One of the most widely discussed and conserved responses to predatorial threat across many phyla is the state of behavioural immobility known as ‘freezing’. Freezing is also a stereotypical response observed in laboratory settings, where escape is often restricted by the behavioural context. Because it can become rapidly associated with learned threats, freezing presents an ideal behavioural parameter for assessing levels of conditioned fear and (by extension) learning and memory processes. Almost any researcher who uses rodent behavioural readouts will have come across freezing, and when browsing posters at big neuroscience conferences it can feel that every other poster sports a plot in which quantified freezing behaviour is used to demonstrate the effect of a manipulation. In modern systems neuroscience, the availability of novel recording and image-analysis tools has recently driven a renewed interest in and expansion of the behavioural testing toolkit to encompass many other rodent behavioural patterns. However, freezing remains one of the most widely used rodent behavioural readouts.

Behavioural immobility as a fear response was described in the 19th century by Darwin and, a century later, characterized as ‘crouching’ in rodents by Robert and Caroline Blanchard. Yet, it was Michael Fanselow, together with his mentor Richard Bolles, who — in his seminal paper from 1980 — first systematically operationalized freezing, defined as the absence of movement despite respiration, as a particular component of learned fear.