Fig. 4. space cadet swimming can be phenocopied by lesioning two rhombomere 3 commissures. Dorsal views of the head of 120 hpf larvae stained with a neurofilament antibody. A tungsten needle was used to separate the nervous system along the midline, the red line indicates the rostrocaudal extent of the cut. The swimming pattern of the operated larvae was analyzed 2 to 16 hours later. Severing midbrain commissures (A) produces mainly larvae with wild-type swimming, while severing the hindbrain along its entire length (B) phenocopies the space cadet swimming defect. Further lesions were restricted to different hindbrain regions, encompassing only the two commissures in rhombomere 3 (red line in C), and between the two rhombomere 3 commissures and the Mauthner neuron (green line in C). (D) Non- operated control larva, the arrows pointing to the rhombomere 3 commissures. (E) Larva in which only these two commissures but not the adjacent axons of the Mauthner neuron (M) were severed.