Seeing beyond the midline: the role of the contralateral isthmotectal projection in the leopard frog

The ground level visual field of each eye of the leopard frog includes the entire ipsilateral 180-deg field and approximately 60 deg into the frontal contralateral field. When one eye is covered with an opaque patch, a frog responds to prey stimuli over the entire field of the other eye. Nevertheless, when one optic nerve is cut, the animal responds to prey in the ipsilateral hemifield of the connected eye, but only responds as far as about 30 deg past the frontal midline. If one optic tract is cut, the animal does not respond to prey past the frontal midline. We hypothesized that the responses past the frontal midline might be mediated by input from contralaterally projecting isthmotectal fibers. These fibers originate in the nucleus isthmi, a posterior midbrain structure. We found that when we placed an opaque patch over one eye and either ablated the ipsilateral nucleus isthmi, or cut crossing isthmotectal fibers in the optic chiasm, or blocked input to nucleus isthmi by ablating the ipsilateral tectal lobe, animals did not respond to prey stimuli past the frontal midline. We found that when we placed an opaque patch over one eye and cut crossing optic fibers in the anterior part of the optic chiasm (sparing crossing isthmotectal fibers), animals responded to prey stimuli in the nasal half of the seeing eye's contralateral frontal field. Our results suggest that contralaterally projecting isthmotectal fibers enable the frog to respond to stimuli past the frontal midline. We suggest a one-dimensional model of how nucleus isthmi influences tectal function.