Individual differences in fear memory expression engage distinct functional brain networks

Fearful stimuli elicit a mix of active (e.g., evasion) and passive (e.g., freezing) behaviors in a wide range of species, including zebrafish ( Danio rerio ). How these different responses are encoded in the brain, and the extent to which individuals vary in their fear responses, is not clear. To investigate this, we first developed a contextual fear conditioning paradigm in adult zebrafish where fish associate a specific tank with an aversive pheromone, conspecific alarm substance (CAS). We collected data from over 300 fish across four different strains (AB, TU, TL, and WIK) and both sexes finding that, as expected, fish exhibit a mix of active and passive responses during CAS exposure and during memory expression. We also found that behavior fell into four distinct groups: non-reactive, evaders, evading freezers, and freezers, with the evading freezer and freezer groups most clearly associated with memory formation. Background strain and sex also influenced how fish respond to CAS, with males more likely to increase evasive behaviors in response to CAS and the TU strain more likely to be non-reactive during recall. Finally, using whole-brain activity mapping, we identified the brain regions associated with active and passive fear responses during memory expression and how the functional brain networks of evading freezers and freezers differed. Freezing behavior was associated with widespread neural activity that was particularly strong in the cerebellum, reticular formation, and parts of the telencephalon associated with olfactory processing. Evasive behavior was associated with an increase in the activity of visual threat detection and a decrease in the activity of brain regions related to foraging and navigation. Network analysis revealed that animals with high freezing and low evasion (i.e., freezers) had strong interactions between the pallium and cerebellum as well as strong interconnectivity between visual (pretectal) and thalamic analog (preglomerular) nuclei. Animals that mix freezing with evasive behaviors (i.e., evading freezers) had higher subpallial connectivity to regions involved in autonomic function and stress responses such as the hypothalamus and preoptic areas.