Daniel Watkins and Dr. Harold Miller, Psychology
Recent approaches have interpreted self-control in terms of temporal factors. Most recent models posit that self-control amounts to a choice between a smaller reward that is available sooner and a larger reward that is available later. The model is applicable to both humans and nonhumans. Additionally, the model characterizes the relation between reward value and delay to reward in terms of a hyperbolic discounting function (Green and Myerson, 2004). Figure 1 (adapted from Domjan, 2006) depicts this relation in terms of a small reward available sooner than a large reward. Both curves in the figures are of the form:
where V signifies reward value, A is the amount of reward, D is the delay to reward, and K is the discount parameter.
Of particular note in Figure 1 is the fact that the two hyperbolic curves intersect. By definition, the values of each reward are equivalent at this point and thus its identification as the indifference point. It becomes both theoretically and practically important because, according to the model, it should be possible at any time in advance of the indifference point to produce a commitment response, that is, a response that would mean forgoing the small, sooner reward in favor of the large, later alternative.
Our experimental interest was two-part. First, we sought to establish a procedure for use with pigeons by which we could identify the indifference point when the choice was between two rewards differing in amount as well as delay. Second, we wanted to determine whether the indifference point was affected by body weight, and if so, how the value of K in Equation 1 might be varied to reflect this additional motivational factor.
The free-feeding weight of each pigeon was originally determined by daily weighings over a 2-week period in which each had constant access to mixed grain in the home cage. Subsequently the weight of each pigeon was reduced to 85% of its free-feeding weight. This was the initial value in the following series of body weights at which the same procedure would be utilized: 85%, 80%, 95%, 90%, and 100%.
Our method in testing the pigeons can be seen in figure 2 where a discrete concurrent chain schedule was used. The more the pigeon choose the right disk the longer the delay would increase for the larger reward. The time delay that would be required for the pigeon to fluctuate back and forth between the quicker smaller reward and the delayed larger reward is the point of indifference.
Our initial results (refer to figure 3) were very encouraging as they seemed to indicate that there was a substantial difference between body weight as a function of free feeding weight. However, we were unable to replicate these results in a repeat trial. As such we are continuing to test the pigeons and are looking to replicate our initial findings. This project is anticipated to continue for a minimum of 18 months.