Figure 1.
Location of the isthmo optic nucleus (ION) in the brainstem.
Photomicrographs of coronal sections through the brainstem of different species of birds showing the location of the isthmo optic nucleus (ION). A shows the absence of ION in a seabird (Procellariiformes), the Short-tailed Shearwater (Puffinus tenuirostris). The coronal section is through the brainstem, at the level of the trochlear nucleus (IV), where the ION is usually found in other birds. B to D show the ION in (B) an owl (Strigiformes), the Northern Hawk Owl (Surnia ulula); (C) a parrot (Psittaciformes), the Galah (Eolophus roseicapilla); (D) a songbird (Passeriformes), the Eastern Spinebill (Acanthorhynchus tenuirostris). Imc = nucleus isthmi magnocellularis; Ipc = nucleus isthmi parvocellularis; TeO = optic tectum; Cb = Cerebellum; LLv = ventral part of the lateral lemniscus; Slu = nucleus semi lunaris. Scale bars in A and C = 400 µm, in B = 600 µm.
Figure 2.
Variation of the complexity of the cytoarchitectonic organization of ION: Categories 1–3.
Photomicrographs of coronal sections through the isthmo optic nucleus (ION) showing the variation of the complexity of the cytoarchitectonic organization of ION in different species of birds. A to C shows species that belong to category 1 of ION complexity (see methods). A, a diurnal raptor (Falconiformes), the Swainson’s Hawk (Buteo swainsoni); B an owl (Strigiformes), the Northern Hawk Owl, (Surnia ulula); C a hummingbird (Apodiformes), the Long-tailed Hermit (Phaethornis superciliosus); D and E shows species that belong to category 2 of ION complexity. D a woodpecker (Piciformes), the Yellow-bellied Sapsucker (Sphyrapicus varius); E a songbird (Passeriformes), the Rusty Blackbird (Euphagus carolinus); F and G shows species that belong to category 3 of ION complexity. F the Spotted Pardalote (Pardalotus punctatus); G the Zebra Finch (Taeniopygia guttata). F and G are both songbirds. Scale bars = 100 µm.
Figure 3.
Variation of the complexity of the cytoarchitectonic organization of ION: Categories 4 and 5.
Photomicrographs of coronal sections through the isthmo optic nucleus (ION) showing the variation of the complexity of the cytoarchitectonic organization of ION in different species of birds (see methods). A to C shows species that belong to category 4 of ION complexity. A shows a Galliform, the Spruce Grouse (Falcipennis canadensis); B showa songbird, the Scarlet Robin (Petroica multicolor); C shows a Columbiform, the Wonga Pigeon (Leucosarcia melanoleuca); D to F shows species that belong to category 5 of ION complexity. D a Coraciiform, the Belted Kingfisher (Megaceryle alcyon); E a songbird, the Superb Lyrebird (Menura novaehollandiae); F the Rock Pigeon (Columba livia). Scale bars = 100 µm, in E and C = 200 µm.
Figure 4.
Variation of relative volume of ION and relation to ION complexity.
A, Scatterplot of the isthmo optic nucleus (ION) volume plotted as a function of brain minus ION volume for all species examined (see table S1). n indicates to the number of species measured in each order. B, Bar graph of the relative size of ION expressed as a percentage of total brain volume grouped by the ION cytoarchitectonical complexity categories; the error bars indicate standard error. C. Phylogenetic relations among orders of birds surveyed in this study based on Hackett et al. (2008; 60]). The black bar graphs represent the relative size of ION expressed as a percentage of total brain volume for the different groups of birds. The error bars indicate standard error. The asterisk (*) indicates the groups in which a lower field myopia has been described [105]–[108]. The black diamond (♦) indicates species where a lack of lower field myopia has been described [109]. The colored bars represent the number of species that were examined of each ION cytoarchitectoncial organization complexity category in each order (see results, Table S1, Fig. 2). An = Anseriformes (red full circles); Ap = Apodiformes (empty orange circle); Ca = Caprimulgiforms; Ch = Charadriiforms (empty light blue circle); Ci = Ciconiiformes; Co = Columbiforms (dark green full circles); Cr = Coraciiforms; F = Falconiforms; G = Galliformes (dark blue full circle); Gr = Gruiformes; Pa = Passerifomes (empty brown circles); Pi = Piciforms; Ps = Psittaciformes (full yellow circle); St = Strigiforms (full black circle).
Figure 5.
ION cells numbers and cells density variation among birds.
A, Scatterplot of the cell numbers of ION plotted as a function of ION volume for all species examined (see table S1). The n between parentheses indicates the number of species measured in each order. B, Bar graph of the residuals of ION cell numbers against the ION volume (A) for different groups of birds; the error bars indicate standard error. C Scatterplot of the cell density (cells/mm3) in the ION, plotted as a function of brain volume for all species examined (see table S1). D, Bar graph of the residuals of ION cell density against the brain volume (C) for different groups of birds; the error bars indicate standard error. E, Bar graph of ION absolute cells numbers grouped by the ION cytoarchitectonic complexity categories; the error bars indicate standard error. n indicates to the number of species measured in each order. An = Anseriformes (red full circles); Ap = Apodiformes (empty orange circle); Ca = Caprimulgiformes; Ch = Charadriiformes (empty light blue circle); Ci = Ciconiiformes; Co = Columbiformes (dark green full circles); Cr = Coraciiformes; F = Falconiformes; G = Galliformes (dark blue full circle); Gr = Gruiformes; Pa = Passerifomes (empty brown circles); Pi = Piciformes; Ps = Psittaciformes (full yellow circle); St = Strigiformes (full black circle).