Functional implications of species differences in the size and morphology of the isthmo optic nucleus (ION) in birds
Date
2013
Authors
Gutierrez-Ibanez, Cristian I.
Iwaniuk, Andrew N.
Lisney, Thomas J.
Faunes, Macarena
Marin, Gonzalo J.
Wylie, Douglas R.
Journal Title
Journal ISSN
Volume Title
Publisher
Public Library of Science
Abstract
In birds, there is a retinofugal projection from the brain to the retina originating from the isthmo optic nucleus (ION) in the
midbrain. Despite a large number of anatomical, physiological and histochemical studies, the function of this retinofugal
system remains unclear. Several functions have been proposed including: gaze stabilization, pecking behavior, dark
adaptation, shifting attention, and detection of aerial predators. This nucleus varies in size and organization among some
species, but the relative size and morphology of the ION has not been systematically studied. Here, we present a comparison
of the relative size and morphology of the ION in 81 species of birds, representing 17 different orders. Our results show that
several orders of birds, besides those previously reported, have a large, well-organized ION, including: hummingbirds,
woodpeckers, coots and allies, and kingfishers. At the other end of the spectrum, parrots, herons, waterfowl, owls and
diurnal raptors have relatively small ION volumes. ION also appears to be absent or unrecognizable is several taxa, including
one of the basal avian groups, the tinamous, which suggests that the ION may have evolved only in the more modern
group of birds, Neognathae. Finally, we demonstrate that evolutionary changes in the relative size and the cytoarchitectonic
organization of ION have occurred largely independent of phylogeny. The large relative size of the ION in orders with very
different lifestyles and feeding behaviors suggest there is no clear association with pecking behavior or predator detection.
Instead, our results suggest that the ION is more complex and enlarged in birds that have eyes that are emmetropic in some
parts of the visual field and myopic in others. We therefore posit that the ION is involved in switching attention between
two parts of the retina i.e. from an emmetropic to a myopic part of the retina.
Description
Sherpa Romeo green journal: open access
Keywords
Isthmo optic nucleus , ION , Species differences , Birds , Retinofugal pathways , Retina
Citation
Gutierrez-Ibanez, C., Iwaniuk, A. N., Lisney, T. J., Faunes, M., Marin, G. J., & Wylie, D. R. (2012). Functional implications of species differences in the size and morphology of the isthmo optic nucleus (ION) in birds. PLoS ONE, 7(5), e37816. doi:10.1371/journal.pone.0037816