Abstract
Orientation selectivity has been described in neurons of V1 for all mammals investigated thus far. Despite the ubiquitous expression of V1 orientation selectivity, distinctions exist across mammalian clades with regard to its functional organization. In primates orientation selectivity is organized in discrete columns across V1 whereas in rodents and lagomorphs orientation selectivity lacks clear organization or is organized in clusters. The overwhelming majority of studies of V1 to date have been conducted in carnivorans, primates, and rodents, which are members of superclades Laurasiatheria and Euarchontoglires. To gain a more complete understanding of the evolutionary trajectory of the functional organization of the neocortex, members of other mammalian superclades must be examined. Xenarthra, represented by anteaters, sloths, and armadillos, is considered to be either the basal eutherian superclade (placental mammals) or a sister superclade to Afrotheria, with an origin approximately 100 million years ago. Here we used the nine-banded armadillo (Dasypus novemcinctus) to determine whether the columnar organization that is the hallmark of visual cortex in carnivorans and primates is apparent in xenarthrans. After initially mapping the retinotopy of armadillo V1 using multi-unit recordings, we recorded single unit responses to drifting grating of different orientations. Drifting gratings (0.02-0.04 cycles per degree) at the preferred orientations activate armadillo V1 neurons strongly, and neurons are significantly orientation tuned using the OSI metric (mean OSI = 0.15 +/- 0.16). To determine whether a functional organization for orientation selectivity exists, we compared the orientation preference of neurons recorded along our electrode penetrations. We did not find evidence for a functional organization for orientation selectivity in armadillos, as the difference between neurons' orientation preferences was not related to the distance between recordings. These results suggest that the functional organization found in carnivorans and primates emerged through convergent evolution and does not reflect a shared mammalian functional organization.
Meeting abstract presented at VSS 2016