Reactivity

Reactivity describes how strongly a neural or physiological system responds to a given stimulus — the amplitude of the response, not merely whether one occurs. A highly reactive amygdala produces a larger signal when it encounters a threatening face than a less reactive one would to the same input; the difference is one of degree, not kind.

In emotional neuroscience the term most often appears as amygdala reactivity or threat reactivity, both of which name the same idea applied to fear- and threat-processing circuits. Individual reactivity varies considerably: some people show consistently larger amygdala responses to emotional stimuli than others, a pattern linked to traits such as neuroticism and to conditions like anxiety disorders.

Reactivity also describes whole physiological systems — heart rate reactivity, cortisol reactivity, and skin conductance reactivity all express the same principle: how far a system moves from baseline in response to a challenge.

How it works · its role

When a threatening or emotionally salient stimulus reaches the brain, sensory signals arrive at the amygdala — a structure involved in tagging events as significant. From there the amygdala relays to the hypothalamus and the autonomic nervous system, initiating the stress response: raised heart rate, cortisol release, and heightened alertness.

The prefrontal cortex counterbalances this. Through inhibitory connections it can dampen amygdala firing, a process called top-down regulation. High reactivity may reflect a stronger initial amygdala signal, weaker prefrontal inhibition, or both; the balance between them shapes the intensity and duration of an emotional response.

A common caveat: the BOLD signal used in fMRI studies is an indirect measure of neural activity, not a direct readout of firing rate. A larger scan signal means stronger regional blood flow, not necessarily a more vivid subjective experience. The relationship between measured reactivity and what a person actually feels is real but imperfect.

Relevance to substances & effects

Many psychoactive substances produce their characteristic effects partly by shifting reactivity in emotional-processing circuits. Benzodiazepines enhance GABA — the brain's main inhibitory neurotransmitter — which reduces firing across stress-related circuits and measurably lowers amygdala reactivity to threatening stimuli. This suppression is the mechanistic basis of their anxiolytic effect.

MDMA releases serotonin and promotes oxytocin signalling. Studies have found reduced amygdala responses to threatening or hostile faces after MDMA administration, a shift thought to underlie the lowered social defensiveness and emotional openness users commonly report.

SSRIs reduce amygdala reactivity to negative stimuli over weeks of use — a change that appears to precede or accompany mood improvement in many people. Cannabis shows the opposite pattern in some individuals: higher THC doses can increase amygdala reactivity, particularly in those predisposed to anxiety, which is consistent with cannabis-related paranoia as a reported effect.

Serotonergic psychedelics present a more complex picture. They appear to alter how emotional salience is processed across several circuits simultaneously, and their net effect on reactivity varies with dose, set, and setting. Research into this remains active and does not yet resolve to a simple increase or decrease.