Proper response to external stimuli is essential for survival. Upon aversive stimuli animals display a stereotypical sequence of motor responses consisting of flight and/or freeze followed by recovery to baseline activity. While these motor responses cannot be executed simultaneously, they do however, occur sequentially. Aberrant response to aversive stimuli leads to depression, anxiety or addiction in humans. While studies have identified neuronal circuits that mediate aversive behavior in distinct brain regions, the biological correlates for how one circuit is selected over the other, a symmetry-breaking step known as competitive selection, is yet unclear. The evolutionarily conserved habenulo-interpeduncular nucleus (Hb-IPN) pathway has emerged as a crucial circuit that mediates fear and stress-related behaviors. This pathway is composed of two distinct circuits, the cholinergic and the peptidergic non-cholinergic, that innervate adjacent domains in the IPN. We recently found a hardwired mode of negatively correlated activity between the cholinergic and non-cholinergic circuits, whereby the synchronized activation of cholinergic neurons inhibits non-cholinergic neuron activity. This occurs through retrograde GABAB signaling at the IPN via GABAB receptors located on the non-cholinergic terminals. An aversive stimulus, electric shock also induces this mode of negatively correlated activity. We hypothesize that different external stimuli induce competitive selection in the habenulo-interpeduncular nucleus pathway to modulate the stereotypical motor response. By taking advantage of the small size and transparency of the zebrafish larval brain to easily monitor and modify neuronal activity in head-fixed animals during behavior, this project aims to understand how an evolutionarily conserved asymmetric structure in the brain that integrates various external stimuli utilizes an atypical mode of competition to modulate motor responses to aversive stimuli.