^A Rats were trained to find food. Even with highly salient cues in each corner (different smells, numbers of colored lights, etc.), rats who were disoriented each time before being placed in the box were unable to break that 180-degree rotational symmetry.²⁹ Rats who were not disoriented were easily able to break the symmetry. See my earlier book Beyond the Cognitive Map for a detailed discussion of the rat experiment and what is known about its neurophysiological correlates and mechanisms.

^B I know that when I’m writing a grant application or working really hard on a paper, I often find that I need a regular infusion of severe sugar to keep my writing stamina up. You can tell I’m working hard when I’ve got a donut for my midmorning snack.

^C As discussed in Appendix B, fMRI measurements depend on increased blood flow because “highly active” neurons require additional resources (but not additional oxygen) from the blood. Although it is not known what all of those additional resources are, a likely candidate is glucose.⁴⁰ So, when a neural structure increases its activity (whatever that actually means⁴¹), the brain accommodates those needs by increasing blood flow. However, the increased flow is much larger than the increased oxygen needs, so with increased blood flow and the same oxygen needs, more oxygen is left in the bloodstream, which is what is detected by fMRI and similar imaging systems. Increased activity can also be tracked by using a radioactive glucose molecule, 2-deoxyglucose. The radioactive 2-deoxyglucose is injected into an animal’s bloodstream and is then taken up by particularly active neurons. The presence of the marked 2-deoxyglucose can be detected postmortem in certain tissues, which have been “highly active.” 2-deoxyglucose tends to mark the same structures that fMRI does. The fact that fMRI identifies certain structures as “involved” in certain tasks suggests that different abilities draw on different parts of the brain and that the glucose hypothesis may affect more systems than just the self-control systems.

^D The term “prefrontal” comes from the idea that it is in the “front of the frontal cortex.” Because this part of the cortex is one of the most differentiated between primates and other mammals and between humans and other primates,⁴⁶ it was historically identified as the locus of the abilities deemed to be uniquely human (such as the “superego”⁴⁷). However, as anatomical studies have become more detailed and more developed, it has become possible to identify homologous areas in the frontal brain regions of other animals as well.⁴⁸ (A homologous structure is one that serves the same purpose between two species and shares an evolutionary history. For example, no one would deny that the rat heart, the monkey heart, and the human heart are all homologous structures. As we have seen throughout this book, many brain structures are conserved between these species as well.) Similarly, as we have begun to develop self-control tasks that can be used in animals (such as the stop-signal task), we have begun to identify specific pathways, structures, and information-processing mechanisms through which self-control works.⁴⁹ The exact details of the homology between species and the exact details of their respective roles in decision-making are an area of active research.⁵⁰