Sex Differences in Strength of Fear Response

While the strength of the fear system can be inferred in part from the conscious experience of fear, it is also instructive to measure the activity of the relevant brain areas directly. Enhanced excitatory activation of some or all of these areas might be expected to increase the intensity of fear. The strength of the fear system has been assessed chiefly via neuroimaging studies in which images of faces showing different emotions (including fear and anger) are presented and regional activation in the viewer’s brain is recorded. Most studies identify a priori regions of interest, but increasingly sophisticated statistical programs and imaging techniques allow researchers to investigate structural, functional and temporal connectivity within and between brain circuits. The interpretation of findings is complicated by the difficulty (some would say impossibility) of mapping the correspondence between structure and function. For example, signals reflecting changes in blood-oxygen levels (so called BOLD signals) indicate regions that are active in response to threat, but these regions could be registering threat, augmenting the fear response, suppressing the fear response, supporting decision-making processes, or a number of other concurrent functions. Nonetheless, some key circuits associated with fear have consistently been identified. These include sub-cortical structures such as the amygdala (located within the medial temporal lobe and associated with fear registration) and the hypothalamus (located at the base of the brain, guiding our autonomic responses). Areas of the mid-brain such as the periaqueductal grey have a role in the expression of behavioural fear while broader regions such as the ventromedial and orbitofrontal cortices show heightened activity in relation to fear regulation, task monitoring, future simulation and action decisions. Appendix B provides more specific details regarding the various neural structures covered within the chapters of this text and their associations with the components of the fear system.