The Visual System of the Florida Garfish, <i>Lepisosteus platyrhincus</i>(Ginglymodi) (Part 2 of 2)

Abstract

Infusion of cobaltous-lysine into the optic nerve of juvenile Florida garfish reveals that the preoptic area, pretectum, thalamus and the mediorostral and ventrolateral poles of the optic tectum each receive bilateral, monosynaptic input. The bilateral input to the mediorostral pole of the tectum projects via the dorsal optic tract and terminates in the rostral half of the tectum. The bilateral input to the ventrolateral tectum projects via the ventral optic tract and extends the whole length of the tectum. The incidence of direct ipsilateral input to the tectum suggests binocular vision may play a functional role in the survival of this species. In order to test this hypothesis, we examined the retinal distribution of ipsilateral retinal ganglion cells that project to the mediorostral tectum and the size and location of the binocular visual field. Cell distribution was examined by retrograde labelling with rhodamine-conjugated dextran amine delivered by microinjections into the mediorostral pole of the right optic tectum. In the ipsilateral retina, ganglion cells are distributed in a narrow temporoventral area with a mean of 0.44±0.14 x 10³ cells per mm² apposing the retinal margin. In the contralateral retina, ganglion cells are also distributed within this temporoventral region with a mean peak density of 2.33 ± 0.47 x 10³ cells per mm². Three broad classes of ipsilaterally projecting retinal ganglion cells (orthotopic cells within the ganglion cell layer, displaced cells within the inner nuclear layer, and giant cells within the ganglion and inner nuclear layers) are identified, intermingled in both the ipsilateral and contralateral retinae after retrograde labelling from the mediorostral pole of the tectum. Ophthalmoscopic mapping of the monocular and binocular visual fields reveals two small frontal wedges of binocular overlap. A dorsal wedge (12° wide) extends from approximately 7° above the horizontal axis to 10° beyond the vertical axis, and a ventral wedge (20° wide) extends approximately 10° below the horizontal axis to 15° beyond the vertical axis. The dorsal binocular visual field is subtended by the temporoventral region of the retina possessing both ipsilaterally and contralaterally projecting ganglion cells which terminate in the mediorostral pole of the optic tectum. Therefore, the partial decussation of retinal ganglion cell axons at the optic chiasm brings information from corresponding regions of the binocular visual field into register in the mediorostral pole of the optic tectum. In addition, the size of the binocular visual field matches the relative proportion of the retina containing ipsilaterally projecting ganglion cells. The ontogeny and phylogeny of ipsilateral retinofugal projections in actinopterygians is discussed in relation to the putative secondary pathways mediating binocular vision.