; heroin, Madden et al., 1997) . </li></ul><ul><li>But, the magnitude effect has been found with different </li></ul><ul><li>amounts of delayed hypothetical beer, soda, and candy bars </li></ul><ul><li>(Estle et al., 2006) . </li></ul>
  8. 8. <ul><li>It may be because the range of amounts used are typically smaller for non-humans than for humans. </li></ul><ul><li>In Green et al. (2004) , the range of amounts for pigeons </li></ul><ul><li>was from 5 to 32 pellets; the range for rats was from 5 to 20 </li></ul><ul><li>pellets. </li></ul><ul><li>In Richards et al. (1997) , the range of amounts for rats was </li></ul><ul><li>from 100 to 200 µL of water. </li></ul><ul><li>For humans, the range in Green et al. (1997) was from </li></ul><ul><li>$100 to $100,000, as one example. </li></ul>
  9. 9. <ul><li>It may be due to differences in the quality of the rewards. </li></ul><ul><li>It could be argued that large amounts of money can buy </li></ul><ul><li>qualitatively different goods than small amounts of money, </li></ul><ul><li>and that preference among these goods may also be </li></ul><ul><li>disparate. </li></ul><ul><li>If a reward is highly preferred, then one might be more </li></ul><ul><li>willing to wait for the larger reward (that is, the subjective </li></ul><ul><li>value would be greater). </li></ul><ul><ul><li>Adults, children, and pigeons show a greater proportion of choice of the </li></ul></ul><ul><ul><li>delayed reward when only that alternative yields the more highly preferred reward (Forzano & Logue, 1995; King & Logue, 1990) . </li></ul></ul>
  10. 10. <ul><li>There is some empirical support that reinforcer quality is a dimension of reinforcer value. </li></ul><ul><ul><li>Miller (1976) found matching on conc VI VI schedules in pigeons when the alternatives yielded different grains. Estimates of bias indexed preference among the grains. </li></ul></ul><ul><ul><li>Behavioral contrast has been found when components of a multiple schedule differ in reinforcer quality (in rats, Beninger & Kendall, 1975; in pigeons, Ettinger, McSweeney, & Norman,1981) . </li></ul></ul><ul><li>This leads to an alternative method for investigating magnitude effects in non-humans; using consumable rewards that differ in quality . </li></ul>
  11. 11. <ul><li>organisms will choose a high-quality reward over a low-quality reward. </li></ul><ul><li>the discounted value of the high-quality reward should be greater than the discounted value of the low-quality reward, all else being equal. </li></ul>If reward quality affects reward value, then:
  12. 12. Two goals of the present experiment: <ul><li>Replicate the finding of the lack of a magnitude effect in rats with rewards that differ in amount . </li></ul><ul><li>Extend the investigation of the magnitude effect in discounting to include rewards that differ in quality . </li></ul>
  13. 13. Preference Tests <ul><li>5 rats </li></ul><ul><li>2 test types: Operant choice and Home cage free access </li></ul><ul><li>3 occasions: before discounting, after 1 st 4 conditions, and after discounting </li></ul><ul><li>3 reward types: sucrose, precision, cellulose </li></ul><ul><li>Reward types presented in pairs </li></ul><ul><li>Percent choice or consumed measured </li></ul>
  14. 18. Preference Tests <ul><li>All rats showed an overwhelming preference for precision and sucrose pellets over cellulose pellets on all occasions for both test types. </li></ul><ul><li>Inconsistent preference between precision and sucrose pellets. </li></ul><ul><ul><li>Could be due to differential exposure to the pellet types at the time of testing. </li></ul></ul>
  15. 19. Discounting Procedure <ul><li>5 rats </li></ul><ul><li>3 reward qualities: precision, cellulose, sucrose pellets </li></ul><ul><li>2 amounts: 10 and 30 pellets </li></ul><ul><li>4 delays at each reward quality and amount: 2, 4, 8, and 24 seconds </li></ul><ul><li>6 total conditions (3 qualities x 2 amounts); each rat experienced at least 5 conditions </li></ul>
  16. 20. Discounting Procedure <ul><li>Trials consisted of choice between an immediate, smaller amount of food reward, and a delayed, larger amount of food reward. </li></ul>
  17. 21. Discounting Procedure <ul><li>Daily sessions composed of blocks of trials </li></ul><ul><li>4 trials per block </li></ul><ul><li>Trials 1 & 2 were forced choice </li></ul><ul><ul><li>These trials ensured that the animal experienced each outcome. </li></ul></ul><ul><li>Trials 3 & 4 were free choice </li></ul><ul><ul><li>These trials were used to asses preference and to determine the adjustment to be made to the immediate outcome. </li></ul></ul>
  18. 22. Discounting Procedure <ul><li>During the free choice trials within a block, if the rat chose the immediate alternative twice, then the amount of the immediate alternative decreased by one pellet for the next block of trials. </li></ul><ul><li>If the rat chose the delayed alternative twice, then the amount of the immediate alternative increased by one pellet for the next block of trials. </li></ul><ul><li>If the rat chose each once, then the immediate amount remained the same for the next block. </li></ul>
  19. 23. Discounting Procedure <ul><li>When the rat chose each alternative equally often, it was indifferent between the two alternatives. </li></ul><ul><li>The amount received immediately that is equal in value to the amount received after a delay is an estimate of the Subjective Value of the delayed alternative. </li></ul><ul><li>The simple hyperbola was fit to the Subjective Values for each condition to arrive at an estimate of k. </li></ul>