Discussion

Experiment 2 showed rates of discounting and response times were again highly varied between participants. Only one participant reached a corner solution, suggesting that the 60 second delay was more effective in obtaining indifference points than the 30 second delay used in Experiment 1. However, as participants’ choices towards LL rewards remained relatively high, an even greater increase in the size of the delay would be necessary in future studies.

Several participants rapidly reached a point at which they chose consistently, however, their rate of discounting increased and they responded more quickly in the final trials. The data therefore showed a decrease in participants’ tolerance of the delay towards the end of the session. This might suggest that these participants were averse to the length of the session (a characteristic common of individuals with ADHD: Sonuga-Barke, Taylor, Sembi & Smith, 1992). Therefore, it seems reasonable to propose that 20 trials were too many for some participants. As each participant's IP was calculated from the final ten trials, it is likely that IPs lacked validity. Several participants (P2, P4, P5, P6, P7, P8 and P9) seemed to reach a point at which they responded consistently either

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before or on trial number 6, suggesting that IPs could effectively be obtained from fewer choice trials. Therefore, reducing the number of choice trials might produce more valid discounting data and response time (RT) data.

The results of Experiment 2 supported those reported in Experiment 1 with respect to response times, showing that participants responded significantly more slowly when making impulsive choices (SS rewards) and faster when making self-controlled choices (LL rewards). It was considered possible that this was affected by participants choosing the more immediate reward when presented with rewards equal in value; however re-analysis of the data provided evidence to suggest that the presentation of this option had no effect on RT. Correlational analyses revealed no significant correlations between IP and RT, possibly as a result of the heterogeneous and small sample size reducing the power of the statistical analyses.

As in Experiment 1, participants seemed to understand the task requirements, and became familiar with the adjustment procedure. Participants also appeared to understand the delay and reward amounts, and were motivated by the monetary reward. However, as several participants became less delay tolerant in the final trials, the potential use of the current CCT for repeated testing across a range of delays seems doubtful. Removal of the additional graphics used in Experiment 1 slightly reduced participants' distraction from the task. However, some participants continued to be distracted by the joystick and asked irrelevant questions throughout.

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Although the adjusting procedure increased and decreased the SS values depending on the previous choice, it was discovered after the data were collected, that the SS value of 3p was incorrectly adjusted to 2p (rather than 1p when 3p SS was chosen) and 4p (rather than 5p when 3p SS was rejected and 10p LL chosen). Furthermore, it was necessary for the SS value to be increased or decreased by similar magnitudes according to each participant’s previous choices, resulting in the reward options converging on a point at which the participant was indifferent between the LL and SS rewards. However, because SS adjustments were made by adding or subtracting half of the previous SS value, when participants reached larger SS values, choosing SS (e.g. 10p SS) would result in larger decreases of the SS value in the next choice trial. Therefore, participants making the self-controlled choices and reaching the option of 10p LL versus 10p SS, most participants made the logical and efficient choice of choosing SS, which they would be punished for as SS returned to the initial value of 5p (Figure 2.7). It seems reasonable to suggest that it would have been more appropriate for the SS reward to have reduced to 8p rather than 5p, allowing participants to respond more consistently between smaller fluctuations.

It was expected that participants would reach a point at which they would respond to the reward options consistently and therefore, calculating each participant’s IP from the final ten SS rewards they were presented with was initially considered a valid method to obtain IPs. However it could be argued that this did not turn out to be the case, as the calculation of IPs only used SS values and therefore did not account for participants such as P1 and P4, who chose all LL rewards towards the end of the task (Figure 2.7). It is likely that IPs for these participants were larger than those actually calculated. Nevertheless,

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the calculation of IP for participants who solely chose LL in the last ten trials or only chose SS when it was equal to LL (P3 and P7) was more representative of these participants' responding than simply averaging SS values in Experiment 1.

As can be seen from P7 in Figure 2.7, following the initial 5p SS versus 10p LL option, two LL choices were required to reach the 10p SS versus 10p LL option. However, on choosing the SS reward in this option, he was presented with the initial choice trial of 5p SS versus 10p LL, rather than returning to the 10p LL versus 8p SS option. In comparison, P4 in Figure 2.7 was presented with 10p LL versus 2p SS in trial number 4 and responded relatively consistently from this point, alternating his choices from 10p (when presented with 2p SS versus 10p LL option) and 3p (when presented with 10p LL versus 3p SS option). Therefore, to produce consistent responding, participants with larger IPs had to choose two LL rewards to obtain larger amounts thus requiring more effort to respond consistently. It is possible that this increased the rate at which participants became averse to the session duration mentioned earlier.

In addition, initial choices may have shaped responding, as choosing several SS values early on meant participants had to endure a slow increase of SS values by choosing the LL reward until they could reach their IP. Again, it is possible that participants were affected by such effortful responding, but having been punished for making these early choices (seeing the SS reward decrease rapidly) chose LL in the remaining trials to avoid low SS values and to continue being offered higher SS values (e.g., P3). Since IPs were calculated from SS values chosen in the last 10 trials, it could be argued that such IP data lacked

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validity. In light of the present findings the CCT adjustment procedure required re-programming to ensure identical progressions between increased SS reward amounts and decreased SS reward amounts.