Anuran amphibians possess a very large binocular visual field. This field projects topographically to the mesencephalic optic tectum, the main visual center in lower vertebrates. Since retinal efferents in frogs and toads are completely crossed at the optic chiasma, information from one eye to the ipsilateral tectum must use an indirect route. This route involves an initial passage to the contralateral tectum, a relay into the subjacent mesencephalic nucleus isthmi, and subsequently a re-crossing isthmotectal linkage via the optic chiasma / post-optic commissural system. At the tectal level, the so-called visuotectal projection  from the ipsilateral eye is in-register with the projection from the contralateral eye. Because this direct projection undergoes dramatic changes throughout development and is able to regenerate throughout life, the isthmo-tectal linkage must be plastic enough in order to maintain the normal congruence between the two visual maps. Current works on nucleus isthmi and related projections isthmi deal therefore with visual function and plasticity. The present paper reviews the major morphological, physiological and behavioural characteristics of the nucleus isthmi in amphibians. Basic data from all classes of vertebrates are given for comparison. For mechanisms underlying developmental plasticity, the reader is referred to a recent extensive review .
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