The brain’s perception of the emotional valence of an external event as a stressor is dependent on the current environment and modulated by previous experiences (memory), current physiological state, traits (e.g., neuroticism), and genotype. The brain generates outputs to the autonomic nervous systems (ANS), the hypothalamo-pituitary-adrenal (HPA) axis, the immune system, gene expression and epigenetics (overall increasing time duration of stress activation components from left to right. These responses directly affect the body but also feedback to the brain. Learning includes assessment of risks and rewards and it can be clinical designed to reduce reactivity, e.g., allergy therapy or mindfulness meditation. b). Example of self-reinforcing stress response system that is pathological if in a non-threatening environment. Normally, while stress activation from a loud non-threatening noise may initially activate a stress response, response to repeated loud noise will be attenuated through negative feedback (e.g., habituation). In PTSD emotional memories and the stress activation itself may contribute to an auto-reinforcing positive feedback loop. As mentioned in the text and , this PTSD attractor basin may be entered secondary to a single severe negative event via a catastrophic dynamical systems event. This pathological transition is more likely in those with predispositions, e.g., neurotransmitter alterations such as depression.