"The Neuroscience of Delayed Gratification" by Lindsey Jay
Just when you are about to sit down to do work, you suddenly hear a ping: a notification from your phone. Do you read the notification, or do you get to work, suppressing the urge to check it? Suppressing that urge is known as delay of gratification, the “act of resisting an impulse to take an immediately available reward in the hope of obtaining a more-valued reward in the future.” But are some people better at delaying gratification than others? What is the nature of delayed gratification as a developing trait? We will explore whether delay of gratification is a persistent or dynamic trait in people as well as look at what pathways are involved.
The Original Experiment
The classic experiment testing the persistence of delay of gratification is Walter Mischel’s “marshmallow test.” The researcher places a marshmallow in front of a child. The child can wait for some time (~15 minutes) until the researcher returns with a second marshmallow (together with the first, a bigger reward) or the child can eat the single marshmallow at any time and forgo the larger reward. When they compared the children’s delay times to their Scholastic Aptitude Test (SAT) scores ten years later, they found that longer delay times significantly correlated with higher SAT scores. This finding suggested that those who could delay longer in favor of a higher-value reward when they were young retained this tendency in their study habits when they grew older.
To test whether the tendency to delay persisted even further into adulthood, as well as to study which brain regions were active in participants who tended to delay rewards, Casey et al. conducted a follow-up study on nearly 60 of the same participants in their mid-forties. Participants were presented faces of males and females and were instructed to push a button (“go”) for one sex but to not push the button (“nogo”) for the other sex. However, some faces were happy (instead of neutral or fearful), which served as a distractor, since seeing a happy face reflexively elicits a “go” response in humans as a positive reward cue. The real test, then, lay in suppressing the “go” response when viewing a happy face in a “nogo” stimulus (i.e. male or female).
In their results, those with low delay of gratification from childhood were consistently less able to suppress a “go” response to a “nogo” happy face, showing lower impulse control. fMRI scans of those with high delay of gratification were subsequently compared between “nogo” happy face trials (requiring suppression) and “go” trials. They found that the right inferior frontal gyrus, a part of prefrontal cortex in the brain involved in executive decision making, was more active for “nogo” trials with correct responses, consistent with previous findings that prefrontal cortex is involved in response inhibition. On the other hand, fMRI scans of those with low delay of gratification displayed greater activity in the ventral striatum, which has been implicated in processing rewarding cues such as a happy face, compared to the group with higher delay of gratification during a “nogo” happy face task. These studies confirmed that delay behavior in these individuals persists into their adulthood. Additionally, higher activity in the inferior frontal gyrus may aid in suppression of the impulsive “go” response and higher activity in the ventral striatum may override the rational prefrontal cortex in favor of the more impulsive response in poor delayers. Thus, the key to higher self-control may lie in the prefrontal cortex effectively exercising control over the limbic reward pathway, which includes the striatum.
Role of the Reward Pathway
What about the interactions in the brain’s reward circuits? Abela et al. studied the interactions between the nucleus accumbens (NAc) and ventral hippocampus (vHC) versus those between NAc and orbitofrontal cortex (OFC) in delayed gratification behavior. Rats had either their vHC-NAc or OFC-NAc lesioned (cut), some contralaterally (i.e. disconnection) to abolish direct and indirect connections, and some ipsilaterally to disrupt only one hemisphere. They then participated in a delay-discounted task, where they could choose between two identical white panels: one immediately dispensed one pellet and the other dispensed four pellets after a delay. The delay for the reward gradually increased as the number of trials increased.
They found that only a contralateral vHC-NAc lesion lowered the rats’ tolerance for delays. These rats significantly preferred the small, immediate reward to the larger, delayed reward, suggesting that the vHC-NAc connection is involved in interpreting the trade-off between the costs and payoff of waiting. On the other hand, there was no difference in reward choice between rats with an OFC-NAc disconnection, those with an OFC-NAc ipsilateral lesion, those with an ipsilateral vHC-NAc lesion and controls (where no cuts were made). These findings suggest that the vHC-NAc interaction, and not the OFC-NAc connection, is important for decisions that involve waiting (i.e. future rewards). Thus, disrupting this pathway may affect the modulation of firing patterns of dopamine neurons in the ventral tegmental area, a part of the midbrain involved in reward pathways. However, since the OFC-NAc disconnection did not affect decision-making, the altered behavior may not be due to dopamine dysregulation, and it is unclear exactly what the vHC-NAc disconnection altered. It might be valuable to further investigate the cellular interactions in the vHC-NAc area, perhaps through extracellular recordings of the ventral tegmental area in rats during the delayed-discounted task.
Future Directions and Conclusions
For future studies, it may be useful to study the brain activity along the reward pathway, which includes the forebrain, medial forebrain bundle, substantia nigra, and ventral midbrain. There may also be applications in trying to teach young children impulse control to try to shape them towards delayed gratification behavior and potentially better working and studying habits. These studies have only begun to scratch the surface of the neural mechanisms and persistence of delayed gratification behavior. As the field of neuroscience grows, we will begin to explore these questions more deeply.
 Conti, Regina. "Delay of gratification." Encyclopædia Britannica. December 21, 2015. Accessed November 19, 2017. https://www.britannica.com/topic/delay-of-gratification.
 Mischel, W., Y. Shoda, and MI Rodriguez. "Delay of gratification in children." Science. May 26, 1989. Accessed November 19, 2017. http://science.sciencemag.org/content/244/4907/933.long.
 Casey, B. J., Leah H. Somerville, Ian H. Gotlib, Ozlem Ayduk, Nicholas T. Franklin, Mary K. Askren, John Jonides, Marc G. Berman, Nicole L. Wilson, Theresa Teslovich, Gary Glover, Vivian Zayas, Walter Mischel, and Yuichi Shoda. "Behavioral and neural correlates of delay of gratification 40 years later." Proceedings of the National Academy of Sciences of the United States of America. September 06, 2011. Accessed November 19, 2017. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3169162/.
 Abela, Andrew R., Yiran Duan, and Yogita Chudasama. "Hippocampal interplay with the nucleus accumbens is critical for decisions about time." The European journal of neuroscience. September 2015. Accessed November 19, 2017. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5233438/.