This new paper may help you in your search for answers:

Exp Clin Psychopharmacol. 2014 Jul 7. Predictive Validity of Delay Discounting Behavior in Adolescence: A Longitudinal Twin Study.

Isen JD, Sparks JC, Iacono WG.

Their abstract: Abstract

A standard assumption in the delay discounting literature is that individuals who exhibit steeper discounting of hypothetical rewards also experience greater difficulty deferring gratification to real-world rewards. There is ample cross-sectional evidence that delay discounting paradigms reflect a variety of maladaptive psychosocial outcomes, including substance use pathology. We sought to determine whether a computerized assessment of hypothetical delay discounting (HDD) taps into behavioral impulsivity in a community sample of adolescent twins (N = 675). Using a longitudinal design, we hypothesized that greater HDD at age 14-15 predicts real-world impulsive choices and risk for substance use disorders in late adolescence. We also examined the genetic and environmental structure of HDD performance. Individual differences in HDD behavior showed moderate heritability, and were prospectively associated with real-world temporal discounting at age 17-18. Contrary to expectations, HDD was not consistently related to substance use or trait impulsivity. Although a significant association between HDD behavior and past substance use emerged in males, this effect was mediated by cognitive ability. In both sexes, HDD failed to predict a comprehensive index of substance use problems and behavioral disinhibition in late adolescence. In sum, we present some of the first evidence that HDD performance is heritable and predictive of real-world temporal discounting of rewards. Nevertheless, HDD might not serve as a valid marker of substance use disorder risk in younger adolescents, particularly females.

Thank you so much, Beatrice. This is a very clarifying study. 

It seems here that Delayed Discounting is separable from impulsivity, so they are distinct processes.

I will read this paper carefully, because I am still wondering what the causal process is. It seems that computation and choice are two different processes or stages. 

One very interesting paper on this topic is 

2008_Kalis_Cogn.Aff.Behav.Neurosc_Weakness of will, akrasia, and the neuropsychiatry of decision making

Dear Maria, thank you so much!!

I am sure this will help me with my doubts regarding temporal discounting and choice. It seems a very exhaustive and focused review on this topic. 

I will write here again if I have any new doubts. 

Thanks again!

Dear all, after thinking about this a little bit more, I would like to change the question. 

What I really want to know is if impulsivity, measured by tasks of delay discounting (wanting something now instead of a bigger reward in the future) is the same as disinhibition; the incapacity to withhold a response, measured in Go/No-Go tasks. I made a quick literature search and I did not really find any specific paper on this. 

In behavioral experiments, they could be both confounded, but my idea is that impulsivity is due to an overestimation of rewards, probably involving frontal dopamine; while disinhibition is due to an incapacity to inhibit or select inappropriate responses, probably involving serotonin. In this sense, I think that it is not the same to ask subjects between two options, options being a small reward now and a big reward later (delay discounting), than asking subjects to do or not do something (disinhibition). 

My hypothesis is that subjects with disinhibition would perform poorly on both delay discounting and Go/No-Go tasks, while impulsive subjects would only perform poorly on delay discounting tasks. 

Any help? Thanks in advance

The mediating role of impulsivity between mindfulness  (the "now") and trait disinhibition is clarified in:

Appetite. 2011 Apr;56(2):241-8. doi: 10.1016/j.appet.2010.12.007. A cross-sectional investigation of trait disinhibition and its association with mindfulness and impulsivity. Lattimore P, Fisher N, Malinowski P. [email protected]

Abstract

Two online surveys were conducted to assess the relationship between trait disinhibition, impulsivity, mindfulness and adverse psychological symptoms. In study 1 adult females (n=196; mean age=21 yrs) completed the Three-Factor Eating Questionnaire (TEFQ-R21), the Hospital Anxiety and Depression Scale and a measure of dispositional mindfulness. In study 2 adult females (n=190; mean age=26 yrs) completed the same measures as in study 1 with the addition of the Barratt Impulsivity Scale. In both studies it was predicted that mindfulness would be negatively related to trait disinhibition controlling for adverse psychological symptoms. The second study addressed the additional hypothesis that the relationship between mindfulness and trait disinhibition would be mediated by impulsivity. Regression analyses indicated that mindfulness was negatively related to and explained 11% of variation in trait disinhibition (study 1). This relationship was replicated and extended in study 2 whereby impulsivity mediated the relationship between mindfulness and trait disinhibition.

This complicates your new hypotheses. In one of my papers on drug usage and impulsivity,  Barratt Impulsivity Scale was also used.

At some level, impulsivity, could be defined as behavior that is inconsistent with a either a more proximal or distant behavior goal.  So as you sustain and maintain goal oriented behavior, you are not impulsive.  For persons with substance misuse, particularly those who are dependent, there are multiple competing contingencies (I and to be drug free, I don't want to disappoint people, I want to feel better,  I like the feeling of being high, I want to feel in control, etc.).  In ADHD the child has multiple competing contingencies (watching the teacher, watching other kids, noticing something out the window, turning in the chair, etc.). All of these behaviors may be in competition with the directed behavior goal, silent reading, for example.  Impulsivity or distractibility is perhaps an effort to finding something more rewarding.  Presumably, stimulant medications for ADHD influence the brain to register it is getting enough reward for maintaining the current behavior so that searching for better reward is not necessary.  We conducted a study evaluating the ability of ADHD children to conform to matching law. (Taylor D, Lincoln AJ, Foster SL. Impaired behavior regulation under conditionsof concurrent variable schedules of reinforcement in children with ADHD. J AttenDisord. 2010 Jan;13(4):358-68. Epub . PubMed PMID: 19520999.) 

Devaluation of delayed rewards is normative in pretty much every species studied (where in rodents etc, the delays to reward are acutely experienced).  In the drug abuse field, unusually severe discounting is the most consistent finding in behavioral economics. DD is thought to be *one manifestation* of impulsivity.  A seminal paper by John Evenden in Psychopharmacology 1999 lays out how diverse the construct of concept of impulsivity is.  See also a paper I did with Alan Swann and colleagues that suggests that impulsivity can be roughly divided into choice tasks like DD and failure to inhibit behavior in rapid stimulus-evaluation and response tasks (PMID 12062883).  Harriet de Wit at UChicago also published a typology of impulsivity.  Importantly, measures of impulsivity are generally higher across the board in severe populations, like poly-substance-addicted people who've cooked their frontal cortex, but measures of impulsivity frequently correlate poorly with each other within-subject.  Especially dicey are correlations between lab performance and paper-and-pencil measures.  Even paper and pencil measures can differ a lot.  The Barratt Impulsiveness Scale captures Ernie Barratt's more "cold cognitive" tempo-based concept, wheras the UPPS scale captures more of an affect-driven, negative-urgency based tendency to act rashly.  Bottom line- it's a diverse, messy construct.

Dear Alan, 

thank you very much for your contribution. 

I guess then that we are then talking about the same thing, because being able to pursue long term goals implies inhibiting short-term rewarding behaviors, as you have illustrated with your examples. 

Thank you James, the papers you mentioned are of great value. I hope to have a better idea of this concept once I read them. 

Although it seems to be a messy construct, it is always better to know little things than knowing nothing. 

Thank you all again for your contributions

Thank you so much, Stephan! I had not found it, so this will be very helpful. 

Just in case anyone else interested in the broader topic of Decision Making, I would recommend this paper

2011_Rogers_Neuropsychopharmacology_the roles of dopamine and serotonin in decision making-evidence from pharmacological experiments in humans