Distribution of retinogeniculate cells in the Tammar wallaby in relation to decussation at the optic chiasm

Abstract

Partial decussation of the optic nerve in mammals is related to the laterofrontal placement of the eyes. To investigate this relationship in the wallaby (Macropus eugenii), injections of wheat germ agglutinin-conjugated to horseradish peroxidase were made into one dorsal lateral geniculate nucleus to label retinal ganglion cell bodies in both retinas. Contralaterally, labelled ganglion cells were present across the nasotemporal axis, except for the far temporal retina where they were absent or very sparsely scattered compared with the density of labelled cells at similar nasal eccentricities in the same retinas. Ipsilaterally, labelling was confined to the temporal retina. Cell counts confirmed a visual streak and an area centralis in the contralateral projection. Diameters of labelled cells ranged from 9 microm to 30 microm with a hint of three categories of cells based on size. Only the large alpha-type cells were easily separated. Measurement of the acceptance angles of the eye in the anaesthetised animal showed about 15% of the horizontal visual field of each eye projects into a region of binocular overlap giving a binocular field of 50 degrees . The uniocular visual field extends from -25 degrees (nasally) to + 162 degrees (temporally) in azimuth, giving the wallaby a monocular visual field width of 187 degrees and a total visual field width of 324 degrees . In elevation, field ranges from 70 degrees inferior to +120 degrees superior, encompassing 190 degrees in the vertical plane. The wallaby shows partial decussation of optic nerve fibres projecting to the lateral geniculate nucleus that could allow stereopsis, plus an extensive panoramic field.