Pattherns of cytochrome oxidase activity in the visual cortex of a South American opossum

The normal pattern of cytochrome oxidase (CO) activity in the posterior cortical areas of the South American opossum (Didelphis marsupialis aurita) was assessed both in horizontal section of flattened cortices and in transversal cortical sextions. the tangential distribution of CO activity was uniformly high in the strate cortex. In the peristriate region alternating bands of dense and weak staining occupied all the cortical layers with the exception of layer I. This observation suggests the existence of a functional segregation of visual processing in the peristriate cortex of the opossum similar to that present in phylogenetically more recent groups

The nucleus of the optic tract of the opossum

This paper reviews anatomical and electrophysiological data on the nucleus of the optic tract (NOT) of the opossum, a nucleus in the afferent branch of the horizontal optokinetic reflex. It is proposed that subcortical routes are essential for responses from the two eyes: a direct retinal projection from the contralateral eye and a commissural pathway between the two NOTs for the ipsolateral eye. In the latter case there's evidence that the commisural axons have a relay on inhibitory neurones. This circuit accounts for the differences in response pattern under monocular condition: temporo-nasal motion of the visual stimulus elicits excitation in the contralateral NOT, resulting in inhibition of the ipsolateral nucleus, while naso-temporal motion promotes inhibition in the contralateral nucleus, releasing the ipsolateral nucleus from the commissural input.

Horizontal optokinetic reflex in the opussum Didelphis marsupialis aurita

Electro-oculographic recordings were performed in 10 opossums. The optokinetic reflex was elicited by projecting a random dot stimulus on a cylindrical screen moving horizontally from left to right or right to left at various constant speeds. Binocular stimulation yelded the same response as the temporal to nasal monocular condition. The nasal to temporal monocular response was always less than to the opposite direction: 50% at 3 degrees/s and 15% at 18 degress/s. These results are discussed in a comparative context

Evidence for recrossing of the pathway from the retina to the ipsilateral nucleus of the optic tract

Single-unit recordings of the nucleous of the optic tract (NOT) under visual stimulation were performed in 5 opossums. Most of the units were directionally selective. Receptive fields for the contralateral eye lie mainly in the contralateral field while those for the ipsilateral eye were mainly in the ipsilateral field. As the nasal does not project ipsilaterally, recrossing must occur in the pathway from the retina to the ipsilateral NOT. Possible sites for this recrossing are discussed