Investigations into the source of binocular input to the nucleus of the optic tract in an Australian marsupial, the wallaby Macropus eugenii

Recordings from direction-selective neurons in the nucleus of the optic tract (NOT) of the marsupial wallaby, Macropus eugenii, show that 53% of cells are sensitive to visual stimulation of both eyes. Anatomical tracing studies using horseradish peroxidase reveal many retinal terminals in the contralateral NOT but very few in the ipsilateral nucleus. There was no convincing evidence of cortical inputs to the ipsilateral NOT despite large injections of tracer into the visual cortex. During visual stimulation in the visual field of the contralateral eye with moving patterns, the excitatory responses in the NOT generated by ipsiversive motion (right-to-left when recording from the left NOT) were usually larger than the inhibitory responses produced by contraversive motion. Conversely, during ipsilateral eye stimulation, the negative motion components to contraversive motion were usually larger than the positive components to ipsiversive motion. This response pattern resembles that observed in the NOT of the American opossum, Didelphis aurita, where binocularity appears to arise through a commissural subcortical pathway that connects the two nuclei and inverts the directional tuning of the transmitted signals. We propose that the lack of significant input from the ipsilateral eye and cortex in the wallaby suggests that binocularity must arise from another pathway, possibly a commissural route between the nuclei. As directional information appears not to be carried by the internucleus pathway in rats and cats, our results suggest that binocularity in the NOT arises from different sources in marsupials as compared to eutherians