Coulson, G., Koffer, K., Coulson, V., & Went, H. (2011). Positive Reinforcement as a Correlation Between Lever Pressing and Inerease in Frequency of food Pellet Delivery in rats. Mexican Journal of Behavior Analysis, 2(2), 165–175. https://doi.org/10.5514/rmac.v2.i2.25267