The effect of delay, utility, and magnitude on delay discounting in an animal model of Attention-Deficit/Hyperactivity Disorder (ADHD): a systematic review

Abstract

The delay discounting paradigm involves choosing between a small, immediate reinforcer (SS) or larger, delayed reinforcer (LL). Children with ADHD tend to choose the SS reinforcer more often than controls, which is interpreted as impulsivity. Studies on an animal model of ADHD, the Spontaneously Hypertensive Rat (SHR), show the same pattern, with SHR preferring the SS reinforcer. However, it is not entirely clear why this pattern exists. It has been proposed that ADHD children tend to be delay averse, i.e. that they actively seek to reduce trial time. An alternative hypothesis is that ADHD children struggle to see the long-term utility of their choices. We reviewed data from eight SHR studies on delay discounting and investigated which hypothesis was the best predictor of LL preference. Results found that SHRs and controls do not differ in overall performance on the delay discounting task, regardless of whether the independent variable is delay between response and reinforcer, magnitude of the reinforcer, or utility of the large reinforcer. However, if utility is held constant while the response-reinforcer delay is manipulated, SHRs show a steeper discounting curve than controls.

Full text

The effect of delay, utility, and magnitude on delay discounting in
an animal model of Attention-Deficit/Hyperactivity Disorder
(ADHD): a systematic review
The delay discounting paradigm involves choosing between a small, immediate reinforcer (SS) or larger, delayed reinforcer (LL). Children
with ADHD tend to choose the SS reinforcer more often than controls, which is interpreted as impulsivity. Studies on an animal model of
ADHD, the Spontaneously Hypertensive Rat (SHR), show the same pattern, with SHR preferring the SS reinforcer. However, it is not entirely
clear why this pattern exists. It has been proposed that ADHD children tend to be delay averse, i.e. that the time between response and
reinforcer is something they opt to avoid. An alternative hypothesis is that ADHD children struggle to see the long-term utility of their choices.
We reviewed data from eight SHR studies on delay discounting and investigated which hypothesis was the best predictor of LL preference.
Results found that SHRs and controls do not differ in overall performance on the delay discounting task, regardless of whether the dependent
variable is delay between response and reinforcer, magnitude of the reinforcer, or utility of the large reinforcer. However, if utility is held
constant while the response-reinforcer delay is manipulated, SHRs show a steeper discounting curve than controls. The evidence suggests
the possibility that SHRs may be delay averse.
Delay discounting involves choosing
between a small, immediate reinforcer (SS)
and a larger, delayed reinforcer (LL). In
ADHD studies, ADHD children will typically
show a steeper discounting curve than
controls, displaying impulsivity (Demurie et
al., 2012).
It is hypothesised that ADHD children are
delay averse, because they actively seek to
reduce trial time by choosing the smaller
reinforcer (Sonuga-Barke et al., 1992).
Top: Percentage of LL choices as a function of response-reinforcer delay
for the large reinforcer. Significant only at 12 seconds.
Center: Distribution of LL choices by utility.
Bottom: Preliminary data from a meta-analysis on SHR and delay
discounting. Final weighted d may change pending full data collection.
We collected data from eight studies on
delay discounting that used SHR as
participants (Adriani et al., 2003; 2004; Fox
et al., 2008; Garcia & Kirkpatrick, 2013;
Hand et al., 2009; Íbias & Pellón,
2011;2014; Pardey et al., 2009).
Data was analysed in terms of utility (long-
term value of the large reinforcer relative to
the small reinforcer), response-reinforcer
delay, and magnitude of the large reinforcer.
Preliminary data on a meta-analysis is also presented.
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The effect of delay, utility, and magnitude on delay discounting in
an animal model of Attention-Deficit/Hyperactivity Disorder
(ADHD): a systematic review
0%
10%
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90%
100%
0 1 3 6 10 12 20 24 30 60
% of LL choices
Response-Reinforcer Delay
Percentage of LL choices as a function of strain and RRD
SHR
WKY
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00
% of LL choices
Utility
Percentage of LL choices as a function of strain and
flexible utility
SHR
WKY
Overall, the SHRs and controls (WKY rats)
did not differ in their percentage of LL
choices regardless of whether the measure
was response-reinforcer delay, magnitude of
the reinforcer, or utility (all ps > .09).
Thus, contrary to what is often reported in
the literature, the SHRs do not differ in their
performance relative to controls, and
therefore do not display the same impulsivity
as observed in ADHD children.
There was one exception to this pattern:
If the utility was held constant, then SHRs
would show a steeper discounting curve
than controls as response-reinforcer delay
increased. This finding may suggests that
SHRs are delay averse compared to
controls, similar to ADHD children (Fox et
al., 2008; Hand et al., 2009).
However, this manipulation was only
possible by manipulating the inter-trial
interval. A possibility therefore exists that
this pattern reflects a sensitivity to inter-trial
delays rather than response-reinforcer
delays.
Furthermore, the data on which this
exception was based on only one set of rats.
Department of Behavioral Sciences
The Spontaneously Hypertensive Rat (SHR)
is a validated animal model of ADHD
(Sagvolden et al., 2000). While SHRs tend
to show the same impulsive behaviour as
ADHD children in the delay-discounting
paradigm (e.g. Fox et al., 2008), we cannot
be certain that they display this behaviour
for the same reason. There are two primary
hypotheses that attempts to explain
discounting in SHRs:
1) Delay aversion hypothesis: The SHRs
are delay averse and choose the SS
option more frequently as the response-
reinforcer delay increases, because they
want to avoid the waiting between their
response and the reinforcer.
2) Optimality hypothesis: The SHRs are not
able to see the long-term value, or utility,
of their choices, and so choosing the SS
option more frequently is a result of them
discounting the utility of the options more
than controls do.
r = .57
r = .50
-8.00
-6.00
-4.00
-2.00
0.00
2.00
4.00
6.00
8.00
Effect size g
Distribution of effect sizes (g)
g= -0.175
SHR ->
controls ->