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). Yet, how individuals vary in these responses, and how such differences are encoded in the brain, remains unclear. Here, we developed a contextual fear conditioning paradigm in which adult zebrafish learned to associate a specific tank with an aversive pheromone, conspecific alarm substance (CAS). Behavior during memory expression comprised of a mix of active and passive responses that clustered into four behavioral clusters: non-reactive, evaders, evading freezers, and freezers with the two high-freezing groups most clearly reflecting fear memory formation. We tested memory in four strains (AB, TU, TL, and WIK), and both sexes, finding that they shaped fear memory responses: males were more likely to increase evasion during CAS exposure, whereas TU fish were more likely non-reactive during recall. Whole-brain activity mapping during memory expression revealed that freezing engaged widespread neural activity that was particularly strong in the cerebellum, reticular formation, and parts of the telencephalon associated with olfactory processing. Evasion recruited visual threat detection regions and suppressed foraging and navigation circuits. Functional network analysis revealed that fish that froze, but had low levels of evasion (i.e., freezers) during memory expression had strong pallial-cerebellar connections and heightened interconnectivity between visual (pretectal) and thalamic like (preglomerular) nuclei. Animals that mixed freezing with evasive behaviors (i.e., evading freezers) had higher subpallial connectivity with autonomic and stress-regulatory regions like the hypothalamus and preoptic areas. Together, these findings demonstrate that distinct behavioral types during fear memory recall are supported by divergent brain-wide networks.