Relationships of the visual cortex in the marsupial brush-tailed possum, Trichosurus vulpecula, a horseradish peroxidase and autoradiographic study.

The ascending and descending relationships of visual cortex in Trichosurus were determined using HRP and autoradiographic methods. The visual thalamus, LGNd and LP, was found to project to three cytoarchitecturally distinct areas of cortex with each of these regions displaying differing patterns of connectivity with other centres. The striate and peristriate cortices received homotypically organized projections from LGNd as well as projections from both divisions of LP. The posterior parietal cortex received projections from both LGNd and LP as well as from the latero-intermediate, posterior, ventroanterior and intralaminar thalamic nuclei. The cortical projection fields of LGNd and LP were co-extensive. Descending projections to thalamic centres were reciprocal with the exception of a descending only projection to the thalamic reticular nucleus. The striatogeniculate projection was homotypically organized as was the striatocollicular projection which extended only to the superficial layers of that nucleus. The posterior parietal projection to the superior colliculus involved the deeper layers of that nucleus. The entire visual cortex projected to the pretectum and to the pontine nuclei. The organization of visual cortex in Trichosurus differs from that reported in the American marsupial, Didelphis, in that the LGNd and LP complexes possess a very broad and co-extensive cortical projection, a finding substantially different to what has been reported in placental mammals as well. The organization of thalamic relationships with the posterior parietal cortex would also appear to be unusual with this region receiving convergent projections from at least seven separate thalamic nuclei.