Quantifying Delay Discounting Across Species: Successes and Caveats for Human to Rat Translation

Heightened delay discounting has been linked to various psychopathologies, including substance use disorders, ADHD and mood disorders. Identifying the neural circuits and genetics related to delay discounting, a recognized economic cost-benefit tradeoff behavior, through translational research is therefore critical for understanding its linkage to complex psychopathological conditions. As research addressing delay discounting has expanded with human and nonhuman animals, different analytic traditions have developed. To evaluate whether measures used in human studies can be effectively translated into nonhuman animal research, we examined delay discounting using an adjusting-amount procedure in male and female rats choosing between a small, immediate sucrose solution reward ("smaller sooner") and 150 µl delivered following a delay (0-24 second delay; "larger later"). Adopting strategies developed from human studies, the percentage of nonsystematic data was calculated and found to be similar to past human studies. Some functions fit rodent data well (hyperbolic, exponential), but others fit less well (hyperboloid, quasi-hyperbolic). Incorporating side bias, a consideration particularly relevant to rodent procedures, improved all function fits. Area-Under-the-Curve and ED50 indices also benefited from the inclusion of side bias. These results support the idea that quantitative measures can be translated between species but suggest modifications may be necessary for summary statistics. The implications of these modifications on interspecies translation and data interpretation remain unknown.