The retina of the leopard frog projects topographically to
the superficial neuropil of the entire contralateral tectum.
In the rostromedial neuropil of the tectum, there is a map of
the binocular region of the visual field seen from the ipsilateral
eye that is in register with the map of the binocular region
of the visual field seen from the contralateral eye. The
ipsilateral eye projects indirectly to the tectum through nucleus
isthmi (n. isthmi), a midbrain tegmental structure. N. isthmi
receives input from the ipsilateral optic tectum and sends
projections bilaterally that cover both tectal lobes. Previous
workers have not been able to find visual activity from the
ipsilateral eye in the caudolateral optic tectum, representing
the monocular visual field of the contralateral eye. We show
electrophysiologically that across the entire extent of n. isthmi
there are two superimposed maps, one map representing the entire
visual field of the contralateral eye, the other map representing
the binocular visual field of the ipsilateral eye. We also studied
the behavioral consequences of localized lesions to n. isthmi
and compared them to the behavioral consequences of localized
lesions to the optic tectum representing equivalent areas of
the visual field. Lesions to the optic tectum produce scotomas
in the corresponding portion of the visual field. Lesions to
n. isthmi, even medial n. isthmi representing the superior visual
field, lead to scotomas in the temporal-most portion of the
contralateral ground level visual field. Thus, the representation
of visual space in n. isthmi is not a simple copy of the tectal
representation of visual space.