General Discussion

In support of findings by Ainslie (1975) and by Neef et al. (1993), Experiments 1 and 2 found that there was large variation between participants' discounting. The CCT, the delay, the adjusting procedure and the reward amounts were sufficiently understood by all participants, with the exception of one participant in Experiment 1. Increasing the delay to the LL reward from 30 seconds in Experiment 1 to 60 seconds in Experiment 2 reduced the number of corner solutions, but tolerance of the 60 second delay was still high suggesting the need to increase delays further in future studies. The experimenter noted a reduction in the number of participants whose attention appeared distracted from the CCT in Experiment 2 after the removal of additional graphics included in Experiment 1. It appeared the monetary reward provided the necessary motivation for participants to choose the larger delayed reward and the CCT was sufficiently engaging for participants to produce episodes of consistent responding during the 20 choice trials.

Although a small number of participants required the full twenty choice trials to reach a point at which they responded consistently, 13 out of the total 19 participants in Experiments 1 and 2 started responding consistently either before or on choice trial number 6. Furthermore, several participants in Experiments 1 and 2 decreased their tolerance of the delay and tended to respond more quickly toward the end of the session, suggesting participants

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were averse to the session duration. Therefore, it seems reasonable to propose a reduction in the number of choice trials presented to participants in future studies. Some participants had difficulties progressing from the practice (no- choice) task to the real (choice) task in Experiment 2, possibly due to the different requirements of the practice and main tasks. Consequently, providing a practice task more similar to the main task was likely to be more useful to participants in preparing and familiarising them with the main task.

In contrast to initial expectations and to findings reported by McCown et al. (1993) and by Mitchell (1999), both Experiments 1 and 2 found that participants responded more slowly when choosing the SS reward (the most impulsive). It is possible that this was because participants were highly motivated by the monetary reward but averse to the delay, and therefore expended more time considering their options when presented with increased SS values. It seems reasonable to consider the effect the CCT had on the validity of RT data in Experiments 1 and 2. For example, given the lack of similarity between the practice CCT and the main CCT participants may have responded more slowly in the initial choice trials whilst they became familiar with making choices in the main task. Furthermore, as several participants increased their discounting and responded more quickly to options towards the end of the testing session, it seemed possible that RT was affected by session duration. Therefore, RT was confounded by choice trial number and consequently the RT data in Experiments 1 and 2 lacked validity, and further research concerning the relation between response time and choice is required.

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In examining the SS data obtained from Experiment 1 and Experiment 2, it became apparent that there were problems with the adjusting method. Firstly, the program was required to round adjusted amounts to whole values (rounding up when a value includes the decimal .5 or above, and down if the value includes a decimal .49 or below). However, the adjustment procedure used in Experiments 1 and 2 inconsistently rounded values, causing incorrect SS values to be presented. For example (see Figure 2.11), if a participant chose 5p SS in the initial choice option, the SS reward amount in the next trial would be calculated as half of the previous SS amount (rounded to 3p) subtracted from the previous SS value, therefore presenting 10p LL versus 2p SS in the following trial (the correct adjustment). However, if a participant chose 3p SS when presented with 10p LL versus 3p SS, the following option they were presented with was 2p. Calculated correctly, this SS reward amount should have been 1p (half of 3p SS is 1.5p (rounded to 2p) subtracted from 3p SS).

Secondly, the calculated SS adjustments decreased more rapidly than they increased, requiring more LL choices to obtain the higher SS values (see Figure 2.11 below). Therefore, it was necessary to apply an alternate adjusting method that generated similar progressions between SS reward amounts. The current adjustment procedure calculated SS adjustments using the SS value, resulting in larger adjustments occurring with larger SS reward amounts. A possible alternative to this procedure that would generate a more progressive increase and decrease of SS reward amounts, involved calculating adjustments using previous adjustments.

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Figure 2.11 Choice options calculated by the adjustment procedure used and presented in Experiments 1 and 2.

Research by Du et al. (2002) provided evidence to support the effective use of an adjusting procedure that would enable this. As mentioned in chapter 1, a number of adjusting methods have been developed. However, the DAA designed by Du et al. took only six choice trials to converge on an indifference point at a single delay. Although initial concerns were that such small numbers of trials would force participants towards an invalid IP, the majority of participants in Experiments 1 and 2 responded consistently (i.e. made the same choices time after time) to options by choice trial 6, with evidence suggesting several participants were averse to the session duration. In consideration of research by Kowal, Yi, Erisman and Bickel (2007) and in addition to difficulties adolescents with BESD are reported to have in sustaining attention (see section 1.2), it is reasonable to propose that IPs would be more efficiently obtained from

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six trials rather than the 20 trials presented to participants in Experiments 1 and 2.

Experiments 1 and 2 provided evidence to suggest the CCT was understood and effective in producing indifference points to measure delay discounting in adolescents with BESD. Despite initial reports by staff regarding participants’ lack of motivation for money, money functioned as a suitable reward and was sufficiently understood by all participants. Participants produced varied rates of discounting, with several participants producing highly self-controlled responding and others responding more impulsively (choosing more SS rewards). Increasing the LL delay produced fewer corner solutions in Experiment 2 and increasing the delay further was proposed for future studies in the present project to bracket participants' responding more precisely. However, several problems with the CCT arose and re-programming was

required to obtain more valid measures of adolescents’ discounting to

determine the effectiveness of the CCT as a measure of impulsivity. Additional RT data was required to explore more accurately whether impulsivity was related to RT. Such evidence could provide insight into the degree of awareness involved in impulsive choice, and consequently offer evidence that impulsivity might be reduced through increased self-awareness.

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