Pérez Manrique, T., J. H. Diaz Barbosa, and J. C. Pulido Vega. “Behavioral Contrast in Relational Operants With Human Participants”. Acta Colombiana de Psicología, vol. 12, no. 2, July 2009, pp. 115-26, https://actacolombianapsicologia.ucatolica.edu.co/index.php/acta-colombiana-psicologia/article/view/283.