Background

How the brain processes danger and then reacts to it has been studied extensively. Recently there has been a shift towards using more ethologically relevant behaviors. This emerging field of “neuroethology” combines behavioral information in tasks that are relevant to the species with real-time recordings of neural activity data from specific cell populations in the brain.

In the late 1980’s, Blanchard et al. proposed a fear taxonomy (Blanchard et al., 1986), in which they contemplated the idea that responses to fear were coordinated in an organized fashion that balanced threat with opportunity. Those studies provided direct evidence that variables such as predator proximity or escape opportunities influence how an individual behaves in response to a putative threat. For example, changes in the distance between a predator and prey, known as a change in threat imminence, initiate changes in the behavior of the prey. Specifically, the prey might exhibit a passive response to avoid detection when the predator is distant, but rapidly transition to flight mode to avoid capture as the predator approaches.

More recently, investigators have explored experimental approaches that are more naturalistic both in terms of the stimuli that are presented and in the opportunities an animal has to execute an action. An intriguing approach, that relies exclusively on visual stimulation to elicit innate fear responses, has been developed for rodents. Pioneered by Yilmaz and Meister (Yilmaz and Meister, 2013), it uses a dark visual stimulus that looms overhead and then increases in size, mimicking the rapid advance of a flying predator. This was paired with a rudimentary shelter in an open arena, thus providing mice an opportunity to find safety. Under these conditions, when exposed to an expanding disk overhead, mice engage in one of two different strategies: they either show a decrease in motor activity (freezing) or they initiate an escape response that targets the shelter. The probability of executing a given behavior is strongly dependent on the various aspects of the experimental parameters.

While many protocols for eliciting a fear response are species specific, the looming visual stimulus can be used across multiple species. Without having to change the physical characteristics of the stimulus, a looming and advancing black disk elicits innate defensive responses from zebra-fish (De Marco et al., 2016) to humans (Mobbs et al., 2007) and, as shown in this protocol, in mice (Evans et al., 2018 ; Daviu et al., 2020). This protocol is highly reproducible and also allows for the simultaneous acquisition of neural activity using single fiber photometry (Daviu et al., 2020) or single-cell data using head-mounted, miniature endoscopes. This new methodological approach may help advance translational knowledge about how different species compute threats and execute innate defensive behaviors.