The eyes of weakly electric gymnotid fish are poorly developed in comparison to those of most diurnal teleosts. The tectum and pretectum, despite their usual association with the visual system, are large and well differentiated in gymnotids. We have studied retinal projections in gymnotids in order to define the visual components of the mesencephalon and diencephalon and thus allow comparison with other teleosts in which retinofugal fibers have been extensively mapped. Retinofugal projections reported in this work are based on the anterograde transport of conjugated wheat germ agglutinin horseradish peroxidase, following injection into the posterior chamber of the eye of Apteronotus leptorhynchus (brown ghost knife fish). The results show a remarkable similarity to those of non-electroreceptive teleosts. Although the optic nerves appear to cross completely at the optic chiasm, close scrutiny shows a slender recrossing fascicle which continues from the contralateral tractus opticus medialis through the rostroventral hypothalamus to reach the ipsilateral side, providing a scanty projection to the n. opticus hypothalamicus, n. anterior periventricularis, n. dorsolateralis thalami, and n. commissurae posterioris. A few fibers ascend via the tractus opticus dorsomedialis to the rostral dorsomedial part of the stratum fibrosum et griseum superficiale of the ipsilateral tectum. The main body of the retinal projections in Apteronotus are to the following contralateral target areas: preoptic area, n. opticus hypothalamicus, n. anterior periventricularis, n. dorsolateralis thalami, n. pretectalis, area pretectalis, n. corticalis, n. commissurae posterioris, n. geniculatus lateralis, area and n. ventrolateralis thalami, caudal dorsal tegmentum and the tectum opticum. The retinotectal projection is modest in comparison to that of more vision dependent fish and terminates mainly in the upper half of the stratum fibrosum et griseum superficiale; hardly any retinal fibers reach the caudalmost tectum.
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